Authors: Yu Yue, Zhang Shikai, Wang Min, Wu Peng
Key words: frozen dough, influencing factors, improver
Abstract:As a semi-finished product, frozen dough can maintain dough performance and extend the shelf life of dough through frozen storage, and is widely used in food chain processing. This paper reviews the factors affecting the quality of frozen dough, discusses the difficulties in the current research on frozen dough in China, and aims to provide a reference for the development of frozen dough in China.
Frozen dough refers to the semi-finished product obtained by freezing technology in order to facilitate storage and transportation during the production of products. Semi-finished products are refrigerated, defrosted when they need to be used, and then the remaining production process is carried out.
Traditional noodle food processing generally has two modes: the first is on-site production and sales, which is difficult to achieve intensive operation, and the quality of the food produced is uneven, and it is difficult to achieve standardization; The second is to distribute the finished products to various regions for sale after centralized production and unified processing, but in the process of transportation and storage, it will affect the freshness and flavor of food, so that the quality of products will decline. The emergence of frozen dough, in the molding, awakening, baking and other links to add people cold chain process, can make the bread processing operation and the late baking worker g49% sequence separated, break through the constraints of the traditional production mode, can make the product to achieve large-scale, standardized production at the same time, but also to ensure the quality of the product.
Frozen dough has a strong advantage, which can reduce production costs and labor while ensuring quality. At present, it has been used in the baking industry at home and abroad, and has been rapidly developed in the world.
With the rapid development of China's food industry, the application of frozen dough in food processing has become a hot spot for research. This paper summarizes the research of domestic and foreign scholars on the quality of frozen dough and its influencing factors, analyzes the current problems in the theoretical research and application of frozen dough, and on this basis, the research direction of frozen dough in China is prospected, which provides a reference for better realizing its industrialization development.
1 Factors influencing the quality of frozen dough
Generally speaking, the quality of the dough in the frozen storage process will be affected by many factors, for example, the temperature fluctuation will lead to the destruction of the protein structure of the dough, and the quality of the dough product will also crack due to the loss of moisture, which will seriously affect the quality of the frozen dough product, so the study of the influencing factors of the quality of the frozen dough is crucial.
1.1 Flour strength
Wheat flour is the main component of frozen dough, and its function is to make the dough gas-holding and viscoelastic. The size of the gluten force of wheat flour is a key parameter for measuring the quality of the dough after freezing. Lu Jie et al. found that the fermentation vitality of high gluten dough is higher than that of low gluten dough. Huang Minsheng et al. found that the influence of wheat flour on the quality of the dough is far greater than the effect of protein content. The optimal flour gluten strength required for different products varies by 4%. According to the needs, in the production of Chinese pastry or bread, in order to obtain a higher gluten strength, in the production should be selected with good gluten strength of high gluten flour, at lower temperatures, can also reduce the damage of the opposite group air holding through a strong gluten network.
1.2 Starch
By scanning electron microscopy to observe the aging characteristics and network structure of frozen glutinous rice starch, it was found that the low temperature environment in the frozen storage process played a role in promoting the recrystallization of amylose, so that the amylopectin eventually formed an orderly crystal structure; Ye Xiaofeng et al. used liquid nitrogen environment quick-frozen dough to determine the content of thawed dough amylose, and on the 5th day, the amylose content decreased sharply, and the long-term freezing made the amylose content fluctuate; Zhang Hua et al. found that the frozen storage by directly extracting wheat starch for freezing was found. Wheat starch granules with damaged surface after freezing will increase solubility and viscosity, reduce the expansion force and gelatinization temperature, indicating that the morphological and physical and chemical properties of the frozen starch granules have changed significantly.
By analyzing the experimental results of different instruments and methods on different types of starch, it can be found that frozen storage has a certain impact on the basic composition of wheat starch microstructure, crystal structure, gelatinization characteristics and thermodynamic properties, and further affects the cooking characteristics and baking characteristics of wheat flour. In future research, we can use the synergy between starch and pectin to improve the hardness and elasticity of starch texture by using the compound system, and play a better protective role in the performance of frozen dough.
1.3 Protein
During the processing of dough, frozen storage can affect the secondary structure of gluten protein, wheat gluten and wheat gliadin, change its molecular mass, and ultimately affect the quality characteristics and product quality of frozen dough. Pan Zhili et al. studied the structure and functional characteristics of the gluten freezing and thawing cycle in the skin of dumplings, and found that the irregular curl and a- heli content in the secondary structure of wheat gluten were reduced, and the protein functional characteristics showed a decreasing trend. Zhao Yan: After experimental observation of the changes in the secondary structure of wheat protein in the four storage microenvironments, it was found that the prolongation of storage time would reduce the a-helix and β-turns of the secondary structure of the protein, increase the β-folding, and transform each other between the three; In the study of Li Xuehong et al., the dough was placed in the environment of -18 °C for frozen storage, and the changes of gliadin and gluten in wheat protein and their secondary structures were studied, and it was found that the a-helix and β-corners were reduced, the β-folding was increased, and the elasticity and viscosity of the opposite group were affected. In addition, the researchers also used high performance liquid phase molecular exclusion chromatography (SE-HPLC) to quantitatively analyze the molecular mass of gluten, found that the soluble protein content increased significantly after freezing, and further verified that wheat grain macromolecular protein (GMP) depolymerization occurs during freezing.
1.4 Moisture
Fluctuations in temperature during freezing can cause water to recrystallize, destroy the gluten protein structure, and degrade the quality of frozen dough. Frozen dough in the freezing process, with the extension of the freezing time, the surface moisture of the frozen dough dry consumption, the internal moisture slowly moved to the surface, resulting in the loss of moisture in the dough, with the extension of time, the moisture reduction makes the surface crack, affecting the quality and appearance of the frozen dough. When the temperature fluctuates, the ice crystals in the frozen dough continue to melt and recombine, and recrystallization occurs, and the ice crystals produced by the recrystallization are usually large, which will destroy the structure of the gluten network, thereby affecting the quality of the frozen dough and the finished product. Studies have found that extracellular polysaccharides can increase the moisture content of frozen gluten, delay the dehydration of gluten and the migration rate of bound water in the gluten matrix. In addition, adding sodium carboxymethyl cellulose with different degrees of substitution to the frozen dough can increase the content of bound water in the dough, and the highly substituted sodium carboxymethyl cellulose inhibits the formation of ice crystals better.
In the process of dough making, wheat protein needs the action of water to form gluten, and temperature fluctuations during freezing will cause water to form ice crystals, affecting the interaction between water and protein, causing damage to the fermentation characteristics of the dough and the network structure of gluten, resulting in a decrease in the quality of the finished product. In future studies, we should innovate refrigeration technology, properly control the temperature during frozen storage, strengthen the constant temperature effect of the freezer, reduce temperature fluctuations, weaken the migration of water in the dough, and improve the quality of freezing.
1.5 Yeast
The yeast fermentation process produces CO2 to inflate the dough, and the other products produced give the bread a rich aroma and good color. When refrigerating, the moisture in the yeast cells will form ice crystals, if the temperature fluctuation is larger, it will lead to recrystallization, so that the ice crystal volume increases, the gluten network is seriously damaged, destroying the yeast, reducing its gas production capacity, and ultimately reducing product quality.
At present, there are many types of yeast on the market, and the different choices required due to the product are also different. Wolt et al. found that when making ready-to-use dough, the initial awakening time of dry yeast is shorter than that of fresh yeast, but in the dough that has been stored for a longer time, the awakening time of dry yeast is longer. Bulk liquid yeast mixes better with wheat flour, reduces the time required for pre-fermentation, reduces metabolic activity and improves its resistance to freezing. On the other hand, compensating for the damage caused by the freezing of yeast is usually 0.5 times more than the usual amount by increasing the amount of yeast added. The yeast used to make frozen dough is different, and the characteristics of various aspects are also different, and the final resulting bun has obvious differences in texture characteristics, color, specific capacity and sensory; Overall, steamed buns made from low-sugar and sugar-resistant active dry yeast are of better quality and more stable in all respects.
During frozen dough processing, the freezing damage of yeast is the main factor causing the quality of the finished product to decrease. During storage and transportation, violent fluctuations in temperature can lead to an increase in the size of ice crystals, exacerbating the damage of freezing to yeast, reducing its freezing tolerance, destroying the dough structure, and causing the quality of the dough to decrease. Therefore, reducing the damage of yeast during the freezing process and improving its frost resistance are the key research contents to improve the quality of frozen dough.
1.6 Processing technology
In the process of dough processing, many processes have a great impact on the quality of the product, for example, watana and freezing are important parts of the frozen dough production process, and its success will be able to directly affect the quality of the dough. And the surface should pay attention to the proportion of water, too much and too little will affect the quality of the finished product. Refrigeration technology allows the dough to be stored for a long time, which is conducive to transport and storage, but the low temperature will reduce the quality of the finished bread. When processing frozen dough, in order to obtain a higher quality product, it is necessary to effectively control the freezing speed and time. When frozen storage, both quick-frozen and slow freezing will form ice crystals, and slow freezing allows water to flow out of the cells in enough time to form extracellular ice, resulting in yeast cell dehydration, affecting yeast viability and dough structure. As the freezing rate increases, the loss of the basic components of the dough decreases, and the ice crystals produced by the use of quick-freezing are small, almost do not damage the tissue, and have little impact on the quality of the finished product of the frozen dough. At present, pre-fermentation and freezing systems before freezing are the most studied in the freezing process. No fermentation before freezing or rapid fermentation can reduce the sensitivity of yeast and make it more frost resistant in subsequent freezing processes. Wang Xianlun recorded the development relationship between cracking and shrinking of frozen dough with fermentation time through experiments, and found that the optimal fermentation time of dough was 10min.
1.7 Modifiers
Compared with fresh dough products, frozen dough products have problems such as long fermentation time, small size, poor texture performance, and reduced taste. The use of modifiers is extremely important in improving the quality of frozen dough, and can play a good protective role in the use of yeast, moisture of frozen dough and frozen dough, thereby improving the quality of the finished product of frozen dough. At present, the commonly used modifiers mainly include food glue, emulsifiers, enzyme preparations, modified starch, and antifreeze agents.
1.7.1 Food glue
Food glue is a kind of water-soluble gel-forming food modifier, with thickening, adhesion, gelling and stabilization and other effects, hydrophilic groups can make the dough form a polymer complex, so that the dough has good rheological properties. During the freezing process of the dough, the presence of food glue can reduce the amount of movable water, avoid the formation of large ice crystals, and thus weaken the degree of damage of ice crystals to yeast cells and gluten networks.
Xanthan gum is a commonly used food glue in food processing, which has strong water absorption and has a good protective effect on gluten protein. The network structure of xanthan gum has a certain supporting role in the gluten protein system, which can compensate for the damage suffered by gluten protein during the dough freezing process. In addition, xanthan gum can not only form a composite network structure with wheat gluten, but also disperse water in the gluten network structure through water absorption, hindering the growth and migration of ice crystals during the freezing process, thereby improving the freezing stability of gluten protein.
The study found that xanthan gum can change the water retention and rheological properties of the dough, thereby repairing the deterioration of the texture caused by the freezing of the dough, so that the hardness of the final product is reduced. On the other hand, xanthan gum can reduce the gelatinization temperature of frozen and cooked noodles, reduce the energy consumption required by the curing process, and improve the quality structure. However, the use of xanthan gum in some processing processes is not obvious, and it is not suitable for intermittent baking. In future studies, by adding konjac gum, pectin, guar gum, etc., using their synergistic effect, you can maximize the water retention capacity of the dough, reduce the number of recrystallization of free water, reduce the negative impact of the freeze-thaw cycle on the frozen dough, and make the finished product have a better texture.
1.7.2 Emulsifiers
Emulsifier is a surfactant that can form an insoluble complex, which can effectively reduce the degree of starch aging and improve the flexibility of the dough, so that the processed food is loose and soft. Emulsifiers reduce the formation of large ice crystals, protect yeast cells, and reduce damage to the structure of the gluten network. Studies such as Selomulyo have found that compound emulsifiers improve the quality of frozen dough better than mono-emulsifiers. We should make full use of the synergy of compound emulsifiers to strengthen the formation of frozen dough protein network structure, enhance the elasticity and tensileity of dough gluten, reduce the loss of moisture and freeze cracking, and improve product quality.
1.7.3 Enzyme preparations
Enzyme preparation is a protein with enzymatic characteristics extracted from an organism, which is a natural and safe food additive that can catalyze certain reactions in the processing of frozen dough, so that the rheological properties of the dough are improved and the quality of the finished product is improved. Through the use of enzyme preparations, particularly good results have been achieved in terms of quality improvement in both frozen dough and baked products. In the frozen dough without enzymatic preparations, the tissue structure is uneven, the freezing cracking phenomenon is more serious, the network structure between proteins is relatively poor, the elasticity and ductility of gluten are not good, and the tensile performance is also reduced. The finished product produced by this has cracks, poor internal organization, not delicate, poor elasticity, and no chewiness.
Glutamine aminotransferase is an enzyme that can catalyze the transfer of phthalocyanins within or between protein molecules. Covalent crosslinking between proteins or polypeptides changes the gelling capacity, hydraulic holding and thermal stability of proteins, and improves the structure and functional properties of proteins. The addition of glutamine transaminases to the noodles can enhance the gluten strength and gelling capacity, reduce the degree of damage to the gluten network of ice crystals, and improve the fermentation vitality of frozen dough. Yang Xuan et al. pointed out that the content of gliadin and gluten in the frozen dough with glutamine transaminases added decreased significantly, and the relative content of deep bound water in the non-fermented dough increased after the freeze-thaw cycle.
Amylase can decompose the damaged starch in wheat flour into simpler compounds such as dextrin, maltose or glucose, providing more nutrients for yeast to meet the needs of yeast fermentation, thereby improving the fermentation characteristics of yeast in frozen dough, enabling yeast to reproduce and grow better, and the finished product is more loose and soft. The addition of a-amylase can change the spatial structure of protein, significantly improve the organoleptic quality of frozen dough products, and add 0.01% amylase can significantly enhance the gluten strength of frozen dough and increase the volume of bread. In addition, experiments by Cen Tao et al. found that adding some xylanase to the frozen dough can effectively improve its ductility and gluten elasticity.
With people's attention to food safety issues, the safety of food additives has received more and more attention, but enzyme preparations will be inactivated during baking and are considered safe for human health. In future studies, the research on the influence of other biological enzyme preparations on the quality of frozen dough can be strengthened, and the mixed synergistic effect of enzyme preparations with other food additives can be further studied, so as to effectively prevent the phenomenon of food additives exceeding the standard.
1.7.4 Antifreeze
Antifreeze can play a good protective role for yeast in frozen dough, enhance the fermentation vitality of yeast, and it has a significant effect in inhibiting the migration and loss of water in frozen dough, which in turn can hinder the formation of ice crystals and recrystallization, protect the gluten network structure, and improve the quality of frozen dough. Commonly used antifreeze agents are antifreeze protein trehalose, F-99 and so on.
1.7.4.1 Antifreeze proteins
Antifreeze proteins, also known as ice structure proteins, are a class of polypeptides produced by certain organisms to resist changes in the external environment, which can effectively inhibit the growth of ice crystals. Studies have found that adding antifreeze protein from different sources to frozen foods can extend the storage time of frozen foods and improve the quality of thawing. Zhang et al. found that the dough added with oat antifreeze protein has a strong fermentation force, the gluten matrix is not damaged by temperature fluctuations and freezing, and the structural characteristics of the dough are enhanced, but with the prolongation of storage time, the improvement effect is gradually reduced. Ji Chengyu and other studies have shown that antifreeze protein can inhibit the breaking of disulfide bonds and changes in secondary structure, reduce the recrystallization of ice crystals, prevent water dispersion of the dough, and maintain the water holding capacity of the dough. Antifreeze proteins have a good protective effect on frozen dough and do not react with other additives or the food itself. Although antifreeze protein can improve the fermentation power of the dough and has a broad space for development in frozen food, it is still very difficult to achieve the industrial production of antifreeze protein at this stage, and the high cost of antifreeze protein production is also an important problem that needs to be solved.
1.7.4.2 Trehalose
The chemical properties of trehalose are very stable, with a strong anti-dehydration effect, which can effectively prevent biofilms, proteins, etc. from being damaged in cold environments. Trehalose has endogenous and exogenous components, and the antifreeze ability of yeast is mainly related to endogenous trehalose. The higher the endogenous trehalose content, the stronger the frost resistance of yeast cells. Exogenous trehalose provides energy for yeast while cooperating with the body's cryonic stress response to repair damaged cells. Endogenous trehalose and exogenous trehalose can reduce the degree of cell freeze damage, but endogenous trehalose has a better antifreeze effect than exogenous trehalose. The antifreeze mechanism of trehalose has not yet been determined, and there are three more recognized hypotheses of "water substitution", "glassy" or "priority exclusion". However, as a non-reducing disaccharide, trehalose is non-toxic and harmless, has cold and drought resistance characteristics, and can play a good protective role in yeast cell activity, so as to improve the fermentation capacity of the dough and improve the quality of frozen dough products, which is a good dough improvement additive.
1.7.4.3 F-99
F-99 is a food modifier specially developed for the production of bread by low temperature frozen dough method, suitable for non-fully fermented and fully fermented frozen dough, which can stabilize the structure of the gluten network, enhance the antifreeze performance of yeast, improve the air holding capacity of the dough, and prolong the storage time of frozen dough.
2 Problems and measures in the study of frozen dough
China's frozen dough technology is still in the process of exploration and improvement. In the production process, the food produced by frozen dough is more likely to shrink, harden, crack, change color, etc. compared with the products made on the spot in the store. At present, the main problems of frozen dough are weak frost resistance of yeast and the reduced gas holding capacity of the dough.
2.1 Yeast has weak frost resistance
The yeast is suitable for growing in an environment of 20 ~ 30 ° C, while the frozen dough needs to be processed in an environment of -18 ~ -40 ° C, and the temperature is not suitable for growth. Moreover, the water in the yeast cells forms crystals, and the longer it is refrigerated, the larger the crystallization, the more serious mechanical damage to the dough, which is easy to cause the dough to crack. If the refrigeration temperature fluctuates greatly, it will cause secondary damage to the yeast, causing more damage to the yeast cells, while the survival rate of yeast cells will decrease, and the activity, fermentation power and gas production capacity of the surviving yeast will decrease. If the dough freezes and thaws repeatedly, all indicators will drop. Frozen dough releases glutathione when thawed, an ingredient that damages the gluten network structure, weakening its air holding and skeleton support. Therefore, in the process of freezing dough making, suitable frost-resistant yeast should be selected to reduce the degree of damage during the freezing process, and the frost resistance of yeast can be improved by using additives, and additives can also maintain the rheological and thermophysical properties of the dough, improve the frost resistance and fermentation ability of yeast. Optimizing frozen storage conditions also protects yeast activity and the structure of the dough network, thus reducing freezing damage caused by ice crystals. In addition, new refrigeration technologies such as ultrasonic assisted freezing can simultaneously accelerate the freezing process and produce fine and uniform ice crystals that protect the dough network structure.
2.2 Refrigeration reduces the air holding capacity of the dough
During the frozen storage of the dough, the ice crystals generated by the water destroy the gluten structure, making the dough gas holding less. In order to improve the air holding capacity of frozen dough, measures such as improving the freezing process, adding dough quality improvers and protecting yeast from freeze can be improved to a certain extent. In terms of refrigeration process, advanced refrigeration equipment is used to reduce temperature fluctuations and reduce the damage to the ice crystals. For dough conditioners, the improvement of various dosages can be analyzed to determine the optimal choice. Gluten can also be added to repair the damaged gluten network to strengthen the skeleton support and improve dough holding. Mono and diglycerides diacetylate bitartrate is a commonly used bread quality improver, which can be connected with hydrophilic groups and hydrophobic groups in gluten to build a new dough network structure, so that the agitation and holding of the dough is improved, the volume of bread is increased, and collapse is prevented.
3 Outlook
China's frozen dough technology has been developing rapidly since it has been widely recognized. At present, the research of jelly dough in foreign countries is relatively deep, although the development of the country is also very rapid, but the research on its system is not mature enough and comprehensive, the quality of the final frozen products is still not as good as the fresh products, to promote the development of China's food processing to the direction of industrialization and standardization, to meet people's continuous pursuit of food freshness and safety, will be the key development direction of frozen dough in the future.
In future studies, attention should be paid to improving the freezing tolerance of yeast, while optimizing the freezing rate and avoiding temperature fluctuations, so that the yeast activity is well maintained and the recrystallization of the moisture in the frozen dough is prevented. In addition, novel freezing technology can speed up the freezing process and improve heat and mass transfer, such as ultrasonic assisted freezing has shown a positive impact on the performance of the opposite group and cell viability, but there are very few studies at present, and its mode of action and influence mechanism need to be studied at a deeper level to improve the quality of frozen dough to promote industrial and standardized production. Existing methods are mostly used alone, and these methods can be combined to further enhance the vitality of yeast, protect the dough network structure and provide a fresher taste to the final product.