Frozen dough first appeared around 1950, and after decades of development, it gradually matured in the 1980s. At present, frozen dough technology is most widely used in bread production in Europe and the United States. The study of frozen dough in traditional fermented noodle products in China is still in the early stage of research, and compared with fresh products made on site, products produced by frozen dough technology are prone to problems such as dry cracks, collapsed skin, discoloration, small volume and hard texture.

If frozen dough can be widely used in the industrialized production of fermented pasta such as buns, it will greatly improve the production efficiency, reduce the production cost, improve the quality of the product, and at the same time, it can promote the development of the Chinese fast food industry, which will bring considerable economic benefits for food enterprises.

Production process of frozen dough buns

Frozen dough technology is widely used in the production of Western-style bread, for the traditional Chinese fermented noodle products have not been relatively perfect process, can be based on the common frozen dough bread or frozen dough steamed bread production process is improved, applied in the frozen dough bun production.

The advantage of the unawakened frozen dough method is that the dough can be stored at -18℃ for 200d, but the dough of wheat flour, yeast and improver requirements are higher, and the quality of the dough will be reduced by long time storage; pre-awakened frozen dough method has a shorter storage time, and the degree of awakening of the dough needs to be strictly controlled, and excessive awakening will cause damage to the network structure of the gluten; the frozen buns method has high production costs, strict requirements of frozen storage environment, and strict requirements of frozen storage environment, and the process of frozen dough bun production is summarized in Table 1. The production cost of frozen buns is high, and the requirements for the freezing environment of buns are strict, and the quality of buns decreases after re-steaming.

Changes in quality of frozen dough buns during freezing

The damage to the gluten network structure and the damage to the yeast cells caused by the ice crystals formed by the moisture in the dough during freezing are the main reasons for the deterioration of the quality of frozen dough. Moisture in the dough during freezing will form ice crystals to fill in the gluten network structure, the volume of ice crystals increases with the increase of freezing time, the temperature difference and the presence of vapor pressure will also cause ice crystals to move inside the gluten protein, which will cause damage to the network structure of gluten, ice crystals recrystallization will occur after the extrusion of the gluten network will have a crushing effect of the destruction of the gluten network, gluten proteins undergo polymer depolymerization, disulfide bonds The breakage leads to the decrease of the cleavage temperature of gluten protein and wheat gluten with the increase of refrigerating time, and the decrease of thermal stability.

The α-helical structure is the main supporting skeleton of gluten proteins, and in freezing storage the α-helical structure will be transformed to the β-turn and β-folded structure, and the degree of binding of wheat glutenolysis proteins to water will be weakened. Ice crystals damage the surface structure of starch, and an increase in damaged starch leads to a decrease in deeply bound water, which further leads to an increase in freezable water.

The number of surviving yeasts and their ability to ferment is an extremely important factor for frozen dough. The ice crystals produced inside the cell during freezing will cause damage to the microstructure of the yeast cell, while the ice crystals outside the cell will cause changes in the osmotic pressure inside and outside the cell, leading to water loss of yeast, which will lead to a decrease in the activity of the yeast, a decrease in the fermentation ability or even the death of the yeast, and these factors will result in the unfavorable changes of the crust of the buns such as the appearance of the dead surface, a decrease in the specific volume, and a decrease in the texture; the glutathione of the dead yeast will destroy the disulfide bonds leading to a decrease in the gluten of the dough.

Improvement of the quality of frozen dough buns

I. Wheat flour
Wheat flour with high gluten protein content should be selected to improve the strength of gluten and attenuate the damage caused by ice crystals and glutathione to the gluten network. It has been found that frozen dough made from high gluten flour has the best overall quality and low gluten flour has the worst.

When the settling value of wheat flour was 44.8 mL, the landing value was 637 s, and the mass fraction of wet gluten was 36.0%, this wheat flour was suitable for making frozen dough buns. Adding potato flour, soya bean dregs flour and konjac flour to the flour can improve the quality of frozen dough, soya bean dregs flour can attenuate the weakening of gluten in the freezing process, and konjac flour can inhibit the growth of ice crystals, and the use of these three also reduces the transfer of water in the dough and enhances the strength of the gluten network, which improves the resistance to freezing and at the same time increases the nutrition of the buns.

Second, yeast
Some researchers have already selected yeast strains through specific culture to improve the antifreeze property of yeast in frozen dough, Xue Meicui et al. found that the survival rate of yeast showed a significant positive correlation with the content of alginose and glycerol, so according to the content of alginose and glycerol, the yeast with good antifreeze property can be initially screened, Lu Xuechun et al. found that the AY005 (brewer’s yeast) had the highest survival rate and good fermentation ability after being stored at -18 ℃ for 60 d. The yeast was also found to have good fermentation properties. Lu Xuechun et al. found that AY005 (brewer’s yeast) had the highest survival rate after 60 d of storage at -18℃ and had good fermentation ability.

Ai Yuhuan made a related research on three kinds of yeasts, and found that hypertonic cultured yeast would form a protective film, reduce the loss of cellular heat, attenuate the damage of ice crystals, and inhibit the change of the protein properties of yeast cells, and the morphology of the hypertonic cultured yeast cells was still intact after 30d of freezing and storage of the dough.

Alginose can well improve the antifreeze ability of yeast in frozen dough, the more endogenous alginose, the better the antifreeze of yeast, and exogenous alginose can provide sufficient energy for the metabolism of yeast, and both of these two kinds of alginose can alleviate the damage to yeast caused by low-temperature freezing, and promote fermentation to reduce the phenomenon of dead surface of the baozi dough. Endogenous alginate, glycerol, proline on the freezing damage to the yeast cell wall has a certain repair effect, promote the cell to restore the normal form.

Third, food glue
Food glue has good water holding, thickening, and food glue and wheat gluten protein conjugate with hydrophilic, can improve the water holding capacity of the dough, reduce the migration of water in the dough during freezing, food glue can also promote the combination of starch and gluten network structure, to improve the processing quality of frozen dough, to reduce the damage of ice crystals on the gluten network structure and the yeast, it has been reported that in the dough at the same time add xanthan gum and pectin, can significantly inhibit the freezing of the yeast cell wall. It has been reported that the addition of xanthan gum and pectin to the dough at the same time can significantly inhibit the moisture transfer during freezing and reduce the damage to the dough structure by freezing and thawing;

Recent studies have concluded that pig skin gelatin is a good antifreeze agent, which can be intertwined or linked with the gluten network through -S-S- and hydrophobic bonding to make the gluten network structure resistant to damage caused by ice crystals, and that the collagen hydrolysates of pig skin (CoAPPs) can significantly attenuate water transfer during freezing and thawing and inhibit recrystallization, thus protecting gluten, but due to religious reasons, if the gelatin is added to the dough, it can significantly inhibit water transfer during freezing and reduce damage to the dough structure. However, if pig skin gelatin is added, it needs to be clearly labeled on the package.

Emulsifiers
Commonly used emulsifiers are sodium stearoyl lactylate, lecithin, monoglycerides and sucrose esters, etc., the addition of emulsifiers can improve the toughness and elongation of the dough, of which the sodium stearoyl lactylate, although the mechanical properties of the dough has a small impact, but it can significantly improve the organoleptic quality of the product.

Lipophilic and hydrophilic groups of emulsifiers can be combined with wheat gluten and wheat alcohol soluble protein respectively to improve the fermentation ability and gas holding capacity of dough, and can also be combined with fat and starch to improve the freeze-thaw stability of dough and reduce the migration of water, emulsifiers are also able to inhibit the aging of starch to reduce the phenomenon of hardening of the bun crust due to starch aging; emulsifiers are combined with starch to form an insoluble complex, which can impede the recrystallization of starch, making the dough more difficult and more difficult to handle. The emulsifier combines with starch to form an insoluble complex, hindering the recrystallization of starch, so that the surface tension of water in the dough is reduced, and the ice crystals form a smaller structure, which ultimately reduces the damage to the gluten network structure.

V. Modified starch
Modified starch can reduce the movement of water in the dough, increase the content of bound water, reduce the damage caused by freezing on the gluten network structure, Wang Yannan et al. measured the addition of different amounts of acetic acid ester potato starch dough water holding capacity, proved that the acetic acid ester potato starch can increase the water holding capacity of the dough, and the water holding capacity with the increase in the amount of water added and increased.

Buns in the storage of the phenomenon of soup seepage, can be in contact with the filling on the surface of the crust with phosphate ester double starch modified starch as an isolation film, to achieve the purpose of impeding the seepage of soup, but also to ensure that the buns good taste.

Sixth, enzyme
Enzymes can improve the flour’s powder properties and tensile properties, reduce the frozen water in the frozen dough can be frozen, reduce the formation of ice crystals on the yeast and the dough caused by the damage, thus improving the quality of frozen dough.

Currently, the commonly used enzyme preparations are cellulase, lipase, glucose oxidase and glutamine aminotransferase. Lipase can strengthen the gluten network structure, cellulase and glucose oxidase can make the dough have high gas production, while glucose oxidase can oxidize -SH to generate -S-S-, which has a positive effect on the stabilization of the gluten network. The addition of glutamine aminotransferase significantly improves the survival rate and fermentation ability of yeast.

Other additives
Oxidizing agent can oxidize -SH to -S-S-, consolidate the gluten network structure, and improve the gluten, air holding and toughness of the dough. In addition, the oxidizing agent can oxidize the glutathione in the dead yeast cells, and protect the gluten network structure. Ice structural proteins can inhibit the formation of ice crystals and recrystallization, improve the rheological properties of dough, and increase the water retention of dough. Water-soluble arabinoxylan prevents -S-S- from being reduced, thus protecting the network structure of gluten.

Eight, freezing process
Factors such as freezing temperature, time, refrigeration temperature and refrigeration environment will affect the quality of dough. When the freezing rate is slower, the ice crystal volume is larger, which will cause greater damage to the gluten structure, and when the freezing rate is faster, it will cause damage to the yeast, when the freezing rate is -3.19℃/min, the ice crystals formed in the dough are small and uniformly distributed, which will cause less damage to the dough and ensure better product quality, and the refrigeration temperature is lower than -20℃, which will result in the large number of deaths of the yeast, and the dough will be frozen if it is used in the process of freezing. If the dough is frozen and thawed many times during the use due to temperature fluctuation, the gluten network structure will suffer obvious damage and the quality of the dough will be reduced.

Prospects

Frozen dough technology has significant advantages, can reduce costs, expand productivity, can effectively promote the rapid development of China’s bun industry. Shanghai, Beijing and other places have food enterprises will be frozen dough technology applied in the production of buns, and has produced considerable benefits. With the continuous development of China’s economy and the improvement of people’s living standards, the pace of life is also accelerating, the industrialized production of buns has become an inevitable trend, to strengthen the research on the frozen dough technology of buns can improve the quality of buns, and provide technical support for the industrialized production of buns. The future research direction can also optimize the process of frozen dough buns according to the characteristics of different types of buns; research and development of improved agents for frozen dough buns and other directions.