Effects of Hydroxypropyl Methylcellulose (HPMC)

Effects of Hydroxypropyl Methylcellulose (HPMC) on Processing Properties of Frozen Dough and Related Mechanisms
Improving the processing properties of frozen dough has certain practical significance for realizing large-scale production of high-quality convenient steamed bread. In this study, a new type of hydrophilic colloid (hydroxypropyl methylcellulose, Yang, MC) was applied to frozen dough. The effects of 0.5%, 1%, 2%) on the processing properties of frozen dough and the quality of steamed bread were evaluated to evaluate the improvement effect of HPMC. Influence on the structure and properties of components (wheat gluten, wheat starch and yeast).
The experimental results of farinality and stretching showed that the addition of HPMC improved the processing properties of the dough, and the dynamic frequency scanning results showed that the viscoelasticity of the dough added with HPMC during the freezing period changed little, and the dough network structure remained relatively stable. In addition, compared with the control group, the specific volume and elasticity of the steamed bread were improved, and the hardness was reduced after the frozen dough added with 2% HPMC was frozen for 60 days.
Wheat gluten is the material basis for the formation of dough network structure. Experiments found that the addition of I--IPMC reduced the breakage of Yd and disulfide bonds between wheat gluten proteins during frozen storage. In addition, the results of low-field nuclear magnetic resonance and differential scanning the water state transition and recrystallization phenomena are limited, and the content of freezable water in the dough is reduced, thereby suppressing the effect of ice crystal growth on the gluten microstructure and its spatial conformation. Scanning electron microscope showed intuitively that the addition of HPMC could maintain the stability of gluten network structure.
Starch is the most abundant dry matter in dough, and changes in its structure will directly affect the gelatinization characteristics and the quality of the final product. X. The results of X-ray diffraction and DSC showed that the relative crystallinity of starch increased and the gelatinization enthalpy increased after frozen storage. With the prolongation of frozen storage time, the swelling power of starch without HPMC addition decreased gradually, while the starch gelatinization characteristics (peak viscosity, minimum viscosity, final viscosity, decay value and retrogradation value) all increased significantly; During the storage time, compared with the control group, with the increase of HPMC addition, the changes of starch crystal structure and gelatinization properties gradually decreased.
The fermentation gas production activity of yeast has an important influence on the quality of fermented flour products. Through experiments, it was found that, compared with the control group, the addition of HPMC could better maintain the fermentation activity of yeast and reduce the increase rate of extracellular reduced glutathione content after 60 days of freezing, and within a certain range, The protective effect of HPMC was positively correlated with its addition amount.
The results indicated that HPMC could be added to frozen dough as a new type of cryoprotectant to improve its processing properties and the quality of steamed bread.
Key words: steamed bread; frozen dough; hydroxypropyl methylcellulose; wheat gluten; wheat starch; yeast.
Table of contents
Chapter 1 Preface ................................................................................................................................. 1
1.1 Current status of research at home and abroad………………………………………………………l
1.1.1 Introduction to Mansuiqi……………………………………………………………………………………1
1.1.2 Research status of steamed buns……………………………………………… ． ． ………… 1
1.1.3 Frozen Dough Introduction ................................................................................................. 2
1.1.4 Problems and challenges of frozen dough………………………………………………………….3
1.1.5 Research status of frozen dough……………………………………. .............................................4
1.1.6 Application of Hydrocolloids in Frozen Dough Quality Improvement………………….5
1.1.7 Hydroxypropyl methyl cellulose (Hydroxypropyl methyl cellulose, I-IPMC) ………. 5
112 Purpose and Significance of the Study ................................................................................ 6
1.3 The main content of the study ...................................................................................................7
Chapter 2 Effects of HPMC addition on the processing properties of frozen dough and the quality of steamed bread………………………………………………………………………………………………... 8
2.1 Introduction ...................................................................................................................................... 8
2.2 Experimental materials and methods ........................................................................................8
2.2.1 Experimental materials ................................................................................................................8
2.2.2 Experimental Instruments and Equipment .............................................................................8
2.2.3 Experimental methods ................................................................................................................ 9
2.3 Experimental results and discussion…………………………………………………………………… ． 11
2.3.1 Index of basic components of wheat flour…………………………………………………………….1l
2.3.2 The effect of HPMC addition on the farinaceous properties of dough………………….11
2.3.3 The effect of HPMC addition on the tensile properties of dough………………………… 12
2.3.4 The effect of HPMC addition and freezing time on the rheological properties of dough…………………………. ………………………………………………………………………………………………………….15
2.3.5 Effects of HPMC addition amount and freezing storage time on the freezable water content (GW) in frozen dough………… ……………………………………………………………………………………15
2.3.6 The effect of HPMC addition and freezing time on the quality of steamed bread………………………………………………………………………………………………………………………………………18
2.4 Chapter Summary ..........................................................................................................................21
Chapter 3 Effects of HPMC addition on the structure and properties of wheat gluten protein under freezing conditions………………………………………………………………………………………...................24
3.1 Introduction .....................................................................................................................................24
3.2.1 Experimental materials ............................................................................................................25
3.2.2 Experimental apparatus ...........................................................................................................25
3.2.3 Experimental reagents…………………………………………………………………………. ………………25
3.2.4 Experimental methods ......................................................................................................． 25
3. Results and Discussion ................................................................................................................ 29
3.3.1 The effect of HPMC addition and freezing time on the rheological properties of wet gluten mass………………………………………………………………………………………………………………………….29
3.3.2 The effect of adding amount of HPMC and freezing storage time on the freezable moisture content (CFW) and thermal stability……………………………………………………………………. 30
3.3.3 Effects of HPMC addition amount and freezing storage time on free sulfhydryl content (C vessel) …………………………………………………………………………………………………………. ． 34
3.3.4 Effects of HPMC addition amount and freezing storage time on the transverse relaxation time (N) of wet gluten mass…………………………………………………………………………………35
3.3.5 Effects of HPMC addition amount and freezing storage time on the secondary structure of gluten………………………………………………………………………………………………………………….37
3.3.6 Effects of FIPMC addition amount and freezing time on the surface hydrophobicity of gluten protein…………………………………………………………………………………………………………………… 41
3.3.7 Effects of HPMC addition amount and freezing storage time on the micro-network structure of gluten………………………………………………………………………………………………………………….42
3.4 Chapter Summary ......................................................................................................................... 43
Chapter 4 Effects of HPMC addition on starch structure and properties under frozen storage conditions………………………………………………………………………………………………………………………… 44
4.1 Introduction .............................................................................................................................. ． 44
4.2 Experimental materials and methods ................................................................................． 45
4.2.1 Experimental materials ................................................................................................ ………….45
4.2.2 Experimental apparatus ............................................................................................................45
4.2.3 Experimental method ................................................................................................................45
4.3 Analysis and discussion ..........................................................................................................． 48
4.3.1 Content of basic components of wheat starch ……………………………………………………. 48
4.3.2 Effects of I-IPMC addition amount and frozen storage time on the gelatinization characteristics of wheat starch……………………………………………………………………………………………….48
4.3.3 Effects of HPMC addition and freezing storage time on the shear viscosity of starch paste………………………………………………………………………………………………………………………………………. 52
4.3.4 Effects of HPMC addition amount and frozen storage time on dynamic viscoelasticity of starch paste………………………………………………………………………………………………….55
4.3.5 Influence of HPMC addition amount and frozen storage time on starch swelling ability……………………………………………………………………………………………………………………………………….56
4.3.6 Effects of I-IPMC addition amount and frozen storage time on the thermodynamic properties of starch ………………………………………………………………………………………………………. ． 57
4.3.7 Effects of HPMC addition amount and freezing storage time on the relative crystallinity of starch……………………………………………………………………………………………………………….59
4.4 Chapter Summary .....................................................................................................................． 6 1
Chapter 5 Effects of HPMC addition on yeast survival rate and fermentation activity under frozen storage conditions………………………………………………………………………………………………. ． 62
5.1Introduction ...................................................................................................................................． 62
5.2 Materials and methods ...........................................................................................................． 62
5.2.1 Experimental materials and instruments ............................................................................. 62
5.2.2 Experimental methods . . . ． ． …………………………………………………………………………. 63
5.3 Results and Discussion ..............................................................................................................． 64
5.3.1 The effect of HPMC addition and freezing time on the proofing height of dough…………………………………………………………………………………………………………………………… 64
5.3.2 Effects of HPMC addition amount and freezing time on yeast survival rate…………………………………………………………………………………………………………………………………………65
5.3.3 The effect of adding amount of HPMC and freezing time on the content of glutathione in dough……………………………………………………………………………………………………………66. "
5.4 Chapter Summary .......................................................................................................................． 67
Chapter 6 Conclusions and Prospects ............................................................................................ ………68
6.1 Conclusion ................................................................................................................................. ． 68
6.2 Outlook .........................................................................................................................................． 68
List of illustrations
Figure 1.1 The structural formula of hydroxypropyl methylcellulose………………………. ． 6
Figure 2.1 The effect of HPMC addition on the rheological properties of frozen dough…………………………………………………………………………………………………………………………………….. 15
Figure 2.2 Effects of HPMC addition and freezing time on specific volume of steamed bread……………………………………………………………………………………………………………………………………... 18
Figure 2.3 The effect of HPMC addition and freezing time on the hardness of steamed bread……………………………………………………………………………………………………………………………………... 19
Figure 2.4 The effect of HPMC addition and freezing time on the elasticity of steamed bread………………………………………………………………………………………………………………………………. ． 20
Figure 3.1 The effect of HPMC addition and freezing time on the rheological properties of wet gluten…………………………………………………………………………………………………………………………. 30
Figure 3.2 Effects of HPMC addition and freezing time on the thermodynamic properties of wheat gluten………………………………………………………………………………………………………………. ． 34
Figure 3.3 Effects of HPMC addition and freezing time on free sulfhydryl content of wheat gluten……………………………………………………………………………………………………………………………... ． 35
Figure 3.4 Effects of HPMC addition amount and freezing storage time on the distribution of transverse relaxation time (n) of wet gluten………………………………………………………………………36
Figure 3.5 Wheat gluten protein infrared spectrum of the amide III band after deconvolution and second derivative fitting………………………………………………………………………... 38
Figure 3.6 Illustration ................................................................................................................ ……….39
Figure 3.7 The effect of HPMC addition and freezing time on the microscopic gluten network structure…………………………………………………………………………………………………………... ． 43
Figure 4.1 Starch gelatinization characteristic curve ..............................................................． 51
Figure 4.2 Fluid thixotropy of starch paste ................................................................................． 52
Figure 4.3 Effects of adding amount of MC and freezing time on the viscoelasticity of starch paste……………………………………………………………………………………………………………………... ． 57
Figure 4.4 The effect of HPMC addition and freezing storage time on starch swelling ability……………………………………………………………………………………………………………………………………... 59
Figure 4.5 Effects of HPMC addition and freezing storage time on the thermodynamic properties of starch…………………………………………………………………………………………………………. ． 59
Figure 4.6 Effects of HPMC addition and freezing storage time on XRD properties of starch……………………………………………………………………………………………………………………………………….62
Figure 5.1 The effect of HPMC addition and freezing time on the proofing height of dough…………………………………………………………………………………………………………………………………... 66
Figure 5.2 The effect of HPMC addition and freezing time on the yeast survival rate…………………………………………………………………………………………………………………………………... ． 67
Figure 5.3 Microscopic observation of yeast (microscopic examination) …………………………………………………………………………………………………………………………. 68
Figure 5.4 The effect of HPMC addition and freezing time on glutathione (GSH) content…………………………………………………………………………………………………………………………………... 68
List of forms
Table 2.1 The basic ingredient content of wheat flour…………………………………………………. 11
Table 2.2 The effect of I-IPMC addition on the farinaceous properties of dough……………11
Table 2.3 Effect of I-IPMC addition on dough tensile properties………………………………….14
Table 2.4 The effect of I-IPMC addition amount and freezing time on the freezable water content (CF work) of frozen dough………………………………………………………………………………………….17
Table 2.5 Effects of I-IPMC addition amount and freezing storage time on the texture properties of steamed bread………………………………………………………………………………………………….21
Table 3.1 Content of basic ingredients in gluten…………………………………………………………….25
Table 3.2 Effects of I-IPMC addition amount and freezing storage time on the phase transition enthalpy (Yi IV) and freezer water content (e chat) of wet gluten………………………. 31
Table 3.3 Effects of HPMC addition amount and freezing storage time on the peak temperature (product) of thermal denaturation of wheat gluten…………………………………………. 33
Table 3.4 Peak positions of protein secondary structures and their assignments………….37
Table 3.5 Effects of HPMC addition and freezing time on the secondary structure of wheat gluten…………………………………………………………………………………………………………………………………….40
Table 3.6 Effects of I-IPMC addition and freezing storage time on the surface hydrophobicity of wheat gluten……………………………………………………………………………………………. 41
Table 4.1 Content of basic components of wheat starch…………………………………………………49
Table 4.2 Effects of HPMC addition amount and frozen storage time on the gelatinization characteristics of wheat starch……………………………………………………………………………………………… 52
Table 4.3 Effects of I-IPMC addition and freezing time on the shear viscosity of wheat starch paste…………………………………………………………………………………………………………………………. 55
Table 4.4 Effects of I-IPMC addition amount and frozen storage time on the thermodynamic properties of starch gelatinization……………………………………………………………….60
Chapter 1 Preface
1.1Research status at home and abroad
1.1.1Introduction to Steamed Bread
Steamed bread refers to the food made from the dough after proofing and steaming. As a traditional Chinese pasta food, steamed bread has a long history and is known as "Oriental Bread". Because its finished product is hemispherical or elongated in shape, soft in taste, delicious in taste and rich in nutrients [l], it has been widely popular among the public for a long time. It is the staple food of our country, especially the northern residents. The consumption accounts for about 2/3 of the dietary structure of products in the north, and about 46% of the dietary structure of flour products in the country [21].
1.1.2Research status of steamed bread
At present, the research on steamed bread mainly focuses on the following aspects:
1)Development of new characteristic steamed buns. Through the innovation of steamed bread raw materials and the addition of functional active substances, new varieties of steamed breads have been developed, which have both nutrition and function. Established the evaluation standard for the quality of miscellaneous grain steamed bread by principal component analysis; Fu et a1. (2015) added lemon pomace containing dietary fiber and polyphenols to steamed bread, and evaluated the antioxidant activity of steamed bread; Hao & Beta (2012) studied barley bran and flaxseed (rich in bioactive substances) The production process of steamed bread [5]; Shiau et a1. (2015) evaluated the effect of adding pineapple pulp fiber on dough rheological properties and steamed bread quality [6].
2)Research on the processing and compounding of special flour for steamed bread. The effect of flour properties on the quality of dough and steamed buns and the research on new special flour for steamed buns, and based on this, an evaluation model of flour processing suitability was established [7]; for example, the effects of different flour milling methods on the quality of flour and steamed buns[7] 81; The effect of the compounding of several waxy wheat flours on the quality of steamed bread [9J et al.; Zhu, Huang, &Khan (2001) evaluated the effect of wheat protein on the quality of dough and northern steamed bread, and considered that gliadin/ Glutenin was significantly negatively correlated with dough properties and steamed bread quality [lo]; Zhang, et a1. (2007) analyzed the correlation between gluten protein content, protein type, dough properties and steamed bread quality, and concluded that the content of high molecular weight glutenin subunit (1ligh.molecular-weight, HMW) and total protein content are all related to the quality of northern steamed bread. have a significant impact [11].
3)Research on dough preparation and steamed bread making technology. Research on the influence of steamed bread production process conditions on its quality and process optimization; Liu Changhong et al. (2009) showed that in the process of dough conditioning, process parameters such as water addition, dough mixing time, and dough pH value have an impact on the whiteness value of steamed bread. It has a significant impact on sensory evaluation. If the process conditions are not suitable, it will cause the product to turn blue, dark or yellow. The research results show that during the dough preparation process, the amount of water added reaches 45%, and the dough mixing time is 5 minutes, ~ When the pH value of the dough was 6.5 for 10 min, the whiteness value and sensory evaluation of the steamed buns measured by the whiteness meter were the best. When rolling the dough 15-20 times at the same time, the dough is flaky, smooth, elastic and shiny surface; when the rolling ratio is 3:1, the dough sheet is shiny, and the whiteness of the steamed bread increases [l to; Li, et a1. (2015) explored the production process of compound fermented dough and its application in steamed bread processing [13].
4)Research on quality improvement of steamed bread. Research on the addition and application of steamed bread quality improvers; mainly including additives (such as enzymes, emulsifiers, antioxidants, etc.) and other exogenous proteins [14], starch and modified starch [15], etc. The addition and optimization of the corresponding process It is particularly noteworthy that in recent years, through the use of some exogenous proteins and other additives, gluten-free (free. gluten) pasta products have been developed to meet the requirements of celiac disease (Dietary needs of patients with Coeliac Disease [16.1 cit.
5)Preservation and anti-aging of steamed bread and related mechanisms. Pan Lijun et al. (2010) optimized the composite modifier with good anti-aging effect through experimental design [l do not; Wang, et a1. (2015) studied the effects of gluten protein polymerization degree, moisture, and starch recrystallization on the increase of steamed bread hardness by analyzing the physical and chemical properties of steamed bread. The results showed that water loss and starch recrystallization were the main reasons for the aging of steamed bread [20].
6)Research on the application of new fermented bacteria and sourdough. Jiang, et a1. (2010) Application of Chaetomium sp. fermented to produce xylanase (with thermostable) in steamed bread [2l'; Gerez, et a1. (2012) used two kinds of lactic acid bacteria in fermented flour products and evaluated their quality [221; Wu, et al. (2012) studied the influence of sourdough fermented by four kinds of lactic acid bacteria (Lactobacillus plantarum, Lactobacillus, sanfranciscemis , Lactobacillus brevis and Lactobacillus delbrueckii subsp bulgaricus) on the quality (specific volume, texture, fermentation flavor, etc.) of northern steamed bread [23]; and Gerez, et a1. (2012) used the fermentation characteristics of two kinds of lactic acid bacteria to accelerate the hydrolysis of gliadin to reduce the allergenicity of flour products [24] and other aspects.
7)Research on the application of frozen dough in steamed bread.
Among them, steamed bread is prone to aging under conventional storage conditions, which is an important factor restricting the development of steamed bread production and processing industrialization. After aging, the quality of steamed bread is reduced - the texture becomes dry and hard, dregs, shrinks and cracks, the sensory quality and flavor deteriorate, the digestion and absorption rate decreases, and the nutritional value decreases. This not only affects its shelf life, but also creates a lot of waste. According to statistics, the annual loss due to aging is 3% of the output of flour products. 7%. With the improvement of people's living standards and health awareness, as well as the rapid development of the food industry, how to industrialize the traditional popular staple noodle products including steamed bread, and obtain products with high quality, long shelf life and easy preservation to meet the needs of the growing demand for fresh, safe, high-quality and convenient food is a long-standing technical problem. Based on this background, frozen dough came into being, and its development is still in the ascendant.
1.1.3Introduction to frozen dough
Frozen dough is a new technology for the processing and production of flour products developed in the 1950s. It mainly refers to the use of wheat flour as the main raw material and water or sugar as the main auxiliary materials. Baked, packed or unpacked, quick-freezing and other processes make the product reach a frozen state, and in. For products frozen at 18"C, the final product needs to be thawed, proofed, cooked, etc. [251].
According to the production process, frozen dough can be roughly divided into four types.
a)Frozen dough method: the dough is divided into one piece, quick-frozen, frozen, thawed, proofed, and cooked (baking, steaming, etc.)
b)Pre-proofing and freezing dough method: the dough is divided into one part, one part is proofed, one is quick-frozen, one is frozen, one is thawed, one is proofed and one is cooked (baking, steaming, etc.)
c)Pre-processed frozen dough: the dough is divided into one piece and formed, fully proofed, then cooked (to a certain extent), cooled, frozen, frozen, stored, thawed, and cooked (baking, steaming, etc.)
d)Fully processed frozen dough: the dough is made into one piece and formed, then fully proofed, and then fully cooked-ling but frozen, frozen and stored-thawed and heated.
The emergence of frozen dough not only creates conditions for the industrialization, standardization, and chain production of fermented pasta products, it can effectively shorten processing time, improve production efficiency, and reduce production time and labor costs. Therefore, the aging phenomenon of the pasta food is effectively inhibited, and the effect of prolonging the shelf life of the product is achieved. Therefore, especially in Europe, America, Japan and other countries, frozen dough is widely used in white bread (Bread), French Sweet Bread (French Sweet Bread), small muffin (muffin), bread rolls (Rolls), French baguette (- Stick), cookies and frozen
Cakes and other pasta products have different degrees of application [26-27]. According to incomplete statistics, by 1990, 80% of bakeries in the United States used frozen dough; 50% of bakeries in Japan also used frozen dough. twentieth century
In the 1990s, frozen dough processing technology was introduced into China. With the continuous development of science and technology and the continuous improvement of people's living standards, frozen dough technology has broad development prospects and huge development space
1.1.4Problems and challenges of frozen dough
The frozen dough technology undoubtedly provides a feasible idea for the industrialized production of traditional Chinese food such as steamed bread. However, this processing technology still has some shortcomings, especially under the condition of longer freezing time, the final product will have longer proofing time, lower specific volume, higher hardness, Water loss, poor taste, reduced flavor, and quality deterioration. In addition, due to freezing
Dough is a multi-component (moisture, protein, starch, microorganism, etc.), multi-phase (solid, liquid, gas), multi-scale (macromolecules, small molecules), multi-interface (solid-gas interface, liquid-gas interface), solid-liquid interface) soft material system 1281, so the reasons for the above-mentioned quality deterioration are very complex and diverse.
Most studies have found that the formation and growth of ice crystals in frozen foods is an important factor leading to the deterioration of product quality [291]. Ice crystals not only reduce the survival rate of yeast, but also weaken the gluten strength, affect the starch crystallinity and gel structure, and damage the yeast cells and release the reducing glutathione, which further reduces the gas holding capacity of gluten. In addition, in the case of frozen storage, temperature fluctuations can cause ice crystals to grow due to recrystallization [30]. Therefore, how to control the adverse effects of ice crystal formation and growth on starch, gluten and yeast is the key to solving the above problems, and it is also a hot research field and direction. In the past ten years, many researchers have been engaged in this work and achieved some fruitful research results. However, there are still some gaps and some unresolved and controversial issues in this field, which need to be further explored, such as:
a)How to restrain the quality deterioration of frozen dough with the extension of frozen storage time, especially how to control the influence of the formation and growth of ice crystals on the structure and properties of the three main components of dough (starch, gluten and yeast), is still an issue. Hotspots and fundamental issues in this research field;
b)Because there are certain differences in the processing and production technology and formula of different flour products, there is still a lack of research on the development of corresponding special frozen dough in combination with different product types;
c)Expand, optimize and use new frozen dough quality improvers, which is conducive to the optimization of production enterprises and the innovation and cost control of product types. At present, it still needs to be further strengthened and expanded;
d)The effect of hydrocolloids on the quality improvement of frozen dough products and the related mechanisms still need to be further studied and systematically explained.
1.1.5Research status of frozen dough
In view of the above problems and challenges of frozen dough, the long-term innovative research on the application of frozen dough technology, the quality control and improvement of frozen dough products, and the related mechanism of changes in the structure and properties of material components in the frozen dough system and quality deterioration Such research is a hot issue in the field of frozen dough research in recent years. Specifically, the main domestic and foreign researches in recent years mainly focus on the following points:
i.Study the changes in the structure and properties of frozen dough with the extension of freezing storage time, in order to explore the reasons for the deterioration of product quality, especially the effect of ice crystallization on biological macromolecules (protein, starch, etc.), for example, ice crystallization. Formation and growth and its relationship with water state and distribution; changes in wheat gluten protein structure, conformation and properties [31]; changes in starch structure and properties; changes in dough microstructure and related properties, etc. 361.
Studies have shown that the main reasons for the deterioration of the processing properties of frozen dough include: 1) During the freezing process, the survival of yeast and its fermentation activity are significantly reduced; 2) The continuous and complete network structure of the dough is destroyed, resulting in the air holding capacity of the dough. and the structural strength is greatly reduced.
II. Optimization of frozen dough production process, frozen storage conditions and formula. During the production of frozen dough, temperature control, proofing conditions, pre-freezing treatment, freezing rate, freezing conditions, moisture content, gluten protein content, and thawing methods will all affect the processing properties of frozen dough [37]. In general, higher freezing rates produce ice crystals that are smaller in size and more uniformly distributed, while lower freezing rates produce larger ice crystals that are not uniformly distributed. In addition, a lower freezing temperature even below the glass transition temperature (CTA) can effectively maintain its quality, but the cost is higher, and the actual production and cold chain transportation temperatures are usually small. In addition, the fluctuation of the freezing temperature will cause recrystallization, which will affect the quality of the dough.
III. Using additives to improve the product quality of frozen dough. In order to improve the product quality of frozen dough, many researchers have made explorations from different perspectives, for example, improving the low temperature tolerance of material components in frozen dough, using additives to maintain the stability of the dough network structure [45.56], etc. Among them, the use of additives is an effective and widely used method. Mainly include, i) enzyme preparations, such as, transglutaminase, O [. Amylase; ii) emulsifiers, such as monoglyceride stearate, DATEM, SSL, CSL, DATEM, etc.; iii) antioxidants, ascorbic acid, etc.; iv) polysaccharide hydrocolloids, such as guar gum, yellow Originalgum, gum Arabic, konjac gum, sodium alginate, etc.; v) other functional substances, such as Xu, et a1. (2009) added Ice-structuring Proteins to wet gluten mass under freezing conditions, and studied its protective effect and mechanism on the structure and function of gluten protein [y71.
Ⅳ. Breeding of antifreeze yeast and application of new yeast antifreeze [58-59]. Sasano, et a1. (2013) obtained freeze-tolerant yeast strains through hybridization and recombination between different strains [60-61], and S11i, Yu, & Lee (2013) studied a biogenic ice nucleating agent derived from Erwinia Herbicans used to protect the fermentation viability of yeast under freezing conditions [62J.
1.1.6Application of Hydrocolloids in Frozen Dough Quality Improvement
The chemical nature of hydrocolloid is a polysaccharide, which is composed of monosaccharides (glucose, rhamnose, arabinose, mannose, etc.) through 0 [. 1-4. Glycosidic bond or/and a. 1--"6. Glycosidic bond or B. 1-4. Glycosidic bond and 0 [.1-3. The high molecular organic compound formed by the condensation of glycosidic bond has a rich variety and can be roughly divided into: ① Cellulose derivatives , such as methyl cellulose (MC), carboxymethyl cellulose (CMC); ② plant polysaccharides, such as konjac gum, guar gum, gum Arabic ; ③ seaweed polysaccharides, such as seaweed gum, carrageenan; ④ microbial polysaccharides, such as Xanthan gum .Polysaccharide has strong hydrophilicity because it contains a large number of hydroxyl groups that are easy to form hydrogen bonds with water, and has the functions of controlling the migration, state and distribution of water in the food system. Therefore, the addition of hydrophilic colloids gives food Many functions, properties, and qualities of hydrocolloids are closely related to the interaction between polysaccharides and water and other macromolecular substances. At the same time, due to the multiple functions of thickening, stabilizing, and water retention, hydrocolloids are widely used to include in the food processing of flour products. Wang Xin et al. (2007) studied the effect of adding seaweed polysaccharides and gelatin on the glass transition temperature of dough [631. Wang Yusheng et al. (2013) believed that compound addition of a variety of hydrophilic colloids can significantly change the flow of dough. Change the properties, improve the tensile strength of the dough, enhance the elasticity of the dough, but reduce the extensibility of the dough [delete.
1.1.7Hydroxypropyl methyl cellulose (Hydroxypropyl methyl cellulose, I-IPMC)
Hydroxypropyl methyl cellulose (Hydroxypropyl methyl cellulose, HPMC) is a naturally occurring cellulose derivative formed by hydroxypropyl and methyl partially replacing the hydroxyl on the cellulose side chain [65] (Fig. 1. 1). The United States Pharmacopeia (United States Pharmacopeia) divides HPMC into three categories according to the difference in the degree of chemical substitution on the side chain of HPMC and the degree of molecular polymerization: E (Hypromellose 2910), F (Hypromellose 2906) and K (Hypromellose 2208).
Due to the existence of hydrogen bonds in the linear molecular chain and crystalline structure, cellulose has poor water solubility, which also limits its application range. However, the presence of substituents on the side chain of HPMC breaks the intramolecular hydrogen bonds, making it more hydrophilic [66l], which can quickly swell in water and form a stable thick colloidal dispersion at low temperatures Tie. As a cellulose derivative-based hydrophilic colloid, HPMC has been widely used in the fields of materials, papermaking, textiles, cosmetics, pharmaceuticals and food [6 71]. In particular, due to its unique reversible thermo-gelling properties, HPMC is often used as a capsule component for controlled release drugs; in food, HPMC is also used as a surfactant, Thickeners, emulsifiers, stabilizers, etc., and play a role in improving the quality of related products and realizing specific functions. For example, the addition of HPMC can change the gelatinization characteristics of starch and reduce the gel strength of starch paste. , HPMC can reduce the loss of moisture in food, reduce the hardness of bread core, and effectively inhibit the aging of bread.
Although HPMC has been used in pasta to a certain extent, it is mainly used as an anti-aging agent and water-retaining agent for bread, etc., which can improve product specific volume, texture properties and prolong shelf life [71.74]. However, compared with hydrophilic colloids such as guar gum, xanthan gum, and sodium alginate [75-771], there are not many studies on the application of HPMC in frozen dough, whether it can improve the quality of steamed bread processed from frozen dough. There is still a lack of relevant reports on its effect.

1.2Research purpose and significance
At present, the application and large-scale production of frozen dough processing technology in my country as a whole is still in the development stage. At the same time, there are certain pitfalls and deficiencies in the frozen dough itself. These comprehensive factors undoubtedly restrict the further application and promotion of frozen dough. on the other hand,this also means that the application of frozen dough has great potential and broad prospects, especially from the perspective of combining frozen dough technology with the industrialized production of traditional Chinese noodles (non-)fermented staple food, to develop more products that meet the needs of Chinese residents. It is of practical significance to improve the quality of the frozen dough based on the characteristics of Chinese pastry and the dietary habits, and is suitable for the processing characteristics of Chinese pastry.
It is precisely because the relevant application research of HPMC in Chinese noodles is still relatively lacking. Therefore, the purpose of this experiment is to expand the application of HPMC to frozen dough, and to determine the improvement of frozen dough processing by HPMC through the evaluation of steamed bread quality. In addition, HPMC was added to the three main components of the dough (wheat protein, starch and yeast liquid), and the effect of HPMC on the structure and properties of wheat protein, starch and yeast was systematically studied. And explain its related mechanism problems, in order to provide a new feasible path for the quality improvement of frozen dough, so as to expand the application scope of HPMC in the food field, and to provide theoretical support for the actual production of frozen dough suitable for making steamed bread.
1.3The main content of the study
It is generally believed that dough is a typical complex soft matter system with the characteristics of multi-component, multi-interface, multi-phase, and multi-scale.
Effects of addition amount and frozen storage time on the structure and properties of frozen dough, the quality of frozen dough products (steamed bread), the structure and properties of wheat gluten, the structure and properties of wheat starch, and the fermentation activity of yeast. Based on the above considerations, the following experimental design was made in this research topic:
1)Select a new type of hydrophilic colloid, hydroxypropyl methylcellulose (HPMC) as an additive, and study the addition amount of HPMC under different freezing time (0, 15, 30, 60 days; the same below) conditions. (0%, 0.5%, 1%, 2%; the same below) on the rheological properties and microstructure of frozen dough, as well as on the quality of the dough product - steamed bread (including the specific volume of steamed bread) , texture), investigate the effect of adding HPMC to the frozen dough on the processing properties of the dough and the quality of steamed bread, and evaluate the improvement effect of HPMC on the processing properties of the frozen dough;
2)From the perspective of the improvement mechanism, the effects of different HPMC additions on the rheological properties of wet gluten mass, the transition of water state and the structure and properties of wheat gluten were studied under different freezing storage time conditions.
3)From the perspective of the improvement mechanism, the effects of different HPMC additions on the gelatinization properties, gel properties, crystallization properties, and thermodynamic properties of starch under different freezing storage time conditions were studied.
4)From the perspective of the improvement mechanism, the effects of different HPMC additions on the fermentation activity, survival rate, and extracellular glutathione content of yeast under different freezing storage time conditions were studied.
Chapter 2 Effects of I-IPMC Addition on Frozen Dough Processing Properties and Steamed Bread Quality
2.1 Introduction
Generally speaking, the material composition of dough used for making fermented flour products mainly includes biological macromolecular substances (starch, protein), inorganic water, and yeast of organisms, and is formed after hydration, cross-linking and interaction. A stable and complex material system with a special structure has been developed. Numerous studies have shown that the properties of the dough have a significant impact on the quality of the final product. Therefore, by optimizing the compounding to meet the specific product and it is a research direction to improve the dough formulation and technology of the quality of the product or food for use; on the other hand, improving or improving the properties of dough processing and preservation to ensure or improve the quality of the product is also an important research issue.
As mentioned in the introduction, adding HPMC to a dough system and examining its effects on dough properties (farin, elongation, rheology, etc.) and final product quality are two closely related studies.
Therefore, this experimental design is mainly carried out from two aspects: the effect of HPMC addition on the properties of the frozen dough system and the effect on the quality of steamed bread products.
2.2 Experimental materials and methods
2.2.1 Experimental materials
Zhongyu Wheat Flour Binzhou Zhongyu Food Co., Ltd.; Angel Active Dry Yeast Angel Yeast Co., Ltd.; HPMC (methyl substitution degree of 28%.30%, hydroxypropyl substitution degree of 7%.12%) Aladdin (Shanghai) Chemical Reagent Company; all chemical reagents used in this experiment are of analytical grade;
2.2.2 Experimental instruments and equipment
Instrument and equipment name
BPS. 500CL constant temperature and humidity box
TA-XT Plus physical property tester
BSAl24S Electronic Analytical Balance
DHG. 9070A Blast Drying Oven
SM. 986S dough mixer
C21. KT2134 Induction Cooker
Powder meter. E
Extensometer. E
Discovery R3 Rotational Rheometer
Q200 Differential Scanning Calorimeter
FD. 1B. 50 Vacuum Freeze Dryer
SX2.4.10 muffle furnace
Kjeltee TM 8400 automatic Kjeldahl nitrogen analyzer
Manufacturer
Shanghai Yiheng Scientific Instrument Co., Ltd.
Stab Micro Systems, UK
Sartorius, Germany
Shanghai Yiheng Scientific Instrument Co., Ltd.
Top Kitchen Appliance Technology Co., Ltd.
Guangdong Midea Life Appliance Manufacturing Co., Ltd.
Brabender, Germany
Brabender, Germany
American TA company
American TA company
Beijing Bo Yi kang Experimental Instrument Co., Ltd.
Huang shi Heng feng Medical Equipment Co., Ltd.
Danish FOSS company
2.2.3 Experimental method
2.2.3.1 Determination of basic components of flour
According to GB 50093.2010, GB 5009.5--2010, GB/T 5009.9.2008, GB50094.2010t78-81], determine the basic components of wheat flour - moisture, protein, starch and ash content.
2.2.3.2 Determination of the floury properties of dough
According to the reference method GB/T 14614.2006 Determination of farinaceous properties of dough [821.
2.2.3.3 Determination of tensile properties of dough
Determination of tensile properties of dough according to GB/T 14615.2006 [831.
2.2.3.4 Production of frozen dough
Refer to the dough making process of GB/T 17320.1998 [84]. Weigh 450 g of flour and 5 g of active dry yeast into the bowl of the dough mixer, stir at low speed to fully mix the two, and then add 245 mL of low-temperature (Distilled water (pre-stored in the refrigerator at 4°C for 24 hours to inhibit the activity of yeast), first stir at low speed for 1 min, then at medium speed for 4 min until dough is formed. Take out the dough and divide it into about 180g / portion, knead it into a cylindrical shape, then seal it with a ziplock bag, and put it in. Freeze at 18°C for 15, 30, and 60 days. Add 0.5%, 1%, 2% (w/w, dry basis) HPMC to replace the corresponding proportion of flour quality to make dough, and the rest of the production methods remain unchanged. The 0-day frozen storage (unfrozen storage) was used as the control experimental group.
2.2.3.5 Determination of rheological properties of dough
Take out the dough samples after the corresponding freezing time, put them in a refrigerator at 4 °C for 4 h, and then place them at room temperature until the dough samples are completely melted. The sample processing method is also applicable to the experimental part of 2.3.6.
A sample (about 2 g) of the central part of the partially melted dough was cut and placed on the bottom plate of the rheometer (Discovery R3). First, the sample was subjected to dynamic strain scanning. The specific experimental parameters were set as follows: A parallel plate with a diameter of 40 mm was used, the gap was set to 1000 mln, the temperature was 25 °C, and the scanning range was 0.01%. 100%, the sample rest time is 10 min, and the frequency is set to 1Hz. The Linear Viscoelasticity Region (LVR) of the tested samples was determined by strain scanning. Then, the sample was subjected to a dynamic frequency sweep, and the specific parameters were set as follows: the strain value was 0.5% (in the LVR range), the resting time, the fixture used, the spacing, and the temperature were all consistent with the strain sweep parameter settings. Five data points (plots) were recorded in the rheology curve for each 10-fold increase in frequency (linear mode). After each clamp depression, the excess sample was gently scraped with a blade, and a layer of paraffin oil was applied to the edge of the sample to prevent water loss during the experiment. Each sample was repeated three times.
2.2.3.6 Content of Freezable Water (Content of Freezable Water, CF Internal Determination) in Dough
Weigh a sample of about 15 mg of the central part of the fully melted dough, seal it in an aluminum crucible (suitable for liquid samples), and measure it with a Differential Scanning Calorimetry (DSC). The specific program parameters are set. As follows: first equilibrate at 20°C for 5 min, then drop to .30°C at a rate of 10"C/min, keep for 10 min, and finally rise to 25°C at a rate of 5"C/min, the purge gas is nitrogen (N2) And its flow rate was 50 mL/min. Using the blank aluminum crucible as a reference, the obtained DSC curve was analyzed using the analysis software Universal Analysis 2000, and the melting enthalpy (day) of the ice crystal was obtained by integrating the peak located at about 0°C. Freezable water content (CFW) is calculated by the following formula [85.86]:

Among them, 厶 represents the latent heat of moisture, and its value is 334 J Dan; MC (total Moisture Content) represents the total moisture content in the dough (measured according to GB 50093.2010t78]). Each sample was repeated three times.
2.2.3.7 Steamed Bread Production
After the corresponding freezing time, the frozen dough was taken out, first equilibrated in a 4°C refrigerator for 4 h, and then placed at room temperature until the frozen dough was completely thawed. Divide the dough into about 70 grams per portion, knead it into shape, and then put it into a constant temperature and humidity box, and proof it for 60 minutes at 30°C and a relative humidity of 85%. After proofing, steam for 20 min, and then cool for 1 h at room temperature to evaluate the quality of steamed bread.

2.2.3.8 Evaluation of Steamed Bread Quality
(1) Determination of specific volume of steamed bread
According to GB/T 20981.2007 [871, the rapeseed displacement method was used to measure the volume (work) of the steamed buns, and the mass (m) of the steamed buns was measured using an electronic balance. Each sample was replicated three times.
Steamed bread specific volume (cm3/g) = steamed bread volume (cm3) / steamed bread mass (g)
(2) Determination of texture properties of steamed bread core
Refer to the method of Sim, Noor Aziah, Cheng (2011) [88] with minor modifications. A 20x 20 x 20 mn'13 core sample of the steamed bread was cut from the central area of the steamed bread, and the TPA (Texture Profile Analysis) of the steamed bread was measured by a physical property tester. Specific parameters: the probe is P/100, the pre-measurement rate is 1 mm/s, the mid-measurement rate is 1 mm/s, the post-measurement rate is 1 mm/s, the compression deformation variable is 50%, and the time interval between two compressions is 30 S, the trigger force is 5 g. Each sample was repeated 6 times.
2.2.3.9 Data processing
All experiments were repeated at least three times unless otherwise specified, and the experimental results were expressed as the mean (Mean) ± standard deviation (Standard Deviation). SPSS Statistic 19 was used for analysis of variance (Analysis of Variance, ANOVA), and the significance level was O. 05; Use Origin 8.0 to draw relevant charts.
2.3 Experimental results and discussion
2.3.1 Basic composition index of wheat flour
Tab 2．1 Content of elementary constituent of wheat flour

2.3.2 The effect of I-IPMC addition on the farinaceous properties of dough
As shown in Table 2.2, with the increase of HPMC addition, the water absorption of dough increased significantly, from 58.10% (without adding HPMC dough) to 60.60% (adding 2% HPMC dough). In addition, the addition of HPMC improved the dough stability time from 10.2 min (blank) to 12.2 min (added 2% HPMC). However, with the increase of HPMC addition, both the dough forming time and the dough weakening degree decreased significantly, from the blank dough forming time of 2.10 min and the weakening degree of 55.0 FU, respectively, to the addition of 2% HPMC, the dough forming time was 1. .50 min and weakening degree of 18.0 FU, decreased by 28.57% and 67.27%, respectively.
Because HPMC has strong water retention and water holding capacity, and is more absorbent than wheat starch and wheat gluten [8"01, therefore, the addition of HPMC improves the water absorption rate of the dough. The dough forming time is when the dough consistency reaches 500 The time required for FU, the addition of HPMC reduces the dough formation time, which indicates that the addition of HPMC promotes the formation of the dough. The dough stability time is the time when the dough consistency is maintained above 500 FU, and HPMC increases the dough stability time, which is due to the dough It is caused by the shortening of the forming time and the relative stability of the dough consistency. The degree of weakening represents the difference between the maximum consistency of the dough and the final consistency, and the reduction of the weakening degree of the dough by HPMC shows that HPMC can play a role in stabilizing the consistency of the dough. Dough stability time the increase of α and the decrease of dough weakening degree indicate that under the action of mechanical shearing force, the dough structure added with HPMC is more stable, and these results are similar to the research results of Rosell, Collar, & Haros (2007).

Note: Different superscript lowercase letters in the same column indicate significant difference (P<0.05)

2.3.3 Effect of HPMC addition on dough tensile properties
The tensile properties of the dough can better reflect the processing properties of the dough after proofing, including the extensibility, tensile resistance and stretch ratio of the dough. The tensile properties of the dough are attributed to the extension of the glutenin molecules in the dough extensibility, as the cross-linking of glutenin molecular chains determines the elasticity of the dough [921]. Termonia, Smith (1987) [93] believed that the elongation of polymers depends on two chemical kinetic processes, that is, the breaking of secondary bonds between molecular chains and the deformation of cross-linked molecular chains. When the deformation rate of the molecular chain is relatively low, the molecular chain cannot sufficiently and quickly cope with the stress generated by the stretching of the molecular chain, which in turn leads to the breakage of the molecular chain, and the extension length of the molecular chain is also short. Only when the deformation rate of the molecular chain can ensure that the molecular chain can be deformed quickly and sufficiently, and the covalent bond nodes in the molecular chain will not be broken, the elongation of the polymer can be increased. Therefore, changing the deformation and elongation behavior of the gluten protein chain will have an impact on the tensile properties of the dough [92].
Table 2.3 lists the effects of different amounts of HPMC (O, 0.5%, 1% and 2%) and different proofing 1'9 (45 min, 90 min and 135 min) on the dough tensile properties (energy, stretch resistance, maximum stretch resistance, elongation, stretch ratio and maximum stretch ratio). The experimental results show that the tensile properties of all dough samples increase with the extension of the proofing time except the elongation which decreases with the extension of the proofing time. For the energy value, from 0 to 90 min, the energy value of the rest of the dough samples increased gradually except for the addition of 1% HPMC, and the energy value of all dough samples increased gradually. There were no significant changes. This shows that when the proofing time is 90 min, the network structure of the dough (cross-linking between molecular chains) is completely formed. Therefore, the proofing time is further extended, and there is no significant difference in the energy value. At the same time, this can also provide a reference for determining the proofing time of the dough. As the proofing time prolongs, more secondary bonds between molecular chains are formed and the molecular chains are more closely cross-linked, so the tensile resistance and the maximum tensile resistance increase gradually. At the same time, the deformation rate of molecular chains also decreased with the increase of secondary bonds between molecular chains and the tighter cross-linking of molecular chains, which led to the decrease of the elongation of the dough with the excessive extension of the proofing time. The increase in tensile resistance/maximum tensile resistance and the decrease in elongation resulted in an increase in tensile LL/maximum tensile ratio.
However, the addition of HPMC can effectively suppress the above trend and change the tensile properties of the dough. With the increase of HPMC addition, the tensile resistance, maximum tensile resistance and energy value of the dough all decreased correspondingly, while the elongation increased. Specifically, when the proofing time was 45 min, with the increase of HPMC addition, the dough energy value decreased significantly, from 148.20-J: 5.80 J (blank) to 129.70-J respectively: 6.65 J (add 0.5% HPMC), 120.30 ± 8.84 J (add 1% HPMC), and 110.20-a: 6.58
J (2% HPMC added). At the same time, the maximum tensile resistance of the dough decreased from 674.50-a: 34.58 BU (blank) to 591.80--a: 5.87 BU (adding 0.5% HPMC), 602.70± 16.40 BU (1% HPMC added), and 515.40-a: 7.78 BU (2% HPMC added). However, the elongation of the dough increased from 154.75+7.57 MITI (blank) to 164.70-a: 2.55 m/rl(adding 0.5% HPMC), 162.90-a: 4 .05 min (1% HPMC added), and 1 67.20-a: 1.98 min (2% HPMC added). This may be due to the increase of the plasticizer-water content by adding HPMC, which reduces the resistance to the deformation of the gluten protein molecular chain, or the interaction between HPMC and the gluten protein molecular chain changes its stretching behavior, which in turn affects It improves the tensile properties of the dough and increases the extensibility of the dough, which will affect the quality (eg, specific volume, texture) of the final product.

2.3.4 Effects of HPMC addition amount and freezing storage time on the rheological properties of dough
The rheological properties of dough are an important aspect of dough properties, which can systematically reflect the comprehensive properties of dough such as viscoelasticity, stability and processing characteristics, as well as the changes in properties during processing and storage.

Fig 2．1 Effect of HPMC addition on rheological properties of frozen dough
Figure 2.1 shows the change of storage modulus (elastic modulus, G') and loss modulus (viscous modulus, G") of dough with different HPMC content from 0 days to 60 days. The results showed that with the prolongation of freezing storage time, the G' of the dough without adding HPMC decreased significantly, while the change of G" was relatively small, and the /an Q (G''/G') increased. This may be due to the fact that the network structure of the dough is damaged by ice crystals during freezing storage, which reduces its structural strength and thus the elastic modulus decreases significantly. However, with the increase of HPMC addition, the variation of G' gradually decreased. In particular, when the added amount of HPMC was 2%, the variation of G' was the smallest. This shows that HPMC can effectively inhibit the formation of ice crystals and the increase in the size of ice crystals, thereby reducing the damage to the dough structure and maintaining the structural strength of the dough. In addition, the G' value of dough is greater than that of wet gluten dough, while the G" value of dough is smaller than that of wet gluten dough, mainly because the dough contains a large amount of starch, which can be adsorbed and dispersed on the gluten network structure. It increases its strength while retaining excess moisture.
2.3.5 Effects of HPMC addition amount and freezing storage time on the freezable water content (ow) in frozen dough
Not all the moisture in the dough can form ice crystals at a certain low temperature, which is related to the state of the moisture (free-flowing, restricted, combined with other substances, etc.) and its environment. Freezable water is the water in the dough that can undergo phase transformation to form ice crystals at low temperatures. The amount of freezable water directly affects the number, size and distribution of ice crystal formation. In addition, the freezable water content is also affected by environmental changes, such as the extension of freezing storage time, the fluctuation of freezing storage temperature, and the change of material system structure and properties. For the frozen dough without added HPMC, with the prolongation of freezing storage time, Q silicon increased significantly, from 32.48±0.32% (frozen storage for 0 days) to 39.13±0.64% (frozen storage for 0 days). Tibetan for 60 days), the increase rate was 20.47%. However, after 60 days of frozen storage, with the increase of HPMC addition, the increase rate of CFW decreased, followed by 18.41%, 13.71%, and 12.48% (Table 2.4). At the same time, the o∥ of the unfrozen dough decreased correspondingly with the increase of the amount of HPMC added, from 32.48a-0.32% (without adding HPMC) to 31.73±0.20% in turn. (adding0.5% HPMC), 3 1.29+0.03% (adding 1% HPMC) and 30.44±0.03% (adding 2% HPMC) Water holding capacity, inhibits the free flow of water and reduces the amount of water that can be frozen. In the process of freezing storage, along with recrystallization, the dough structure is destroyed, so that part of the non-freezable water is converted into freezable water, thus increasing the content of freezable water. However, HPMC can effectively inhibit the formation and growth of ice crystals and protect the stability of the dough structure, thus effectively inhibiting the increase of the freezable water content. This is consistent with the change law of the freezable water content in the frozen wet gluten dough, but because the dough contains more starch, the CFW value is smaller than the G∥ value determined by the wet gluten dough (Table 3.2).

2.3.6 Effects of I'IPMC addition and freezing time on the quality of steamed bread
2.3.6.1 Influence of HPMC addition amount and frozen storage time on specific volume of steamed bread
The specific volume of steamed bread can better reflect the appearance and sensory quality of steamed bread. The larger the specific volume of the steamed bread, the larger the volume of the steamed bread of the same quality, and the specific volume has a certain influence on the appearance, color, texture, and sensory evaluation of the food. Generally speaking, steamed buns with larger specific volume are also more popular with consumers to a certain extent.

Fig 2．2 Effect of HPMC addition and frozen storage on specific volume of Chinese steamed bread
The specific volume of steamed bread can better reflect the appearance and sensory quality of steamed bread. The larger the specific volume of the steamed bread, the larger the volume of the steamed bread of the same quality, and the specific volume has a certain influence on the appearance, color, texture, and sensory evaluation of the food. Generally speaking, steamed buns with larger specific volume are also more popular with consumers to a certain extent.
However, the specific volume of the steamed bread made from frozen dough decreased with the extension of the frozen storage time. Among them, the specific volume of the steamed bread made from the frozen dough without adding HPMC was 2.835±0.064 cm3/g (frozen storage). 0 days) down to 1.495±0.070 cm3/g (frozen storage for 60 days); while the specific volume of steamed bread made from frozen dough added with 2% HPMC dropped from 3.160±0.041 cm3/g to 2.160±0.041 cm3/g. 451±0.033 cm3/g, therefore, the specific volume of the steamed bread made from the frozen dough added with HPMC decreased with the increase of the added amount. Since the specific volume of steamed bread is not only affected by the yeast fermentation activity (fermentation gas production), the moderate gas holding capacity of the dough network structure also has an important impact on the specific volume of the final product [96'9 cited. The measurement results of the above rheological properties show that the integrity and structural strength of the dough network structure are destroyed during the freezing storage process, and the degree of damage is intensified with the extension of the freezing storage time. During the process, its gas holding capacity is poor, which in turn leads to a decrease in the specific volume of the steamed bread. However, the addition of HPMC can more effectively protect the integrity of the dough network structure, so that the air-holding properties of the dough are better maintained, therefore, in O. During the 60-day frozen storage period, with the increase of HPMC addition, the specific volume of the corresponding steamed bread decreased gradually.
2.3.6.2 Effects of HPMC addition amount and frozen storage time on the texture properties of steamed bread
TPA (Textural Profile Analyses) physical property test can comprehensively reflect the mechanical properties and quality of pasta food, including hardness, elasticity, cohesion, chewiness and resilience. Figure 2.3 shows the effect of HPMC addition and freezing time on the hardness of steamed bread. The results show that for fresh dough without freezing treatment, with the increase of HPMC addition, the hardness of steamed bread significantly increases. decreased from 355.55±24.65g (blank sample) to 310.48±20.09 g (add O.5% HPMC), 258.06±20.99 g (add 1% t-IPMC) and 215.29 + 13.37 g (2% HPMC added). This may be related to the increase in specific volume of steamed bread. In addition, as can be seen from Figure 2.4, as the amount of HPMC added increases, the springiness of steamed bread made from fresh dough increases significantly, from 0.968 ± 0.006 (blank) to 1, respectively. .020 ± 0.004 (add 0.5% HPMC), 1.073 ± 0.006 (add 1% I-IPMC) and 1.176 ± 0.003 (add 2% HPMC). The changes of the hardness and elasticity of steamed bread indicated that the addition of HPMC could improve the quality of steamed bread. This is consistent with the research results of Rosell, Rojas, Benedito de Barber (2001) [95] and Barcenas, Rosell (2005) [worms], that is, HPMC can significantly reduce the hardness of bread and improve the quality of bread.

Fig 2．3 Effect of HPMC addition and frozen storage on hardness of Chinese steamed bread
On the other hand, with the prolongation of the frozen storage time of frozen dough, the hardness of the steamed bread made by it increased significantly (P<0.05), while the elasticity decreased significantly (P<0.05). However, the hardness of steamed buns made from frozen dough without added HPMC increased from 358.267 ± 42.103 g (frozen storage for 0 days) to 1092.014 ± 34.254 g (frozen storage for 60 days); 

The hardness of the steamed bread made of frozen dough with 2% HPMC increased from 208.233 ± 15.566 g (frozen storage for 0 days) to 564.978 ± 82.849 g (frozen storage for 60 days). Fig 2．4 Effect of HPMC addition and frozen storage on springiness of Chinese steamed bread In terms of elasticity, the elasticity of steamed bread made from frozen dough without adding HPMC decreased from 0.968 ± 0.006 (freezing for 0 days) to 0.689 ± 0.022 (frozen for 60 days); Frozen with 2% HPMC added the elasticity of the steamed buns made of dough decreased from 1.176 ± 0.003 (freezing for 0 days) to 0.962 ± 0.003 (freezing for 60 days). Obviously, the increase rate of hardness and the decrease rate of elasticity decreased with the increase of the added amount of HPMC in the frozen dough during the frozen storage period. This shows that the addition of HPMC can effectively improve the quality of steamed bread. In addition, Table 2.5 lists the effects of HPMC addition and frozen storage time on other texture indexes of steamed bread. ) had no significant change (P>0.05); however, at 0 days of freezing, with the increase of HPMC addition, the Gumminess and Chewiness decreased significantly (P<o.05). , among them, the stickiness is reduced from 336.54±37.24 (without adding HPMC) to 206.62±11.84 (with 2% swollen MC), while the chewiness is reduced from 325.76±34.64 ( No HPMC added) was reduced to 200.78 ± 10.21 (2% HPMC added).

On the other hand, with the prolongation of freezing time, the cohesion and restoring force of steamed bread decreased significantly. For steamed bread made from frozen dough without adding HPMC, its cohesion was increased by O. 86-4-0.03 g (frozen storage 0 days) was reduced to 0.49+0.06 g (frozen storage for 60 days), while the restoring force was reduced from 0.48+0.04 g (frozen storage for 0 days) to 0.17±0.01 (frozen storage for 0 days) 60 days); however, for steamed buns made from frozen dough with 2% HPMC added, the cohesion was reduced from 0.93+0.02 g (0 days frozen) to 0.61+0.07 g (frozen storage for 60 days), while the restoring force was reduced from 0.53+0.01 g (frozen storage for 0 days) to 0.27+4-0.02 (frozen storage for 60 days). In addition, with the prolongation of frozen storage time, the stickiness and chewiness of steamed bread increased significantly. For the steamed bread made from frozen dough without adding HPMC, the stickiness was increased by 336.54+37. 24 (0 days of frozen storage) increased to 1232.86±67.67 (60 days of frozen storage), while chewiness increased from 325.76+34.64 (0 days of frozen storage) to 1005.83+83.95 (frozen for 60 days); however, for the steamed buns made from frozen dough with 2% HPMC added, the stickiness increased from 206.62+1 1.84 (frozen for 0 days) to 472.84. 96+45.58 (frozen storage for 60 days), while chewiness increased from 200.78+10.21 (frozen storage for 0 days) to 404.53+31.26 (frozen storage for 60 days). This shows that the addition of HPMC can effectively inhibit the changes in the texture properties of steamed bread caused by freezing storage. In addition, the changes in the texture properties of steamed bread caused by freezing storage (such as the increase of stickiness and chewiness and the decrease of recovery force) There is also a certain internal correlation with the change of steamed bread specific volume. Thus, dough properties (eg, farinality, elongation, and rheological properties) can be improved by adding HPMC to frozen dough, and HPMC inhibits the formation, growth, and redistribution of ice crystals (recrystallization process), making frozen dough The quality of the processed steamed buns is improved.
2.4 Chapter Summary
Hydroxypropyl methylcellulose (HPMC) is a kind of hydrophilic colloid, and its application research in frozen dough with Chinese-style pasta food (such as steamed bread) as the final product is still lacking. The main purpose of this study is to evaluate the effect of HPMC improvement by investigating the effect of HPMC addition on the processing properties of frozen dough and the quality of steamed bread, so as to provide some theoretical support for the application of HPMC in steamed bread and other Chinese-style flour products. The results show that HPMC can improve the farinaceous properties of the dough. When the addition amount of HPMC is 2%, the water absorption rate of the dough increases from 58.10% in the control group to 60.60%; 2 min increased to 12.2 min; at the same time, the dough formation time decreased from 2.1 min in the control group to 1.5 mill; the weakening degree decreased from 55 FU in the control group to 18 FU. In addition, HPMC also improved the tensile properties of the dough. With the increase in the amount of HPMC added, the elongation of the dough increased significantly; significantly reduced. In addition, during the frozen storage period, the addition of HPMC reduced the increase rate of the freezable water content in the dough, thereby inhibiting the damage to the dough network structure caused by ice crystallization, maintaining the relative stability of the dough viscoelasticity and the integrity of the network structure, thereby improving the stability of the dough network structure. The quality of the final product is guaranteed.
On the other hand, the experimental results showed that the addition of HPMC also had a good quality control and improvement effect on steamed bread made from frozen dough. For the unfrozen samples, the addition of HPMC increased the specific volume of the steamed bread and improved the texture properties of the steamed bread - reduced the hardness of the steamed bread, increased its elasticity, and at the same time reduced the stickiness and chewiness of the steamed bread. In addition, the addition of HPMC inhibited the deterioration of the quality of steamed buns made from frozen dough with the extension of freezing storage time - reducing the degree of increase in the hardness, stickiness and chewiness of the steamed buns, as well as reducing the elasticity of the steamed buns, Cohesion and recovery force decrease.
In conclusion, this shows that HPMC can be applied to the processing of frozen dough with steamed bread as the final product, and has the effect of better maintaining and improving the quality of steamed bread.
Chapter 3 Effects of HPMC addition on the structure and properties of wheat gluten under freezing conditions
3.1 Introduction
Wheat gluten is the most abundant storage protein in wheat grains, accounting for more than 80% of the total protein. According to the solubility of its components, it can be roughly divided into glutenin (soluble in alkaline solution) and gliadin (soluble in alkaline solution). in ethanol solution). Among them, the molecular weight (mw) of glutenin is as high as 1x107Da, and it has two subunits, which can form intermolecular and intramolecular disulfide bonds; while the molecular weight of gliadin is only 1x104Da, and there is only one subunit, which can form molecules Internal disulfide bond [100]. Campos, Steffe, & Ng (1 996) divided the formation of dough into two processes: energy input (mixing process with dough) and protein association (formation of dough network structure). It is generally believed that during dough formation, glutenin determines the elasticity and structural strength of the dough, while gliadin determines the viscosity and fluidity of the dough [102]. It can be seen that gluten protein has an indispensable and unique role in the formation of the dough network structure, and endows the dough with cohesion, viscoelasticity and water absorption.
In addition, from a microscopic point of view, the formation of the three-dimensional network structure of dough is accompanied by the formation of intermolecular and intramolecular covalent bonds (such as disulfide bonds) and non-covalent bonds (such as hydrogen bonds, hydrophobic forces) [103]. Although the energy of the secondary bond
Quantity and stability are weaker than covalent bonds, but they play an important role in maintaining the conformation of gluten [1041].
For frozen dough, under freezing conditions, the formation and growth of ice crystals (crystallization and recrystallization process) will cause the dough network structure to be physically squeezed, and its structural integrity will be destroyed, and microscopically. Accompanied by changes in the structure and properties of gluten protein [105'1061. As Zhao, et a1. (2012) found that with the prolongation of freezing time, the molecular weight and molecular gyration radius of gluten protein decreased [107J, which indicated that gluten protein partially depolymerized. In addition, the spatial conformational changes and thermodynamic properties of gluten protein will affect the dough processing properties and product quality. Therefore, in the process of freezing storage, it is of certain research significance to investigate the changes of water state (ice crystal state) and the structure and properties of gluten protein under different freezing storage time conditions.
As mentioned in the preface, as a cellulose derivative hydrocolloid, the application of hydroxypropyl methylcellulose (HPMC) in frozen dough is not much studied, and the research on its action mechanism is even less.
Therefore, the purpose of this experiment is to use the wheat gluten dough (Gluten Dough) as the research model to investigate the content of HPMC (0, 0.5%) under different freezing storage time (0, 15, 30, 60 days) , 1%, 2%) on the state and distribution of water in the wet gluten system, gluten protein rheological properties, thermodynamic properties, and its physicochemical properties, and then explore the reasons for the changes in the processing properties of frozen dough, and the role of HPMC Mechanism problems, so as to improve the understanding of related problems.
3.2 Materials and methods
3.2.1 Experimental materials
Gluten Anhui Rui Fu Xiang Food Co., Ltd.; Hydroxypropyl Methylcellulose (HPMC, same as above) Aladdin Chemical Reagent Co., Ltd.
3.2.2 Experimental apparatus
Equipment Name
Discovery. R3 Rheometer
DSC. Q200 Differential Scanning Calorimeter
PQ00 1 low-field NMR instrument
722E Spectrophotometer
JSM. 6490LV Tungsten Filament Scanning Electron Microscope
HH digital constant temperature water bath
BC/BD. 272SC refrigerator
BCD. 201LCT refrigerator
ME. 5 Ultra-microelectronic balance
Automatic microplate reader
Nicolet 67 Fourier transform infrared spectrometer
FD. 1B. 50 Vacuum Freeze Dryer
KDC. 160HR high-speed refrigerated centrifuge
Thermo Fisher FC full wavelength scanning microplate reader
PB. Model 10 pH Meter
MYP ll. Type 2 Magnetic Stirrer
MX. S type eddy current oscillator
SX2.4.10 muffle furnace
Kjeltec TM 8400 automatic Kjeldahl nitrogen analyzer
Manufacturer
American TA company
American TA company
Shanghai Niumet Company
Shanghai Spectrum Instrument Co., Ltd.
Nippon Electronics Manufacturing Co., Ltd.
Jintan Jincheng Guosheng Experimental Instrument Factory
Qingdao Haier Group
Hefei Mei ling Co., Ltd.
Sartorius, Germany
Thermo Fisher, USA
Thermo Nicolet, USA
Beijing Bo Yi kang Experimental Instrument Co., Ltd.
Anhui Zhong ke Zhong jia Scientific Instrument Co., Ltd.
Thermo Fisher, USA
certoris germany
Shanghai Mei ying pu Instrument Co., Ltd.
SCILOGEX, USA
Huangshi Hengfeng Medical Equipment Co., Ltd.
Danish FOSS company
3.2.3 Experimental reagents
All chemical reagents used in the experiments were of analytical grade.
3.2.4 Experimental method
3.2.4.1 Determination of basic components of gluten
According to GB 5009.5_2010, GB 50093.2010, GB 50094.2010, GB/T 5009.6.2003t78-81], the contents of protein, moisture, ash and lipid in gluten were determined respectively, and the results are shown in Table 3.1 shown.

3.2.4.2 Preparation of frozen wet gluten dough (Gluten Dough)
Weigh 100 g of gluten into a beaker, add distilled water (40%, w/w) to it, stir with a glass rod for 5 min, and then place it in a 4 "C refrigerator for 1 h to make it fully Hydrate to obtain wet gluten mass. After taking it out, seal it in a fresh-keeping bag, and freeze it for 24 hours at .30℃. Finally, freeze it in a refrigerator at .18℃ for a certain period of time (15 days, 30 days and 60 days). Take the frozen 0-day sample (je, fresh unfrozen wet gluten mass) as the blank control group. Use 0.5%, 1% and 2% HPMC (w/w) to replace the corresponding quality of gluten Prion powder, and the rest of the production steps and freezing treatment remain unchanged, so as to prepare wet gluten dough samples with different HPMC additions.
3.2.4.3 Determination of rheological properties of wet gluten mass
When the corresponding freezing time is over, take out the frozen wet gluten mass and place it in a 4°C refrigerator to equilibrate for 8 hours. Then, take out the sample and place it at room temperature until the sample is completely thawed (this method of thawing the wet gluten mass is also applicable to later part of the experiments, 2.7.1 and 2.9). A sample (about 2 g) of the central area of ​​the melted wet gluten mass was cut and placed on the sample carrier (Bottom Plate) of the rheometer (Discovery R3). Strain Sweep) to determine the Linear Viscoelasticity Region (LVR), the specific experimental parameters are set as follows - the fixture is a parallel plate with a diameter of 40 mill, the gap is set to 1000 mrn, and the temperature is set to 25 °C, the strain scanning range is 0.01%. 100%, the frequency is set to 1 Hz. Then, after changing the sample, let it stand for 10 minutes, and then perform dynamic
Frequency sweep, the specific experimental parameters are set as follows - the strain is 0.5% (at LVR), and the frequency sweep range is 0.1 Hz. 10 Hz, while other parameters are the same as the strain sweep parameters. Scanning data is acquired in logarithmic mode, and 5 data points (plots) are recorded in the rheological curve for every 10-fold increase in frequency, so as to obtain the frequency as the abscissa, the storage modulus (G') and the loss modulus (G') is the rheological discrete curve of the ordinate. It is worth noting that after each time the sample is pressed by the clamp, the excess sample needs to be gently scraped with a blade, and a layer of paraffin oil is applied to the edge of the sample to prevent moisture during the experiment. of loss. Each sample was replicated three times.
3.2.4.4 Determination of thermodynamic properties
According to the method of Bot (2003) [1081, differential scanning calorimeter (DSC Q.200) was used in this experiment to measure the relevant thermodynamic properties of the samples.
(1)Determination of Content of Freezable Water (CF Silicon) in wet gluten mass
A 15 mg sample of wet gluten was weighed and sealed in an aluminum crucible (suitable for liquid samples). The determination procedure and parameters are as follows: equilibrate at 20°C for 5 min, then drop to .30°C at a rate of 10°C/min, keep the temperature for 10 min, and finally increase to 25°C at a rate of 5°C/min, purge the gas (Purge Gas) was nitrogen (N2) and its flow rate was 50 mL/min, and a blank sealed aluminum crucible was used as a reference. The obtained DSC curve was analyzed using the analysis software Universal Analysis 2000, by analyzing the peaks located around 0 °C. Integral to get the melting enthalpy of ice crystals (Yu day). Then, the freezable water content (CFW) is calculated by the following formula [85-86]:

Among them, three, represents the latent heat of moisture, and its value is 334 J/g; MC represents the total moisture content of the wet gluten measured (measured according to GB 50093.2010 [. 78]). Each sample was replicated three times.
(2)Determination of thermal denaturation peak temperature (TP) of wheat gluten protein
Freeze-dry the frozen-storage-treated sample, grind it again, and pass it through a 100-mesh sieve to obtain gluten protein powder (this solid powder sample is also applicable to 2.8). A 10 mg gluten protein sample was weighed and sealed in an aluminum crucible (for solid samples). The DSC measurement parameters were set as follows, equilibrated at 20 °C for 5 min, and then increased to 100 °C at a rate of 5 °C/min, using nitrogen as the purge gas, and its flow rate was 80 mL/min. Using a sealed empty crucible as a reference, and use the analysis software Universal Analysis 2000 to analyze the obtained DSC curve to obtain the peak temperature of thermal denaturation of wheat gluten protein (Yes). Each sample is replicated three times.
3.2.4.5 Determination of free sulfhydryl content (C) of wheat gluten
The content of free sulfhydryl groups was determined according to the method of Beveridg, Toma, & Nakai (1974) [Hu], with appropriate modifications. Weigh 40 mg of wheat gluten protein sample, shake it well, and make it dispersed in 4 mL of dodecyl sulfonate
Sodium Sodium (SDS). Tris-hydroxymethyl aminomethane (Tris). Glycine (Gly). Tetraacetic acid 7, amine (EDTA) buffer (10.4% Tris, 6.9 g glycine and 1.2 g EDTA/L, pH 8.0, abbreviated as TGE, and then 2.5% SDS It was added to the above TGE solution (that is, prepared into SDS-TGE buffer), incubated at 25°C for 30 min, and shaken every 10 min. Then, the supernatant was obtained after centrifugation for 10 min at 4°C and 5000×g. First, the protein content in the supernatant was determined by the Coomassie brilliant blue (G.250) method. Then, to the supernatant was added O. 04 mL of Ellman's reagent (dissolve 5,5'. Dithio-2. Nitrobenzoic acid, DTNB at TGE to measure the solution, 4 rag/ml), after 30 minutes of incubation in a 25 ℃ water bath, add 412 nm absorbance, and the above buffer was used as blank control. Finally, the free sulfhydryl content was calculated according to the following formula:

Among them, 73.53 is the extinction coefficient; A is the absorbance value; D is the dilution factor (1 here); G is the protein concentration. Each sample was replicated three times.
3.2.4.6 Determination of 1H I"2 relaxation time
According to Kontogiorgos, Goff, & Kasapis (2007) method [1111, 2 g of wet gluten mass was placed in a 10 mm diameter nuclear magnetic tube, sealed with plastic wrap, and then placed in a low-field nuclear magnetic resonance apparatus to measure the transverse relaxation time (n), the specific parameters are set as follows: 32 ℃ equilibrium for 3 min, the field strength is 0.43 T, the resonance frequency is 18.169 Hz, and the pulse sequence is Carr-Purcell-Meiboom-Gill (CPMG), and the pulse durations of 900 and 1 800 were set to 13¨s and 25¨s , respectively, and the pulse interval r was as small as possible to reduce the interference and diffusion of the decay curve. In this experiment, it was set to O. 5 m s. Each assay was scanned 8 times to increase the signal-to-noise ratio (SNR), with a 1 s interval between each scan. The relaxation time is obtained from the following integral equation:

Among them, M is the function of the exponential decay sum of the signal amplitude with time (t) as the independent variable; Yang) is the function of the hydrogen proton number density with the relaxation time (D) as the independent variable.
Using the CONTIN algorithm in the Provencher analysis software combined with the Laplace inverse transformation, the inversion is performed to obtain a continuous distribution curve. Each sample was repeated three times
3.2.4.7 Determination of secondary structure of wheat gluten protein
In this experiment, a Fourier transform infrared spectrometer equipped with an attenuated single reflection attenuated total reflection (ATR) accessory was used to determine the secondary structure of gluten protein, and a cadmium mercury telluride crystal was used as the detector. Both sample and background collection were scanned 64 times with a resolution of 4 cm~ and a scanning range of 4000 cmq-500 cm~. Spread a small amount of protein solid powder on the surface of the diamond on the ATR fitting, and then, after 3 turns clockwise, you can start to collect the infrared spectrum signal of the sample, and finally get the wavenumber (Wavenumber, cm-1) as the abscissa, and absorbance as the abscissa. (Absorption) is the infrared spectrum of the ordinate.
Use OMNIC software to perform automatic baseline correction and advanced ATR correction on the obtained full wavenumber infrared spectrum, and then use Peak. Fit 4.12 software performs baseline correction, Fourier deconvolution and second derivative fitting on the amide III band (1350 cm-1.1200 cm'1) until the fitted correlation coefficient (∥) reaches 0. 99 or more, the integrated peak area corresponding to the secondary structure of each protein is finally obtained, and the relative content of each secondary structure is calculated. Amount (%), that is, the peak area/total peak area. Three parallels were performed for each sample.
3.2.4.8 Determination of surface hydrophobicity of gluten protein
According to the method of Kato & Nakai (1980) [112], naphthalene sulfonic acid (ANS) was used as a fluorescent probe to determine the surface hydrophobicity of wheat gluten. Weigh 100 mg gluten protein solid powder sample, disperse it in 15 mL, 0.2M, pH 7.0 phosphate buffered saline (PBS), stir magnetically for 20 min at room temperature, and then stir at 7000 rpm, 4 " Under the condition of C, centrifuge for 10 min, and take the supernatant. Similarly, use Coomassie brilliant blue method to measure the protein content in the supernatant, then according to the measurement results, the supernatant is diluted with PBS for 5 concentration gradients in turn, and the protein concentration is at 0 .02.0.5 mg/mL range.
Absorb 40 I L ANS solution (15.0 mmol/L) was added to each gradient sample solution (4 mL), shaken and shaken well, then quickly moved to a sheltered place, and 200 "L drops of light were drawn from the sample tube with low concentration to high concentration in turn. Add it to a 96-well microtiter plate, and use an automatic microplate reader to measure the fluorescence intensity values with 365 nm as excitation light and 484 am as emission light. Surface Hydrophobicity is linearly fitted with the protein concentration as the abscissa is characterized by the slope value obtained from the curve of the fluorescence intensity as the ordinate. Each sample is paralleled at least three times.
3.2.4.9 Electron microscope observation
After freeze-drying the wet gluten mass without adding HPMC and adding 2% HPMC that had been frozen for 0 days and 60 days, some samples were cut out, sprayed with gold 90 S with an electron sputter, and then placed in a scanning electron microscope (JSM.6490LV). Morphological observation was carried out. The accelerating voltage was set to 20 KV and the magnification was 100 times.
3.2.4.10 Data processing
All results are expressed as mean 4-standard deviation, and the above experiments were repeated at least three times except for scanning electron microscopy. Use Origin 8.0 to draw charts, and use SPSS 19.0 for one. Way analysis of variance and Duncan's multiple range test, the significance level was 0.05.
3. Results and discussion
3.3.1 Effects of HPMC addition amount and freezing storage time on the rheological properties of wet gluten mass
Rheological properties are an effective way to reflect the structure and properties of food materials and to predict and evaluate product quality [113J. As we all know, gluten protein is the main material component that gives dough viscoelasticity. As shown in Figure 3.1, the dynamic frequency sweep (0.1.10 Hz) results show that the storage modulus (elastic modulus, G') of all wet gluten mass samples is greater than the loss modulus (viscous modulus) , G”), therefore, the wet gluten mass showed solid-like rheological characteristics (Figure 3.1, A.D). This result also shows that the intermolecular and intramolecular glutenin The mutual cross-linking structure formed by covalent or non-covalent interaction is the backbone of the dough network structure [114]. At the same time, Sin Qu & Singh (2013) also believed that the rheological properties of dough are related to their protein components [114]. 115]. In addition, with the prolongation of freezing time, the G' and G' moduli of wet gluten doughs with 0%, 0.5% and 1% HPMC added showed different degrees of decrease (Fig. 3.1, 115). A-C), and the degree of decrease was negatively correlated with the addition of HPMC, so that the G and G" moduli of wet gluten doughs with 2% HPMC addition did not show a significant increase with the freezing storage time from 0 to 60 days. Sexual differences (Figure 3.1, D). This indicates that the three-dimensional network structure of the wet gluten mass without HPMC was destroyed by the ice crystals formed during the freezing process, which is consistent with the results found by Kontogiorgos, Goff, & Kasapis (2008), who believed that the prolonged freezing time caused the functionality and stability of the dough structure were seriously reduced.

Fig 3．1 Effect of HPMC addition and frozen storage on rheological properties of gluten dough
Note: Among them, A is the oscillating frequency scanning result of wet gluten without adding HPMC: B is the oscillating frequency scanning result of wet gluten adding 0.5% HPMC; C is the oscillating frequency scanning result of adding 1% HPMC: D is the oscillating frequency scanning result of adding 2% HPMC Wet Gluten Oscillation Frequency Sweep Results.
During frozen storage, the moisture in the wet gluten mass crystallizes because the temperature is lower than its freezing point, and it is accompanied by a recrystallization process over time (due to fluctuations in temperature, migration and distribution of moisture, changes in moisture state, etc.) , which in turn leads to the growth of ice crystals (increase in size), which makes the ice crystals located in the dough network structure destroy their integrity and break some chemical bonds through physical extrusion. However, by comparing with the comparison of groups showed that the addition of HPMC could effectively inhibit the formation and growth of ice crystals, thereby protecting the integrity and strength of the gluten network structure, and within a certain range, the inhibitory effect was positively correlated with the amount of HPMC added.
3.3.2 Effects of HPMC addition amount and freezing storage time on the freezer moisture content (CFW) and thermal stability
3.3.2.1 Effects of HPMC addition amount and freezing storage time on the freezable moisture content (CFW) in wet gluten dough
Ice crystals are formed by the phase transition of freezable water at temperatures below its freezing point. Therefore, the content of freezable water directly affects the number, size and distribution of ice crystals in the frozen dough. The experimental results (Table 3.2) show that as the freezing storage time is extended from 0 days to 60 days, the wet gluten mass Chinese silicon gradually becomes larger, which is consistent with the research results of others [117'11 81]. In particular, after 60 days of frozen storage, the phase transition enthalpy (day) of the wet gluten mass without HPMC increased from 134.20 J/g (0 d) to 166.27 J/g (60 d), that is, the increase increased by 23.90%, while the freezable moisture content (CF silicon) increased from 40.08% to 49.78%, an increase of 19.59%. However, for the samples supplemented with 0.5%, 1% and 2% HPMC, after 60 days of freezing, the C-chat increased by 20.07%, 16, 63% and 15.96%, respectively, which is consistent with Matuda, et a1. (2008) found that the melting enthalpy (Y) of the samples with added hydrophilic colloids decreased compared with the blank samples [119].
The increase in CFW is mainly due to the recrystallization process and the change of the gluten protein conformation, which changes the state of water from non-freezable water to freezable water. This change in moisture state allows ice crystals to be trapped in the interstices of the network structure, the network structure (pores) gradually become larger, which in turn leads to greater squeezing and destruction of the walls of the pores. However, the significant difference of 0w between the sample with a certain content of HPMC and the blank sample shows that HPMC can keep the water state relatively stable during the freezing process, thereby reducing the damage of ice crystals to the gluten network structure, and even inhibiting the quality of the product. deterioration.

3.3.2.2 Effects of adding different contents of HPMC and freezing storage time on the thermal stability of gluten protein
The thermal stability of gluten has an important influence on the grain formation and product quality of thermally processed pasta [211]. Figure 3.2 shows the obtained DSC curve with temperature (°C) as the abscissa and heat flow (mW) as the ordinate. The experimental results (Table 3.3) found that the heat denaturation temperature of gluten protein without freezing and without adding I-IPMC was 52.95 °C, which was consistent with Leon, et a1. (2003) and Khatkar, Barak, & Mudgil (2013) reported very similar results [120m11. With the addition of 0% unfrozen, O. Compared with the heat denaturation temperature of gluten protein with 5%, 1% and 2% HPMC, the heat deformation temperature of gluten protein corresponding to 60 days increased by 7.40℃, 6.15℃, 5.02℃ and 4.58℃, respectively. Obviously, under the condition of the same freezing storage time, the increase of denaturation peak temperature (N) decreased sequentially with the increase of HPMC addition. This is consistent with the change rule of the results of Cry. In addition, for the unfrozen samples, as the amount of HPMC added increases, the N values ​​decrease sequentially. This may be due to the intermolecular interactions between HPMC with molecular surface activity and gluten, such as the formation of covalent and non-covalent bonds [122J].

Note: Different superscript lowercase letters in the same column indicate significant difference (P<0.05) In addition, Myers (1990) believed that a higher Ang means that the protein molecule exposes more hydrophobic groups and participates in the denaturation process of the molecule [1231]. Therefore, more hydrophobic groups in gluten were exposed during freezing, and HPMC could effectively stabilize the molecular conformation of gluten.

Fig 3．2 Typical DSC thermograms of gluten proteins with 0％HPMC(A)；with O．5％HPMC(B)； with 1％HPMC(C)；with 2％HPMC(D)after different time of frozen storage，from 0d to 60d indicated from the lowest curve to the highest one in each graph． Note: A is the DSC curve of wheat gluten without adding HPMC; B is the addition of O. DSC curve of wheat gluten with 5% HPMC; C is the DSC curve of wheat gluten with 1% HPMC; D is the DSC curve of wheat gluten with 2% HPMC 3.3.3 Effects of HPMC addition amount and freezing time on free sulfhydryl content (C-SH) Intermolecular and intramolecular covalent bonds are very important for the stability of dough network structure. A disulfide bond (-S-S-) is a covalent linkage formed by dehydrogenation of two free sulfhydryl groups (.SH). Glutenin is composed of glutenin and gliadin, the former can form intramolecular and intermolecular disulfide bonds, while the latter can only form intramolecular disulfide bonds [1241] Therefore, disulfide bonds are an intramolecular/intermolecular disulfide bond. important way of cross-linking. Compared to adding 0%, O. The C-SH of 5% and 1% HPMC without freezing treatment and the C-SH of gluten after 60 days of freezing have different degrees of increase respectively. Specifically, the face with no HPMC added gluten C. SH increased by 3.74 "mol/g to 8.25 "mol/g, while C.sh, shellfish, with gluten supplemented with 0.5% and 1% HPMC increased by 2.76 "mol/g to 7.25""mol/g and 1.33 "mol/g to 5.66 "mol/g (Fig. 3.3). Zhao, et a1. (2012) found that after 120 days of frozen storage, the content of free thiol groups increased significantly [ 1071. It is worth noting that the C-SH of gluten protein was significantly lower than that of other frozen storage periods when the freezing period was 15 days, which may be attributed to the freezing shrinkage effect of gluten protein structure, which makes the More intermolecular and intramolecular disulfide bonds were locally formed in a shorter freezing time [1161. Wang, et a1. (2014) found that the C-SH of glutenin-rich proteins was also significantly increased after 15 days of freezing. Decreased [1251. However, the gluten protein supplemented with 2% HPMC did not increase significantly except for C-SH, which also decreased significantly at 15 days, with the extension of freezing time.

Fig 3．3 Effect of HPMC addition and frozen storage on the content of free-SH for gluten proteins As mentioned above, freezable water can form ice crystals at low temperatures and distribute in the interstices of the gluten network. Therefore, with the prolongation of freezing time, the ice crystals become larger, which squeezes the gluten protein structure more seriously, and leads to the breakage of some intermolecular and intramolecular disulfide bonds, which increases the content of free sulfhydryl groups. On the other hand, the experimental results show that HPMC can protect the disulfide bond from the extrusion damage of ice crystals, thereby inhibiting the depolymerization process of gluten protein. 3.3.4 Effects of HPMC addition amount and freezing storage time on transverse relaxation time (T2) of wet gluten mass The distribution of Transverse Relaxation Time (T2) can reflect the model and dynamic process of water migration in food materials [6]. Figure 3.4 shows the distribution of wet gluten mass at 0 and 60 days with different HPMC additions, including 4 main distribution intervals, namely 0.1.1 m s (T21), 1.10 m s (T22), 10.100 m s (dead;) and 1 00-1 000 m s (T24). Bosmans et al. (2012) found a similar distribution of wet gluten mass [1261], and they suggested that protons with relaxation times below 10 m s could be classified as rapidly relaxing protons, which are mainly derived from poor mobility the bound water, therefore, may characterize the relaxation time distribution of bound water bound to a small amount of starch, while Dang may characterize the relaxation time distribution of bound water bound to gluten protein. In addition, Kontogiorgos (2007) - t11¨, the "strands" of the gluten protein network structure are composed of several layers (Sheets) about 5 nm apart, and the water contained in these layers is limited water (or Bulk water, phase water), the mobility of this water is between the mobility of bound water and free water. And T23 can be attributed to the relaxation time distribution of restricted water. The T24 distribution (>100 m s) has a long relaxation time, so it characterizes free water with strong mobility. This water exists in the pores of the network structure, and there is only a weak capillary force with the gluten protein system.

Fig 3．4 Effect of FIPMC addition and frozen storage on distributions curves of transverse relaxation time for gluten dough
Note: A and B represent the transverse relaxation time (N) distribution curves of wet gluten with different contents of HPMC added for 0 days and 60 days in freezing storage, respectively
Comparing the wet gluten doughs with different addition amounts of HPMC stored in frozen storage for 60 days and unfrozen storage respectively, it was found that the total distribution area of T21 and T24 did not show a significant difference, indicating that the addition of HPMC did not significantly increase the relative amount of bound water. content, which may be due to the fact that the main water-binding substances (gluten protein with a small amount of starch) were not significantly changed by the addition of a small amount of HPMC. On the other hand, by comparing the distribution areas of T21 and T24 of wet gluten mass with the same amount of HPMC added for different freezing storage times, there is also no significant difference, which indicates that the bound water is relatively stable during the freezing storage process, and has a negative impact on the environment. Changes are less sensitive and less affected.
However, there were obvious differences in the height and area of ​​T23 distribution of wet gluten mass that was not frozen and contained different HPMC additions, and with the increase of addition, the height and area of ​​T23 distribution increased (Fig. 3.4). This change shows that HPMC can significantly increase the relative content of limited water, and it is positively correlated with the added amount within a certain range. In addition, with the extension of freezing storage time, the height and area of ​​T23 distribution of the wet gluten mass with the same HPMC content decreased to varying degrees. Therefore, compared with bound water, limited water showed a certain effect on freezing storage. Sensitivity. This trend suggests that the interaction between the gluten protein matrix and the confined water becomes weaker. This may be because more hydrophobic groups are exposed during freezing, which is consistent with the thermal denaturation peak temperature measurements. In particular, the height and area of ​​the T23 distribution for the wet gluten mass with 2% HPMC addition did not show a significant difference. This indicates that HPMC can limit the migration and redistribution of water, and can inhibit the transformation of the water state from the restricted state to the free state during the freezing process.
In addition, the height and area of ​​the T24 distribution of the wet gluten mass with different contents of HPMC were significantly different (Fig. 3.4, A), and the relative content of free water was negatively correlated with the amount of HPMC added. This is just the opposite of the Dang distribution. Therefore, this variation rule indicates that HPMC has water holding capacity and converts free water to confined water. However, after 60 days of freezing, the height and area of ​​T24 distribution increased to varying degrees, which indicated that the water state changed from restricted water to free-flowing state during the freezing process. This is mainly due to the change of the gluten protein conformation and the destruction of the "layer" unit in the gluten structure, which changes the state of the confined water contained in it. Although the content of freezable water determined by DSC also increases with the extension of freezing storage time, however, due to the difference in the measurement methods and characterization principles of the two, the freezable water and free water are not completely equivalent. For the wet gluten mass added with 2% HPMC, after 60 days of freezing storage, none of the four distributions showed significant differences, indicating that HPMC can effectively retain the water state due to its own water-holding properties and its interaction with gluten. and stable liquidity.
3.3.5 Effects of HPMC addition amount and freezing storage time on the secondary structure of gluten protein
Generally speaking, the secondary structure of protein is divided into four types, α-Spiral, β-folded, β-Corners and random curls. The most important secondary bonds for the formation and stabilization of the spatial conformation of proteins are hydrogen bonds. Therefore, protein denaturation is a process of hydrogen bond breaking and conformational changes.
Fourier transform infrared spectroscopy (FT-IR) has been widely used for high-throughput determination of the secondary structure of protein samples. The characteristic bands in the infrared spectrum of proteins mainly include, amide I band (1700.1600 cm-1), amide II band (1600.1500 cm-1) and amide III band (1350.1200 cm-1). Correspondingly, the amide I band the absorption peak originates from the stretching vibration of the carbonyl group (-C=O-.), the amide II band is mainly due to the bending vibration of the amino group (-N-H-) [1271], and the amide III band is mainly due to the amino bending vibration and .C-N-．Synchronous compound vibration in the same plane of bond stretching vibration, and has a high sensitivity to changes in protein secondary structure [128'1291. Although the above three characteristic bands are all characteristic infrared absorption peaks of proteins, the specific In other words, the absorption intensity of amide II band is lower, so the semi-quantitative accuracy of protein secondary structure is poor; while the peak absorption intensity of amide I band is higher, so many researchers analyze the secondary structure of protein by this band [ 1301, but the absorption peak of water and the amide I band are overlapped at about 1640 cm. 1 wavenumber (Overlapped), which in turn affects the accuracy of the results. Therefore, the interference of water limits the determination of the amide I band in protein secondary structure determination. In this experiment, in order to avoid the interference of water, the relative contents of four secondary structures of gluten protein were obtained by analyzing the amide III band. Peak position (wavenumber interval) of
The attribution and designation are listed in Table 3.4.
Tab 3．4 Peak positions and assignment of secondary structures originated from amide III band in FT-IR spectra

Figure 3.5 is the infrared spectrum of the amide III band of gluten protein added with different contents of HPMC for 0 days after being frozen for 0 days after deconvolution and fitting of the second derivative. (2001) applied the second derivative to fit the deconvoluted peaks with similar peak shapes [1321]. In order to quantify the relative content changes of each secondary structure, Table 3.5 summarizes the relative percentage content of the four secondary structures of gluten protein with different freezing times and different HPMC additions (corresponding peak integral area/peak total area).

Fig 3．5 Deconvolution of amide band III of gluten with O％HPMC at 0 d(A)，with 2％HPMC at 0 d(B)
Note: A is the infrared spectrum of wheat gluten protein without adding HPMC for 0 days of frozen storage; B is the infrared spectrum of wheat gluten protein of frozen storage for 0 days with 2% HPMC added
With the prolongation of frozen storage time, the secondary structure of gluten protein with different additions of HPMC changed to different degrees. It can be seen that both frozen storage and addition of HPMC have an effect on the secondary structure of gluten protein. Regardless of the amount of HPMC added, B. The folded structure is the most dominant structure, accounting for about 60%. After 60 days of frozen storage, add 0%, O. B. Gluten of 5% and 1% HPMC. The relative content of folds increased significantly by 3.66%, 1.87% and 1.16%, respectively, which was similar to the results determined by Meziani et al. (2011) [l33J]. However, there was no significant difference during frozen storage for gluten supplemented with 2% HPMC. In addition, when frozen for 0 days, with the increase of HPMC addition, p. The relative content of folds increased slightly, especially when the addition amount was 2%, p. The relative content of folds increased by 2.01%. D. The folded structure can be divided into intermolecular p. Folding (caused by aggregation of protein molecules), antiparallel p. Folded and parallel p. Three substructures are folded, and it is difficult to determine which substructure occurs during the freezing process
changed. Some researchers believe that the increase in the relative content of the B-type structure will lead to an increase in the rigidity and hydrophobicity of the steric conformation [41], and other researchers believe that p. The increase in folded structure is due to part of the new β-Fold formation is accompanied by a weakening of the structural strength maintained by hydrogen bonding [421]. β- The increase in the folded structure indicates that the protein is polymerized through hydrophobic bonds, which is consistent with the results of the peak temperature of thermal denaturation measured by DSC and the distribution of transverse relaxation time measured by low-field nuclear magnetic resonance. Protein denaturation. On the other hand, added 0.5%, 1% and 2% HPMC gluten protein α-whirling. The relative content of helix increased by 0.95%, 4.42% and 2.03% respectively with the prolongation of freezing time, which is consistent with Wang, et a1. (2014) found similar results [134]. 0 of gluten without added HPMC. There was no significant change in the relative content of helix during the frozen storage process, but with the increase of the addition amount of freeze for 0 days. There were significant differences in the relative content of α-whirling structures.

Fig 3．6 Schematic description of hydrophobic moiety exposure(A)，water redistribution(B)，and secondary structural changes(C)in gluten matrix with the increasing frozen storage time【31’138】

All samples with the extension of freezing time, p. The relative contents of the corners were significantly reduced. This shows that β-turn is very sensitive to freezing treatment [135. 1361], and whether HPMC is added or not has no effect. Wellner, et a1. (2005) proposed that the β-chain turn of gluten protein is related to the β-turn space domain structure of the glutenin polypeptide chain [l 37]. Except that the relative content of random coil structure of gluten protein added with 2% HPMC had no significant change in frozen storage, the other samples were significantly reduced, which may be caused by the extrusion of ice crystals. In addition, when frozen for 0 days, the relative contents of α-helix, β-sheet and β-turn structure of gluten protein added with 2% HPMC were significantly different from those of gluten protein without HPMC. This may indicate that there is an interaction between HPMC and gluten protein, forming new hydrogen bonds and then affecting the conformation of the protein; or HPMC absorbs the water in the pore cavity of the protein space structure, which deforms the protein and leads to more changes between the subunits. close. The increase of the relative content of β-sheet structure and the decrease of the relative content of β-turn and α-helix structure are consistent with the above speculation. During the freezing process, the diffusion and migration of water and the formation of ice crystals destroy the hydrogen bonds that maintain the conformational stability and expose the hydrophobic groups of proteins. In addition, from the perspective of energy, the smaller the energy of the protein, the more stable it is. At low temperature, the self-organization behavior (folding and unfolding) of protein molecules proceeds spontaneously and leads to conformational changes.
In conclusion, when a higher content of HPMC was added, due to the hydrophilic properties of HPMC and its interaction with the protein, HPMC could effectively inhibit the change of the secondary structure of gluten protein during the freezing process and keep the protein conformation stable.
3.3.6 Effects of HPMC addition amount and freezing storage time on the surface hydrophobicity of gluten protein
Protein molecules include both hydrophilic and hydrophobic groups. Generally, the protein surface is composed of hydrophilic groups, which can bind water through hydrogen bonding to form a hydration layer to prevent protein molecules from agglomerating and maintain their conformational stability. The interior of the protein contains more hydrophobic groups to form and maintain the secondary and tertiary structure of the protein through the hydrophobic force. Denaturation of proteins is often accompanied by exposure of hydrophobic groups and increased surface hydrophobicity.
Tab3．6 Effect of HPMC addition and frozen storage On surface hydrophobicity of gluten

Note: In the same row, there is a superscript letter with no M and b, indicating that there is a significant difference (<0.05);
Different superscript capital letters in the same column indicate significant difference (<0.05);
After 60 days of frozen storage, add 0%, O. The surface hydrophobicity of gluten with 5%, 1% and 2% HPMC increased by 70.53%, 55.63%, 43.97% and 36.69%, respectively (Table 3.6). In particular, the surface hydrophobicity of the gluten protein without adding HPMC after being frozen for 30 days has increased significantly (P<0.05), and it is already greater than the surface of the gluten protein with 1% and 2% HPMC added after freezing for 60 days Hydrophobicity. At the same time, after 60 days of frozen storage, the surface hydrophobicity of gluten protein added with different contents showed significant differences. However, after 60 days of frozen storage, the surface hydrophobicity of gluten protein added with 2% HPMC only increased from 19.749 to 26.995, which was not significantly different from the surface hydrophobicity value after 30 days of frozen storage, and was always lower than other the value of the surface hydrophobicity of the sample. This indicates that HPMC can inhibit the denaturation of gluten protein, which is consistent with the results of DSC determination of the peak temperature of heat deformation. This is because HPMC can inhibit the destruction of protein structure by recrystallization, and due to its hydrophilicity,
HPMC can combine with the hydrophilic groups on the protein surface through secondary bonds, thereby changing the surface properties of the protein, while limiting the exposure of hydrophobic groups (Table 3.6).
3.3.7 Effects of HPMC addition amount and freezing storage time on the micro-network structure of gluten
The continuous gluten network structure contains many pores to maintain the carbon dioxide gas produced by the yeast during the proofing process of the dough. Therefore, the strength and stability of the gluten network structure are very important to the quality of the final product, such as specific volume, quality, etc. Structure and sensory assessment. From a microscopic point of view, the surface morphology of the material can be observed by scanning electron microscopy, which provides a practical basis for the change of the gluten network structure during the freezing process.

Fig 3．7 SEM images of the microstructure of gluten dough，(A)indicated gluten dough with 0％ HPMC for 0d of frozen storage；(B)indicated gluten dough with 0％HPMC for 60d；(C)indicated gluten dough with 2％HPMC for 0d；(D)indicated gluten dough with 2％HPMC for 60d．
Note: A is the microstructure of gluten network without adding HPMC and frozen for 0 days; B is the microstructure of gluten network without adding HPMC and frozen for 60 days; C is the microstructure of gluten network with 2% HPMC added and frozen for 0 days :D is the gluten network microstructure with 2% HPMC added and frozen for 60 days
After 60 days of frozen storage, the microstructure of the wet gluten mass without HPMC was significantly changed (Fig. 3.7, A.B). At 0 days, the gluten microstructures with 2% or 0% HPMC showed complete shape, large
Small approximate porous sponge-like morphology. However, after 60 days of frozen storage, the cells in the gluten microstructure without HPMC became larger in size, irregular in shape, and unevenly distributed (Fig. 3.7, A, B), mainly due to the This is caused by the fracture of the "wall", which is consistent with the measurement results of the free thiol group content, that is, during the freezing process, the ice crystal squeezes and breaks the disulfide bond, which affects the strength and integrity of the structure. As reported by Kontogiorgos & Goff (2006) and Kontogiorgos (2007), the interstitial regions of the gluten network are squeezed due to freeze-shrinkage, resulting in structural disruption [138. 1391]. In addition, due to dehydration and condensation, a relatively dense fibrous structure was produced in the spongy structure, which may be the reason for the decrease in free thiol content after 15 days of frozen storage, because more disulfide bonds were generated and frozen storage. The gluten structure was not severely damaged for a shorter time, which is consistent with Wang, et a1. (2014) observed similar phenomena [134]. At the same time, the destruction of the gluten microstructure leads to freer water migration and redistribution, which is consistent with the results of low-field time-domain nuclear magnetic resonance (TD-NMR) measurements. Some studies [140, 105] reported that after several freeze-thaw cycles, the gelatinization of rice starch and the structural strength of the dough became weaker, and the water mobility became higher. Nonetheless, after 60 days of frozen storage, the microstructure of gluten with 2% HPMC addition changed less, with smaller cells and more regular shapes than gluten without HPMC addition (Fig. 3.7, B, D). This further indicates that HPMC can effectively inhibit the destruction of gluten structure by recrystallization.
3.4 Chapter Summary
This experiment investigated the rheology of wet gluten dough and gluten protein by adding HPMC with different contents (0%, 0.5%, 1% and 2%) during freezing storage (0, 15, 30 and 60 days). properties, thermodynamic properties, and effects of physicochemical properties. The study found that the change and redistribution of water state during the freezing storage process significantly increased the freezable water content in the wet gluten system, which led to the destruction of the gluten structure due to the formation and growth of ice crystals, and ultimately caused the processing properties of the dough to be different. Deterioration of product quality. The results of frequency scanning showed that the elastic modulus and viscous modulus of the wet gluten mass without adding HPMC decreased significantly during the freezing storage process, and the scanning electron microscope showed that its microstructure was damaged. The content of free sulfhydryl group was significantly increased, and its hydrophobic group was more exposed, which made the thermal denaturation temperature and surface hydrophobicity of gluten protein significantly increased. However, the experimental results show that the addition of I-IPMC can effectively inhibit the changes in the structure and properties of wet gluten mass and gluten protein during freezing storage, and within a certain range, this inhibitory effect is positively correlated with the addition of HPMC. This is because HPMC can reduce the mobility of water and limit the increase of the freezable water content, thereby inhibiting the recrystallization phenomenon and keeping the gluten network structure and the spatial conformation of the protein relatively stable. This shows that the addition of HPMC can effectively maintain the integrity of the frozen dough structure, thereby ensuring product quality.
Chapter 4 Effects of HPMC Addition on the Structure and Properties of Starch under Frozen Storage
4.1 Introduction
Starch is a chain polysaccharide with glucose as the monomer. key) two types. From a microscopic point of view, starch is usually granular, and the particle size of wheat starch is mainly distributed in two ranges of 2-10 pro (B starch) and 25-35 pm (A starch). From the perspective of crystal structure, starch granules include crystalline regions and amorphous regions (je, non-crystalline regions), and the crystal forms are further divided into A, B, and C types (it becomes V-type after complete gelatinization). Generally, the crystalline region consists of amylopectin and the amorphous region consists mainly of amylose. This is because, in addition to the C chain (main chain), amylopectin also has side chains composed of B (Branch Chain) and C (Carbon Chain) chains, which makes amylopectin appear "tree-like" in raw starch. The shape of the crystallite bundle is arranged in a certain way to form a crystal.
Starch is one of the main components of flour, and its content is as high as about 75% (dry basis). At the same time, as a carbohydrate widely present in grains, starch is also the main energy source material in food. In the dough system, starch is mostly distributed and attached to the network structure of gluten protein. During processing and storage, starches often undergo gelatinization and aging stages.
Among them, starch gelatinization refers to the process in which starch granules are gradually disintegrated and hydrated in a system with high water content and under heating conditions. It can be roughly divided into three main processes. 1) Reversible water absorption stage; before reaching the initial temperature of gelatinization, the starch granules in the starch suspension (Slurry) keep their unique structure unchanged, and the external shape and internal structure basically do not change. Only very little soluble starch is dispersed in the water and can be restored to its original state. 2) The irreversible water absorption stage; as the temperature increases, water enters the gap between the starch crystallite bundles, irreversibly absorbs a large amount of water, causing the starch to swell, the volume expands several times, and the hydrogen bonds between the starch molecules are broken. It becomes stretched and the crystals disappear. At the same time, the birefringence phenomenon of starch, that is, the Maltese Cross observed under a polarizing microscope, begins to disappear, and the temperature at this time is called the initial gelatinization temperature of starch. 3) Starch granule disintegration stage; starch molecules completely enter the solution system to form starch paste (Paste/Starch Gel), at this time the viscosity of the system is the largest, and the birefringence phenomenon completely disappears, and the temperature at this time is called the complete starch gelatinization temperature, the gelatinized starch is also called α-starch [141]. When the dough is cooked, the gelatinization of starch endows the food with its unique texture, flavor, taste, color, and processing characteristics.
In general, starch gelatinization is affected by the source and type of starch, the relative content of amylose and amylopectin in starch, whether starch is modified and the method of modification, addition of other exogenous substances, and dispersion conditions (such as The influence of salt ion species and concentration, pH value, temperature, moisture content, etc.) [142-150]. Therefore, when the structure of starch (surface morphology, crystalline structure, etc.) is changed, the gelatinization properties, rheological properties, aging properties, digestibility, etc. of starch will be affected accordingly.
Many studies have shown that the gel strength of starch paste decreases, it is easy to age, and its quality deteriorates under the condition of freezing storage, such as Canet, et a1. (2005) studied the effect of freezing temperature on the quality of potato starch puree; Ferrero, et a1. (1993) investigated the effects of freezing rate and different types of additives on the properties of wheat and corn starch pastes [151-156]. However, there are relatively few reports on the effect of frozen storage on the structure and properties of starch granules (native starch), which needs to be further explored. Frozen dough (excluding pre-cooked frozen dough) is in the form of ungelatinized granules under the condition of frozen storage. Therefore, studying the structure and structural changes of native starch by adding HPMC has a certain effect on improving the processing properties of frozen dough. significance.
In this experiment, by adding different HPMC contents (0, 0.5%, 1%, 2%) to the starch suspension, the amount of HPMC added during a certain freezing period (0, 15, 30, 60 days) was studied. on starch structure and its gelatinization influence of nature.
4.2 Experimental materials and methods
4.2.1 Experimental materials
Wheat Starch Binzhou Zhongyu Food Co., Ltd.; HPMC Aladdin (Shanghai) Chemical Reagent Co., Ltd.;
4.2.2 Experimental apparatus
Equipment Name
HH digital constant temperature water bath
BSAl24S electronic balance
BC/BD-272SC refrigerator
BCD-201LCT refrigerator
SX2.4.10 muffle furnace
DHG. 9070A Blast Drying Oven
KDC. 160HR high-speed refrigerated centrifuge
Discovery R3 Rotational Rheometer
Q. 200 Differential Scanning Calorimeter
D/MAX2500V type X. ray diffractometer
SX2.4.10 muffle furnace
Manufacturer
Jiangsu Jintan Jincheng Guosheng Experimental Instrument Factory
Sartorius, Germany
Haier Group
Hefei Meiling Co., Ltd.
Huangshi Hengfeng Medical Equipment Co., Ltd.
Shanghai Yiheng Scientific Instrument Co., Ltd.
Anhui Zhongke Zhongjia Scientific Instrument Co., Ltd.
American TA company
American TA company
Rigaku Manufacturing Co., Ltd.
Huangshi Hengfeng Medical Equipment Co., Ltd.
4.2.3 Experimental method
4.2.3.1 Preparation and frozen storage of starch suspension
Weigh 1 g of starch, add 9 mL of distilled water, fully shake and mix to prepare a 10% (w/w) starch suspension. Then place the sample solution. 18 ℃ refrigerator, frozen storage for 0, 15 d, 30 d, 60 d, of which 0 day is the fresh control. Add 0.5%, 1%, 2% (w/w) HPMC instead of the corresponding quality starch to prepare samples with different addition amounts, and the rest of the treatment methods remain unchanged.
4.2.3.2 Rheological properties
Take out the above-mentioned samples treated with the corresponding freezing time, equilibrate at 4 °C for 4 h, and then move to room temperature until they are completely thawed.
(1)Starch gelatinization characteristics
In this experiment, a rheometer was used instead of a fast viscometer to measure the gelatinization characteristics of starch. See Bae et a1. (2014) method [1571] with slight modifications. The specific program parameters are set as follows: use a plate with a diameter of 40 mill, the gap (gap) is 1000 mm, and the rotation speed is 5 rad/s; I) incubate at 50 °C for 1 min; ii) at 5. C/min heated to 95°C; iii) kept at 95°C for 2.5 min, iv) then cooled to 50°C at 5°C/min; v) lastly held at 50°C for 5 min.
Draw 1.5 mL of sample solution and add it to the center of the rheometer sample stage, measure the gelatinization properties of the sample according to the above program parameters, and obtain the time (min) as the abscissa, the viscosity (Pa s) and the temperature (°C) as the starch gelatinization curve of the ordinate. According to GB/T 14490.2008 [158], the corresponding gelatinization characteristic indicators—gelatinization peak viscosity (field), peak temperature (Ang), minimum viscosity (high), final viscosity (ratio) and decay value (Breakdown) are obtained. Value, BV) and regeneration value (Setback Value, SV), wherein, decay value = peak viscosity - minimum viscosity; setback value = final viscosity - minimum viscosity. Each sample was repeated three times.
(2)Steady flow test of starch paste
The above gelatinized starch paste was subjected to the Steady Flow Test, according to the method of Achayuthakan & Suphantharika [1591, the parameters were set to: Flow Sweep mode, stand at 25°C for 10 min, and the shear rate scan range was 1) 0.1 S one. 100S～, 2) 100s～. 0.1 S～, the data is collected in logarithmic mode, and 10 data points (plots) are recorded every 10 times the shear rate, and finally the shear rate (Shear Rate, S-I) is taken as the abscissa, and the shear viscosity ( Viscosity, pa ·s) is the rheological curve of the ordinate. Use Origin 8.0 to perform nonlinear fitting of this curve and obtain the relevant parameters of the equation, and the equation satisfies the power law (Power Law), that is, t/=K), n-I, where M is the shear viscosity (pa ·s), K is the consistency coefficient (Pa ·s), is the shear rate (s. 1), and n is the flow behavior index (Flow Behavior Index, dimensionless).
4.2.3.3 Starch paste gel properties
(1) Sample preparation
Take 2.5 g of amyloid and mix it with distilled water in a ratio of 1:2 to make starch milk. Freeze at 18°C for 15 d, 30 d, and 60 d. Add 0.5, 1, 2% HPMC (w/w) to replace starch of the same quality, and other preparation methods remain unchanged. After the freezing treatment is completed, take it out, equilibrate at 4 °C for 4 h, and then thaw at room temperature until it is tested.
(3)Starch gel strength (Gel Strength)
Take 1.5 mL of sample solution and place it on the sample stage of the rheometer (Discovery.R3), press down the 40 m/n plate with a diameter of 1500 mm, and remove the excess sample solution, and continue to lower the plate to 1000 mm, on motor, the speed was set to 5 rad/s and rotated for 1 min to fully homogenize the sample solution and avoid the sedimentation of starch granules. The temperature scan starts at 25°C and ends at 5. C/min was raised to 95°C, kept for 2 min, and then lowered to 25°C at 5"C/min.
A layer of petrolatum was lightly applied to the edge of the starch gel obtained above to avoid water loss during subsequent experiments. Referring to the Abebe & Ronda method [1601], an oscillatory strain sweep was firstly performed to determine the Linear Viscoelasticity Region (LVR), the strain sweep range was 0.01-100%, the frequency was 1 Hz, and the sweep was started after standing at 25 °C for 10 min.
Then, sweep the oscillation frequency, set the strain amount (strain) to 0.1% (according to the strain sweep results), and set the frequency range to O. 1 to 10 Hz. Each sample was repeated three times.
4.2.3.4 Thermodynamic properties
(1) Sample preparation
After the corresponding freezing treatment time, the samples were taken out, thawed completely, and dried in an oven at 40 °C for 48 h. Finally, it was ground through a 100-mesh sieve to obtain a solid powder sample for use (suitable for XRD testing). See Xie, et a1. (2014) method for sample preparation and determination of thermodynamic properties '1611, weigh 10 mg of starch sample into a liquid aluminum crucible with an ultra-micro analytical balance, add 20 mg of distilled water in a ratio of 1:2, press and seal it and place it at 4 °C In the refrigerator, equilibrated for 24 h. Freeze at 18°C (0, 15, 30 and 60 days). Add 0.5%, 1%, 2% (w/w) HPMC to replace the corresponding quality of starch, and other preparation methods remain unchanged. After the freezing storage time is over, take out the crucible and equilibrate at 4 °C for 4 h.
(3)Determination of gelatinization temperature and enthalpy change
Taking the blank crucible as a reference, the nitrogen flow rate was 50 mL/min, equilibrated at 20 °C for 5 min, and then heated to 100 °C at 5 °C/min. Finally, the heat flow (Heat Flow, mW) is the DSC curve of the ordinate, and the gelatinization peak was integrated and analyzed by Universal Analysis 2000. Each sample was repeated at least three times.
4.2.3.5 XRD measurement
The thawed frozen starch samples were dried in an oven at 40 °C for 48 h, then ground and sieved through a 100-mesh sieve to obtain starch powder samples. Take a certain amount of the above samples, use D/MAX 2500V type X. The crystal form and relative crystallinity were determined by X-ray diffractometer. The experimental parameters are voltage 40 KV, current 40 mA, using Cu. Ks as X. ray source. At room temperature, the scanning angle range is 30--400, and the scanning rate is 20/min. Relative crystallinity (%) = crystallization peak area/total area x 100%, where the total area is the sum of the background area and the peak integral area [1 62].
4.2.3.6 Determination of starch swelling power
Take 0.1 g of the dried, ground and sieved amyloid into a 50 mL centrifuge tube, add 10 mL of distilled water to it, shake it well, let it stand for 0.5 h, and then place it in a 95°C water bath at a constant temperature. After 30 min, after gelatinization is complete, take out the centrifuge tube and place it in an ice bath for 10 min for rapid cooling. Finally, centrifuge at 5000 rpm for 20 min, and pour off the supernatant to obtain a precipitate. Swelling Power=precipitation mass/sample mass [163].
4.2.3.7 Data analysis and processing
All experiments were repeated at least three times unless otherwise specified, and the experimental results were expressed as mean and standard deviation. SPSS Statistic 19 was used for analysis of variance (Analysis of Variance, ANOVA) with a significance level of 0.05; correlation charts were drawn using Origin 8.0.
4.3 Analysis and Discussion
4.3.1 Content of basic components of wheat starch
According to GB 50093.2010, GB/T 5009.9.2008, GB 50094.2010 (78-s0), the basic components of wheat starch - moisture, amylose/amylopectin and ash content were determined. The results are shown in Table 4. 1 shown.
Tap 4．1 Content of constituent of wheat starch

4.3.2 Effects of HPMC addition amount and frozen storage time on the gelatinization characteristics of wheat starch
The starch suspension with a certain concentration is heated at a certain heating rate to make the starch gelatinized. After starting to gelatinize, the turbid liquid gradually becomes pasty due to the expansion of starch, and the viscosity increases continuously. Subsequently, the starch granules rupture and the viscosity decreases. When the paste is cooled at a certain cooling rate, the paste will gel, and the viscosity value will further increase. The viscosity value when it is cooled to 50 °C is the final viscosity value (Figure 4.1).
Table 4.2 lists the influence of several important indicators of starch gelatinization characteristics, including gelatinization peak viscosity, minimum viscosity, final viscosity, decay value and appreciation value, and reflects the effect of HPMC addition and freezing time on starch paste. effects of chemical properties. The experimental results show that the peak viscosity, the minimum viscosity and the final viscosity of starch without frozen storage increased significantly with the increase of HPMC addition, while the decay value and recovery value decreased significantly. Specifically, the peak viscosity gradually increased from 727.66+90.70 CP (without adding HPMC) to 758.51+48.12 CP (adding 0.5% HPMC), 809.754-56.59 CP (adding 1 %HPMC), and 946.64+9.63 CP (adding 2% HPMC); the minimum viscosity was increased from 391.02+18.97 CP (blank not adding) to 454.95+36.90 (adding O .5% HPMC), 485.56+54.0.5 (add 1% HPMC) and 553.03+55.57 CP (add 2% HPMC); the final viscosity is from 794.62.412.84 CP ( Without adding HPMC) increased to 882.24±22.40 CP (adding 0.5% HPMC), 846.04+12.66 CP (adding 1% HPMC) and 910.884-34.57 CP (adding 2 %HPMC); however, the attenuation value gradually decreased from 336.644-71.73 CP (without adding HPMC) to 303.564-11.22 CP (adding 0.5% HPMC), 324.19±2.54 CP (Add
With 1% HPMC) and 393.614-45.94 CP (with 2% HPMC), the retrogradation value decreased from 403.60+6.13 CP (without HPMC) to 427.29+14.50 CP, respectively (0.5% HPMC added), 360.484-41.39 CP (15 HPMC added) and 357.85+21.00 CP (2% HPMC added). This and the addition of hydrocolloids such as xanthan gum and guar gum obtained by Achayuthakan & Suphantharika (2008) and Huang (2009) can increase the gelatinization viscosity of starch while reducing the retrogradation value of starch. This may be mainly because HPMC acts as a kind of hydrophilic colloid, and the addition of HPMC increases the gelatinization peak viscosity due to the hydrophilic group on its side chain which makes it more hydrophilic than starch granules at room temperature. In addition, the temperature range of the thermal gelatinization process (thermogelation process) of HPMC is larger than that of starch (results not shown), so that the addition of HPMC can effectively suppress the drastic decrease in viscosity due to the disintegration of starch granules. Therefore, the minimum viscosity and final viscosity of starch gelatinization increased gradually with the increase of HPMC content.
On the other hand, when the amount of HPMC added was the same, the peak viscosity, minimum viscosity, final viscosity, decay value and retrogradation value of starch gelatinization increased significantly with the extension of freezing storage time. Specifically, the peak viscosity of starch suspension without adding HPMC increased from 727.66±90.70 CP (frozen storage for 0 days) to 1584.44+68.11 CP (frozen storage for 60 days); adding 0.5 The peak viscosity of starch suspension with %HPMC increased from 758.514-48.12 CP (freezing for 0 days) to 1415.834-45.77 CP (freezing for 60 days); starch suspension with 1% HPMC added The peak viscosity of the starch liquid increased from 809.754-56.59 CP (freeze storage for 0 days) to 1298.19-±78.13 CP (frozen storage for 60 days); while the starch suspension with 2% HPMC CP added Gelatinization peak viscosity from 946.64 ± 9.63 CP (0 days frozen) increased to 1240.224-94.06 CP (60 days frozen). At the same time, the lowest viscosity of starch suspension without HPMC was increased from 391.02-41 8.97 CP (freezing for 0 days) to 556.77±29.39 CP (freezing for 60 days); adding 0.5 The minimum viscosity of the starch suspension with %HPMC increased from 454.954-36.90 CP (freezing for 0 days) to 581.934-72.22 CP (freezing for 60 days); the starch suspension with 1% HPMC added The minimum viscosity of the liquid increased from 485.564-54.05 CP (freezing for 0 days) to 625.484-67.17 CP (freezing for 60 days); while the starch suspension added 2% HPMC CP gelatinized The lowest viscosity increased from 553.034-55.57 CP (0 days frozen) to 682.58 ± 20.29 CP (60 days frozen).

The final viscosity of starch suspension without adding HPMC increased from 794.62 ± 12.84 CP (frozen storage for 0 days) to 1413.15 ± 45.59 CP (frozen storage for 60 days). The peak viscosity of starch suspension increased from 882.24 ± 22.40 CP (frozen storage for 0 days) to 1322.86 ± 36.23 CP (frozen storage for 60 days); the peak viscosity of starch suspension added with 1% HPMC The viscosity increased from 846.04 ± 12.66 CP (frozen storage 0 days) to 1291.94 ± 88.57 CP (frozen storage for 60 days); and the gelatinization peak viscosity of starch suspension added with 2% HPMC increased from 91 0.88 ± 34.57 CP
(Frozen storage for 0 days) increased to 1198.09 ± 41.15 CP (frozen storage for 60 days). Correspondingly, the attenuation value of starch suspension without adding HPMC increased from 336.64 ± 71.73 CP (frozen storage for 0 days) to 1027.67 ± 38.72 CP (frozen storage for 60 days); adding 0.5 The attenuation value of starch suspension with %HPMC increased from 303.56±11.22 CP (frozen storage for 0 days) to 833.9±26.45 CP (frozen storage for 60 days); starch suspension with 1% HPMC added The attenuation value of the liquid was increased from 324.19 ± 2.54 CP (freezing for 0 days) to 672.71 ± 10.96 CP (freezing for 60 days); while adding 2% HPMC，the attenuation value of the starch suspension increased from 393.61 ± 45.94 CP (freezing for 0 days) to 557.64 ± 73.77 CP (freezing for 60 days); while the starch suspension without HPMC added The retrogradation value increased from 403.60 ± 6.13 C
P (frozen storage for 0 days) to 856.38 ± 16.20 CP (frozen storage for 60 days); the retrogradation value of starch suspension added with 0.5% HPMC increased from 427 .29±14.50 CP (frozen storage for 0 days) increased to 740.93±35.99 CP (frozen storage for 60 days); the retrogradation value of starch suspension added with 1% HPMC increased from 360.48±41. 39 CP (frozen storage for 0 days) increased to 666.46 ± 21.40 CP (frozen storage for 60 days); while the retrogradation value of starch suspension added with 2% HPMC increased from 357.85 ± 21.00 CP (frozen storage for 60 days). 0 days) increased to 515.51 ± 20.86 CP (60 days frozen).
It can be seen that with the prolongation of freezing storage time, the starch gelatinization characteristics index increased, which is consistent with Tao et a1. f2015) 1. Consistent with the experimental results, they found that with the increase of the number of freeze-thaw cycles, the peak viscosity, minimum viscosity, final viscosity, decay value, and retrogradation value of starch gelatinization all increased to different degrees [166J]. This is mainly because in the process of freezing storage, the amorphous region (Amorphous Region) of starch granules is destroyed by ice crystallization, so that the amylose (the main component) in the amorphous region (non-crystalline region) undergoes phase separation (Phase. separated) phenomenon, and dispersed in the starch suspension, resulting in an increase in the viscosity of starch gelatinization, and an increase in the related attenuation value and retrogradation value. However, the addition of HPMC inhibited the effect of ice crystallization on starch structure. Therefore, the peak viscosity, minimum viscosity, final viscosity, decay value and retrogradation rate of starch gelatinization increased with the addition of HPMC during frozen storage. increase and decrease sequentially.

Fig 4．1 Pasting curves of wheat starch without HPMC(A)or with 2％HPMC①)
4.3.3 Effects of HPMC addition amount and frozen storage time on the shear viscosity of starch paste
The effect of shear rate on the apparent viscosity (shear viscosity) of the fluid was investigated by the Steady Flow Test, and the material structure and properties of the fluid were reflected accordingly. Table 4.3 lists the equation parameters obtained by nonlinear fitting, that is, the consistency coefficient K and the flow characteristic index D, as well as the influence of the addition amount of HPMC and the freezing storage time on the above parameters K gate.

Fig 4．2 Thixotropism of starch paste without HPMC(A)or with 2％HPMC(B)

It can be seen from Table 4.3 that all the flow characteristic indices, 2, are less than 1. Therefore, starch paste (whether HPMC is added or whether it is frozen or not) belongs to Pseudoplastic Fluid, and all show shearing Thinning phenomenon (as the shear rate increases, the shear viscosity of the fluid decreases). In addition, the shear rate scans ranged from 0.1 s, respectively. 1 increased to 100 s ~, and then decreased from 100 s d to O. The rheological curves obtained at 1 sd do not completely overlap, and the fitting results of K, s are also different, so the starch paste is a thixotropic pseudoplastic fluid (whether HPMC is added or whether it is frozen or not). However, under the same freezing storage time, with the increase of HPMC addition, the difference between the fitting results of the K n values ​​of the two scans gradually decreased, which indicates that the addition of HPMC makes the structure of starch paste under shear stress. It remains relatively stable under the action and reduces the "thixotropic ring"
(Thixotropic Loop) area, which is similar to Temsiripong, et a1. (2005) reported the same conclusion [167]. This may be mainly because HPMC can form intermolecular cross-links with gelatinized starch chains (mainly amylose chains), which "bound" the separation of amylose and amylopectin under the action of shearing force. , so as to maintain the relative stability and uniformity of the structure (Figure 4.2, the curve with shear rate as abscissa and shear stress as ordinate).
On the other hand, for the starch without frozen storage, its K value decreased significantly with the addition of HPMC, from 78.240±1.661 Pa ·s n (without adding HPMC) to 65.240±1.661 Pa ·s n (without adding HPMC), respectively. 683±1.035 Pa ·s n (add 0.5% Hand MC), 43.122±1.047 Pa ·s n (add 1% HPMC), and 13.926±0.330Pa·Sn (add 2% HPMC), while the n value increased significantly, from 0.277 ± 0.011 (without adding HPMC) to 0.277 ± 0.011 in turn. 310 ± 0.009 (add 0.5% HPMC), O. 323 ± 0.013 (add 1% HPMC) and O. 43 1 ± 0.0 1 3 (adding 2% HPMC), which is similar to the experimental results of Techawipharat, Suphantharika, & BeMiller (2008) and Turabi, Sumnu, & Sahin (2008), and the increase of n value shows that the addition of HPMC makes the fluid has a tendency to change from pseudoplastic to Newtonian [168'1691]. at the same time, For the starch stored frozen for 60 days, the K, n values showed the same change rule with the increase of HPMC addition.
However, with the prolongation of freezing storage time, the values ​​of K and n increased to different degrees, among which the value of K increased from 78.240 ± 1.661 Pa·s n (unadded, 0 days) to 95.570 ± 1, respectively. 2.421 Pa·s n (no addition, 60 days), increased from 65.683±1.035 Pa ·S n (addition of O. 5% HPMC, 0 days) to 51.384±1.350 Pa ·S n (Add to 0.5% HPMC, 60 days), increased from 43.122±1.047 Pa ·s n (adding 1% HPMC, 0 days) to 56.538±1.378 Pa ·s n (adding 1% HPMC, 60 days) ), and increased from 13.926 ± 0.330 Pa ·s n (adding 2% HPMC, 0 days) to 16.064 ± 0.465 Pa ·s n (adding 2% HPMC, 60 days); 0.277 ± 0.011 (without adding HPMC, 0 days) rose to O. 334±0.014 (no addition, 60 days), increased from 0.310±0.009 (0.5% HPMC added, 0 day) to 0.336±0.014 (0.5% HPMC added, 60 days), from 0.323 ± 0.013 (add 1% HPMC, 0 days) to 0.340 ± 0.013 (add 1% HPMC, 60 days), and from 0.431 ± 0.013 (add 1% HPMC, 60 days) 2% HPMC, 0 days) to 0.404+0.020 (add 2% HPMC, 60 days). By comparison, it can be found that with the increase of the addition amount of HPMC, the change rate of K and Knife value decreases successively, which shows that the addition of HPMC can make the starch paste stable under the action of shearing force, which is consistent with the measurement results of starch gelatinization characteristics. consistent.
4.3.4 Effects of HPMC addition amount and frozen storage time on dynamic viscoelasticity of starch paste
The dynamic frequency sweep can effectively reflect the viscoelasticity of the material, and for starch paste, this can be used to characterize its gel strength (Gel Strength). Figure 4.3 shows the changes of storage modulus/elastic modulus (G') and loss modulus/viscosity modulus (G") of starch gel under the conditions of different HPMC addition and freezing time.

Fig 4．3 Effect of HPMC addition and frozen storage on elastic and viscous modulus of starch paste
Note: A is the change of viscoelasticity of unadded HPMC starch with the extension of freezing storage time; B is the addition of O. The change of viscoelasticity of 5% HPMC starch with the extension of freezing storage time; C is the change of the viscoelasticity of 1% HPMC starch with the extension of freezing storage time; D is the change of the viscoelasticity of 2% HPMC starch with the extension of freezing storage time
The starch gelatinization process is accompanied by the disintegration of starch granules, the disappearance of the crystalline region, and the hydrogen bonding between starch chains and moisture, the starch gelatinized to form a heat-induced (Heat. induced) gel with a certain gel strength. As shown in Figure 4.3, for starch without frozen storage, with the increase of HPMC addition, the G' of starch decreased significantly, while G" had no significant difference, and tan 6 increased (Liquid. 1ike), which shows that during the gelatinization process, HPMC interacts with starch, and due to the water retention of HPMC, the addition of HPMC reduces the water loss of starch during the gelatinization process. At the same time, Chaisawang & Suphantharika (2005) found that, adding guar gum and xanthan gum to tapioca starch, the G' of the starch paste also decreased [170]. In addition, with the extension of the freezing storage time, the G' of starch gelatinized decreased to different degrees. This is mainly because during the frozen storage process of starch, the amylose in the amorphous region of starch granules is separated to form damaged starch (Damaged Starch), which reduces the degree of intermolecular cross-linking after starch gelatinization and the degree of cross-linking after cross-linking. Stability and compactness, and the physical extrusion of ice crystals makes the arrangement of "micelles" (microcrystalline structures, mainly composed of amylopectin) in the starch crystallization area more compact, increasing the relative crystallinity of starch, and at the same time , resulting in insufficient combination of molecular chain and water after starch gelatinization, low extension of molecular chain (molecular chain mobility), and finally caused the gel strength of starch to decline. However, with the increase of HPMC addition, the decreasing trend of G' was suppressed, and this effect was positively correlated with the addition of HPMC. This indicated that the addition of HPMC could effectively inhibit the effect of ice crystals on the structure and properties of starch under frozen storage conditions.
4.3.5 Effects of I-IPMC addition amount and frozen storage time on starch swelling ability
The swelling ratio of starch can reflect the size of starch gelatinization and water swelling, and the stability of starch paste under centrifugal conditions. As shown in Figure 4.4, for starch without frozen storage, with the increase of HPMC addition, the swelling force of starch increased from 8.969+0.099 (without adding HPMC) to 9.282- -L0.069 (adding 2% HPMC), which shows that the addition of HPMC increases the swelling water absorption and makes starch more stable after gelatinization, which is consistent with the conclusion of starch gelatinization characteristics. However, with the extension of frozen storage time, the swelling power of starch decreased. Compared with 0 days of frozen storage, the swelling power of starch decreased from 8.969-a:0.099 to 7.057+0 after frozen storage for 60 days, respectively. .007 (no HPMC added), reduced from 9.007+0.147 to 7.269-4-0.038 (with O.5% HPMC added), reduced from 9.284+0.157 to 7.777 +0.014 (adding 1% HPMC), reduced from 9.282+0.069 to 8.064+0.004 (adding 2% HPMC). The results showed that the starch granules were damaged after freezing storage, resulting in the precipitation of part of the soluble starch and centrifugation. Therefore, the solubility of starch increased and the swelling power decreased. In addition, after freezing storage, starch gelatinized starch paste, its stability and water holding capacity decreased, and the combined action of the two reduced the swelling power of starch [1711]. On the other hand, with the increase of HPMC addition, the decline of starch swelling power gradually decreased, indicating that HPMC can reduce the amount of damaged starch formed during freezing storage and inhibit the degree of starch granule damage.

Fig 4．4 Effect of HPMC addition and frozen storage on swelling power of starch
4.3.6 Effects of HPMC addition amount and frozen storage time on the thermodynamic properties of starch
The gelatinization of starch is an endothermic chemical thermodynamic process. Therefore, DSC is often used to determine the onset temperature (Dead), peak temperature (To), end temperature (T p), and gelatinization enthalpy of starch gelatinization. (Tc). Table 4.4 shows the DSC curves of starch gelatinization with 2% and without HPMC added for different freezing storage times.

Fig 4．5 Effect of HPMC addition and frozen storage on thermal properties of wheat starch pasting
Note: A is the DSC curve of starch without adding HPMC and frozen for 0, 15, 30 and 60 days: B is the DSC curve of starch with 2% HPMC added and frozen for 0, 15, 30 and 60 days

As shown in Table 4.4, for fresh amyloid, with the increase of HPMC addition, starch L has no significant difference, but increases significantly, from 77.530 ± 0.028 (without adding HPMC) to 78.010 ± 0.042 (add 0.5% HPMC), 78.507 ± 0.051 (add 1% HPMC), and 78.606 ± 0.034 (add 2% HPMC), but 4H is significant Decrease, from 9.450 ± 0.095 (without adding HPMC) to 8.53 ± 0.030 (adding 0.5% HPMC), 8.242A: 0.080 (adding 1% HPMC) and 7 .736 ± 0.066 (add 2% HPMC). This is similar to Zhou, et a1. (2008) found that adding a hydrophilic colloid decreased the starch gelatinization enthalpy and increased the starch gelatinization peak temperature [172]. This is mainly because HPMC has better hydrophilicity and is easier to combine with water than starch. At the same time, due to the large temperature range of the thermally accelerated gelation process of HPMC, the addition of HPMC increases the peak gelatinization temperature of starch, while the gelatinization Enthalpy decreases.
On the other hand, starch gelatinization To, T p, Tc, △T and △Hall increased with the extension of freezing time. Specifically, starch gelatinization with 1% or 2% HPMC added had no significant difference after freezing for 60 days, while starch without or with 0.5% HPMC was added from 68.955±0.01 7 (frozen storage for 0 days) increased to 72.340 ± 0.093 (frozen storage for 60 days), and from 69.170 ± 0.035 (frozen storage for 0 days) to 71.613 ± 0.085 (frozen storage for 0 days) 60 days); after 60 days of frozen storage, the growth rate of starch gelatinization decreased with the increase of HPMC addition, such as starch without HPMC added from 77.530 ± 0.028 (frozen storage for 0 days) to 81.028. 408 ± 0.021 (frozen storage for 60 days), while the starch added with 2% HPMC increased from 78.606 ± 0.034 (frozen storage for 0 days) to 80.017 ± 0.032 (frozen storage for 60 days). days); in addition, ΔH also showed the same change rule, which increased from 9.450 ± 0.095 (no addition, 0 days) to 12.730 ± 0.070 (no addition, 60 days), respectively, from 8.450 ± 0.095 (no addition, 0 days) to 12.730 ± 0.070 (no addition, 60 days), respectively. 531 ± 0.030 (add 0.5%, 0 days) to 11.643 ± 0.019 (add 0.5%, 60 days), from 8.242 ± 0.080 (add 1%, 0 days) to 10.509 ± 0.029 (add 1%, 60 days), and from 7.736 ± O. 066 (2% addition, 0 days) rose to 9.450 ± 0.093 (2% addition, 60 days). The main reasons for the above-mentioned changes in the thermodynamic properties of starch gelatinization during the frozen storage process are the formation of damaged starch, which destroys the amorphous region (amorphous region) and increases the crystallinity of the crystalline region. The coexistence of the two increases the relative crystallinity of starch, which in turn leads to an increase in thermodynamic indexes such as starch gelatinization peak temperature and gelatinization enthalpy. However, through comparison, it can be found that under the same freezing storage time, with the increase of HPMC addition, the increase of starch gelatinization To, T p, Tc, ΔT and ΔH gradually decreases. It can be seen that the addition of HPMC can effectively maintain the relative stability of the starch crystal structure, thereby inhibiting the increase of the thermodynamic properties of starch gelatinization.
4.3.7 Effects of I-IPMC addition and freezing storage time on the relative crystallinity of starch
X. X-ray diffraction (XRD) is obtained by X. X-ray diffraction is a research method that analyzes the diffraction spectrum to obtain information such as the composition of the material, the structure or morphology of the atoms or molecules in the material. Because starch granules have a typical crystalline structure, XRD is often used to analyze and determine the crystallographic form and relative crystallinity of starch crystals.
Figure 4.6. As shown in A, the positions of the starch crystallization peaks are located at 170, 180, 190 and 230, respectively, and there is no significant change in the peak positions regardless of whether they are treated by freezing or adding HPMC. This shows that, as an intrinsic property of wheat starch crystallization, the crystalline form remains stable.
However, with the prolongation of freezing storage time, the relative crystallinity of starch increased from 20.40 + 0.14 (without HPMC, 0 days) to 36.50 ± 0.42 (without HPMC, frozen storage, respectively). 60 days), and increased from 25.75 + 0.21 (2% HPMC added, 0 days) to 32.70 ± 0.14 (2% HPMC added, 60 days) (Figure 4.6.B), this and Tao, et a1. (2016), the change rules of the measurement results are consistent [173-174]. The increase in relative crystallinity is mainly caused by the destruction of the amorphous region and the increase in the crystallinity of the crystalline region. In addition, consistent with the conclusion of the changes in the thermodynamic properties of starch gelatinization, the addition of HPMC reduced the degree of relative crystallinity increase, which indicated that during the freezing process, HPMC could effectively inhibit the structural damage of starch by ice crystals and maintain the Its structure and properties are relatively stable.

Fig 4．6 Effect of HPMC addition and frozen storage on XRD properties
Note: A is x. X-ray diffraction pattern; B is the relative crystallinity result of starch;
4.4 Chapter Summary
Starch is the most abundant dry matter in dough, which, after gelatinization, adds unique qualities (specific volume, texture, sensory, flavor, etc.) to the dough product. Since the change of starch structure will affect its gelatinization characteristics, which will also affect the quality of flour products, in this experiment, the gelatinization characteristics, flowability and flowability of starch after frozen storage were investigated by examining starch suspensions with different contents of HPMC added. Changes in rheological properties, thermodynamic properties and crystal structure were used to evaluate the protective effect of HPMC addition on starch granule structure and related properties. The experimental results showed that after 60 days of frozen storage, the starch gelatinization characteristics (peak viscosity, minimum viscosity, final viscosity, decay value and retrogradation value) all increased due to the significant increase in the relative crystallinity of starch and the increase in the content of damaged starch. The gelatinization enthalpy increased, while the gel strength of starch paste decreased significantly; however, especially the starch suspension added with 2% HPMC, the relative crystallinity increase and starch damage degree after freezing were lower than those in the control group Therefore, the addition of HPMC reduces the degree of changes in gelatinization characteristics, gelatinization enthalpy, and gel strength, which indicates that the addition of HPMC keeps the starch structure and its gelatinization properties relatively stable.
Chapter 5 Effects of HPMC Addition on Yeast Survival Rate and Fermentation Activity under Frozen Storage Conditions
5.1 Introduction
Yeast is a unicellular eukaryotic microorganism, its cell structure includes cell wall, cell membrane, mitochondria, etc., and its nutritional type is a facultative anaerobic microorganism. Under anaerobic conditions, it produces alcohol and energy, while under aerobic conditions it metabolizes to produce carbon dioxide, water and energy.
Yeast has a wide range of applications in fermented flour products (sourdough is obtained by natural fermentation, mainly lactic acid bacteria), it can use the hydrolyzed product of starch in the dough - glucose or maltose as a carbon source, under aerobic conditions, using Substances produce carbon dioxide and water after respiration. The carbon dioxide produced can make the dough loose, porous and bulky. At the same time, the fermentation of yeast and its role as an edible strain can not only improve the nutritional value of the product, but also significantly improve the flavor characteristics of the product. Therefore, the survival rate and fermentation activity of yeast have an important impact on the quality of the final product (specific volume, texture, and flavor, etc.) [175].
In the case of frozen storage, yeast will be affected by environmental stress and affect its viability. When the freezing rate is too high, the water in the system will rapidly crystallize and increase the external osmotic pressure of the yeast, thereby causing the cells to lose water; when the freezing rate is too high. If it is too low, the ice crystals will be too large and the yeast will be squeezed and the cell wall will be damaged; both will reduce the survival rate of the yeast and its fermentation activity. In addition, many studies have found that after the yeast cells are ruptured due to freezing, they will release a reducing substance-reduced glutathione, which in turn reduces the disulfide bond to a sulfhydryl group, which will eventually destroy the network structure of gluten protein, resulting in a decrease in the quality of pasta products [176-177].
Because HPMC has strong water retention and water holding capacity, adding it to the dough system can inhibit the formation and growth of ice crystals. In this experiment, different amounts of HPMC were added to the dough, and after a certain period of time after frozen storage, the quantity of yeast, fermentation activity and glutathione content in unit mass of dough were determined to evaluate the protective effect of HPMC on yeast under freezing conditions.
5.2 Materials and methods
5.2.1 Experimental materials and instruments
Materials and Instruments
Angel Active Dry Yeast
BPS. 500CL constant temperature and humidity box
3M solid film colony rapid count test piece
SP. Model 754 UV Spectrophotometer
Ultra-clean sterile operating table
KDC. 160HR high-speed refrigerated centrifuge
ZWY-240 constant temperature incubator
BDS. 200 Inverted Biological Microscope

Manufacturer
Angel Yeast Co., Ltd.
Shanghai Yiheng Scientific Instrument Co., Ltd.
3M Corporation of America
Shanghai Spectrum Scientific Instrument Co., Ltd.
Jiangsu Tongjing Purification Equipment Co., Ltd.
Anhui Zhongke Zhongjia Scientific Instrument Co., Ltd.
Shanghai Zhicheng Analytical Instrument Manufacturing Co., Ltd.
Chongqing Auto Optical Instrument Co., Ltd.
5.2.2 Experimental method
5.2.2.1 Preparation of yeast liquid
Weigh 3 g of active dry yeast, add it to a sterilized 50 mL centrifuge tube under aseptic conditions, and then add 27 mL of 9% (w/V) sterile saline to it, shake it up, and prepare 10% (w/w) yeast broth. Then, quickly move to. Store in a refrigerator at 18°C. After 15 d, 30 d, and 60 d of frozen storage, the samples were taken out for testing. Add 0.5%, 1%, 2% HPMC (w/w) to replace the corresponding percentage of active dry yeast mass. In particular, after the HPMC is weighed, it must be irradiated under an ultraviolet lamp for 30 minutes for sterilization and disinfection.
5.2.2.2 Dough proofing height
See Meziani, et a1. (2012)'s experimental method [17 cited, with slight modifications. Weigh 5 g of frozen dough into a 50 mL colorimetric tube, press the dough to a uniform height of 1.5 cm at the bottom of the tube, then place it upright in a constant temperature and humidity box, and incubate for 1 h at 30 °C and 85% RH, after taking it out, measure the proofing height of the dough with a millimeter ruler (retain two digits after the decimal point). For samples with uneven upper ends after proofing, select 3 or 4 points at equal intervals to measure their corresponding heights (for example, each 900), and the measured height values were averaged. Each sample was paralleled three times.
5.2.2.3 CFU (colony-forming units) count
Weigh 1 g of dough, add it to a test tube with 9 mL of sterile normal saline according to the requirements of the aseptic operation, shake it fully, record the concentration gradient as 101, and then dilute it into a series of concentration gradients until 10'1. Draw 1 mL of dilution from each of the above tubes, add it to the center of the 3M yeast rapid count test piece (with strain selectivity), and place the above test piece in a 25°C incubator according to the operating requirements and culture conditions specified by 3M. 5 d, take out after the end of the culture, first observe the colony morphology to determine whether it conforms to the colony characteristics of yeast, and then count and microscopically examine [179]. Each sample was repeated three times.
5.2.2.4 Determination of glutathione content
The alloxan method was used to determine the glutathione content. The principle is that the reaction product of glutathione and alloxan has an absorption peak at 305 nl. Specific determination method: pipette 5 mL of yeast solution into a 10 mL centrifuge tube, then centrifuge at 3000 rpm for 10 min, take 1 mL of supernatant into a 10 mL centrifuge tube, add 1 mL of 0.1 mol/mL to the tube L alloxan solution, mixed thoroughly, then add 0.2 M PBS (pH 7.5) and 1 mL of 0.1 M, NaOH solution to it, mix well, let stand for 6 min, and immediately add 1 M, NaOH The solution was 1 mL, and the absorbance at 305 nm was measured with a UV spectrophotometer after thorough mixing. The glutathione content was calculated from the standard curve. Each sample was paralleled three times.
5.2.2.5 Data processing
Experimental results are presented as 4-standard deviation of the mean, and each experiment was repeated at least three times. Analysis of variance was performed using SPSS, and the significance level was 0.05. Use Origin to draw graphs.
5.3 Results and Discussion
5.3.1 Influence of HPMC addition amount and frozen storage time on dough proofing height
The proofing height of dough is often affected by the combined effect of yeast fermentation gas production activity and dough network structure strength. Among them, yeast fermentation activity will directly affect its ability to ferment and produce gas, and the amount of yeast gas production determines the quality of fermented flour products, including specific volume and texture. The fermentation activity of yeast is mainly affected by external factors (such as changes in nutrients such as carbon and nitrogen sources, temperature, pH, etc.) and internal factors (growth cycle, activity of metabolic enzyme systems, etc.).

Fig 5．1 Effect of HPMC addition and frozen storage on height of dough proofing
As shown in Figure 5.1, when frozen for 0 days, with the increase in the amount of HPMC added, the proofing height of the dough increased from 4.234-0.11 cm to 4.274 cm without adding HPMC. -0.12 cm (0.5% HPMC added), 4.314-0.19 cm (1% HPMC added), and 4.594-0.17 cm (2% HPMC added) This may be mainly due to HPMC Addition changes the properties of the dough network structure (see Chapter 2). However, after being frozen for 60 days, the proofing height of the dough decreased to varying degrees. Specifically, the proofing height of the dough without HPMC was reduced from 4.234-0.11 cm (freezing for 0 days) to 3 .18+0.15 cm (frozen storage for 60 days); the dough added with 0.5% HPMC was reduced from 4.27+0.12 cm (frozen storage for 0 days) to 3.424-0.22 cm (frozen storage for 0 days). 60 days); the dough added with 1% HPMC decreased from 4.314-0.19 cm (frozen storage for 0 days) to 3.774-0.12 cm (frozen storage for 60 days); while the dough added with 2% HPMC woke up. The hair height was reduced from 4.594-0.17 cm (frozen storage for 0 days) to 4.09-±0.16 cm (frozen storage for 60 days). It can be seen that with the increase of the addition amount of HPMC, the degree of decrease in the proofing height of the dough gradually decreases. This shows that under the condition of frozen storage, HPMC can not only maintain the relative stability of the dough network structure, but also better protect the survival rate of yeast and its fermentation gas production activity, thereby reducing the quality deterioration of fermented noodles.
5.3.2 Effect of I-IPMC addition and freezing time on yeast survival rate
In the case of frozen storage, since the frozen water in the dough system is converted into ice crystals, the osmotic pressure outside the yeast cells is increased, so that the protoplasts and cell structures of the yeast are under a certain degree of stress. When the temperature is lowered or kept at low temperature for a long time, a small amount of ice crystals will appear in the yeast cells, which will lead to the destruction of the cell structure of the yeast, the extravasation of the cell fluid, such as the release of the reducing substance - glutathione, or even complete death; at the same time, the yeast Under environmental stress, its own metabolic activity will be reduced, and some spores will be produced, which will reduce the fermentation gas production activity of yeast.

Fig 5．2 Effect of HPMC addition and frozen storage on survival rate of yeast
It can be seen from Figure 5.2 that there is no significant difference in the number of yeast colonies in samples with different contents of HPMC added without freezing treatment. This is similar to the result determined by Heitmann, Zannini, & Arendt (2015) [180]. However, after 60 days of freezing, the number of yeast colonies decreased significantly, from 3.08x106 CFU to 1.76x106 CFU (without adding HPMC); from 3.04x106 CFU to 193x106 CFU (adding 0.5% HPMC); reduced from 3.12x106 CFU to 2.14x106 CFU (added 1% HPMC); reduced from 3.02x106 CFU to 2.55x106 CFU (added 2% HPMC). By comparison, it can be found that the freezing storage environment stress led to the decrease of the yeast colony number, but with the increase of HPMC addition, the degree of the decrease of the colony number decreased in turn. This indicates that HPMC can better protect yeast under freezing conditions. The mechanism of protection may be the same as that of glycerol, a commonly used strain antifreeze, mainly by inhibiting the formation and growth of ice crystals and reducing the stress of low temperature environment to yeast. Figure 5.3 is the photomicrograph taken from the 3M yeast rapid counting test piece after preparation and microscopic examination, which is in line with the external morphology of yeast.

Fig 5．3 Micrograph of yeasts
5.3.3 Effects of HPMC addition and freezing time on glutathione content in dough
Glutathione is a tripeptide compound composed of glutamic acid, cysteine and glycine, and has two types: reduced and oxidized. When the yeast cell structure is destroyed and died, the permeability of the cells increases, and the intracellular glutathione is released to the outside of the cell, and it is reductive. It is particularly worth noting that reduced glutathione will reduce the disulfide bonds (-S-S-) formed by the cross-linking of gluten proteins, breaking them to form free sulfhydryl groups (.SH), which in turn affects the dough network structure. stability and integrity, and ultimately lead to the deterioration of the quality of fermented flour products. Usually, under environmental stress (such as low temperature, high temperature, high osmotic pressure, etc.), yeast will reduce its own metabolic activity and increase its stress resistance, or produce spores at the same time. When the environmental conditions are suitable for its growth and reproduction again, then restore the metabolism and proliferation vitality. However, some yeasts with poor stress resistance or strong metabolic activity will still die if they are kept in a frozen storage environment for a long time.

Fig 5．4 Effect of HPMC addition and frozen storage on the content of glutathione(GSH)
As shown in Figure 5.4, the glutathione content increased regardless of whether HPMC was added or not, and there was no significant difference between the different addition amounts. This may be because some of the active dry yeast used to make the dough have poor stress resistance and tolerance. Under the condition of low temperature freezing, the cells die, and then glutathione is released, which is only related to the characteristics of the yeast itself. It is related to the external environment, but has nothing to do with the amount of HPMC added. Therefore, the content of glutathione increased within 15 days of freezing and there was no significant difference between the two. However, with the further extension of the freezing time, the increase of glutathione content decreased with the increase of HPMC addition, and the glutathione content of the bacterial solution without HPMC was increased from 2.329a: 0.040mg/ g (frozen storage for 0 days) increased to 3.8514-0.051 mg/g (frozen storage for 60 days); while the yeast liquid added 2% HPMC, its glutathione content increased from 2.307+0 .058 mg/g (frozen storage for 0 days) rose to 3.351+0.051 mg/g (frozen storage for 60 days). This further indicated that HPMC could better protect yeast cells and reduce the death of yeast, thereby reducing the content of glutathione released to the outside of the cell. This is mainly because HPMC can reduce the number of ice crystals, thereby effectively reducing the stress of ice crystals to yeast and inhibiting the increase of extracellular release of glutathione.
5.4 Chapter Summary
Yeast is an indispensable and important component in fermented flour products, and its fermentation activity will directly affect the quality of the final product. In this experiment, the protective effect of HPMC on yeast in frozen dough system was evaluated by studying the effect of different HPMC additions on yeast fermentation activity, yeast survival number, and extracellular glutathione content in frozen dough. Through experiments, it was found that the addition of HPMC can better maintain the fermentation activity of the yeast, and reduce the degree of decline in the proofing height of the dough after 60 days of freezing, thus providing a guarantee for the specific volume of the final product; in addition, the addition of HPMC effectively The decrease of yeast survival number was inhibited and the increase rate of reduced glutathione content was reduced, thereby alleviating the damage of glutathione to dough network structure. This suggests that HPMC can protect yeast by inhibiting the formation and growth of ice crystals.