Study On Quality Effect And Mechanism Of Steamed Bread At Frozen And Storage Temperature

Based on the wheat flour fermentation of steamed bread as the research object,through the analysis of frozen steamed bread,the quality characteristics,water distribution and protein structure changes of steamed bread under different quick-frozen temperatures(-24 °C,-28 °C,-32 °C,-36 °C)were studied and the quick-frozen temperature of steamed bread was determined.The quality differences of steamed bread after quick-frozen stored at five different temperatures of-6 ?,-12 ?,-18 ?,-24 ? and-30 ? were compared.The changes of moisture migration and protein physical and chemical properties during freezing storage were also discussed.The main conclusions are as follows:(1)Effect of quick-frozen temperature on quality and protein structure of fermented flour products.The water loss rate,hardness,chewiness and frozen water content of steamed bread decreased significantly(P<0.05)with the decrease of quick-frozen temperature,while the water holding capacity,specific volume,spring,cohesiveness and total peak area increased significantly(P<0.05).At-32 °C and-36 °C,the moisture and protein of quick-frozen steamed bread were tightly bound,the binding effect on binding water is strong,and the quality of steamed bread was better.From the GMP content and protein secondary structure,the higher the quick-freezing temperature,the higher the GMP content,the transition of ?-helix and ?-turn to random coil and ?-sheet,and the higher the degree of protein depolymerization.Comprehensive consideration steamed bread at-32 °C quick-frozen effect was the best.(2)Effect of different storage temperatures on frozen storage stability of fermented flour products.With frozen steamed bread as the research object,when the frozen storage temperature was lower,the moisture content,specific volume,p H value,bound water and hydrogen proton number in steamed bread increased,and the water loss rate,freezable water content,weak bound water and free water decreased.The lower the freezing temperature,the less water loss and the better the storage stability of steamed bread.With the extension of frozen storage time,it was found that the p H value and water loss of steamed bread increased firstly and then decreased,while the trend of water content was opposite.The specific volume,cohesiveness and spring of steamed bread decreased,and the freezable water content,hardness and chewiness increased.Steamed bread had the highest water loss rate and the lowest water content,free water and hydrogen proton content at 3 weeks.With the extension of frozen storage time,bound water and weak bound water in steamed bread gradually migrate to free water.The quality characteristics of steamed bread were affected by the change of water distribution during frozen storage at different temperatures.The higher the temperature,the faster the water loss of steamed bread during freezing storage,the stability of protein network structure weakened,and the quality of steamed bread deteriorated.The SEM results showed that the higher the temperature was,the longer the frozen storage time was,the more serious the mechanical damage to the microstructure was.The starch granules were partially or completely exposed,and the quality of steamed bread was worse.In summary,the lower the freezing temperature,the smaller the influence on moisture migration,quality characteristics and microstructure of steamed bread,and the worse the stability and quality of steamed bread with the extension of freezing time.(3)The effects of different storage temperatures on protein characteristics of fermented flour products were studied.The higher the freezing storage temperature was,the lower the protein emulsification,emulsion stability,S-S content and total-SH content were,and the higher the free-SH content was.Soluble protein content and water holding capacity of protein increased at first and then decreased,and the trend of surface hydrophobicity was opposite.The emulsifying stability of frozen storage at-30 ? was the lowest within 2 weeks.Emulsification stability and soluble protein content were lowest after 3 weeks storage at-24 °C,while S-S content had no significant change.When stored for 4 weeks at different temperatures,there was no significant difference in the moisture holding capacity of protein in frozen steamed bread,and the emulsification was significantly different at-6 ? compared with other temperatures(P<0.05).During freezing storage at-24 ?,the changes of emulsification,oil retention and soluble protein contents were relatively stable,and the change of protein quality was the least.With the extension of frozen storage time,the recrystallization of ice exposed the hydrophobic groups inside the protein,resulting in the decrease of the water holding capacity of the protein.With the increase of storage time,the oil holding capacity,emulsifying capacity,emulsifying stability and surface hydrophobicity of the protein increased.The destruction of disulfide cross-linking between glutenin protein and gliadin protein increased the soluble protein content with the extension of storage time,and the overall trend of S-S and total-SH content decreased with the increase of frozen storage time.SDS-PAGE results showed that the proteins in steamed bread mainly included gliadin protein and low molecular weight glutenin protein subunits.In the early stage of storage,the migration of protein bands was more obvious at-24 °C,but in the late stage of freezing storage,there was no significant effect on protein subunits at different temperatures.With the increase of storage time,the optical density of the bands on the separation gel decreased significantly,and the content of soluble glutenin decreased.In summary,the protein properties were better with the decrease of temperature,while the properties of the protein decreased with the extension of freezing time.Overall,the change of protein characteristics was the most stable at-24 °C,and was more suitable for long-term frozen storage.