Deterioration Mechanism And The Improvement Of Frozen Dough Steamed Bread

Steamed bread is a kind of traditional fermented flour staple food in China,which is popular with consumers.Applying the new frozen dough technology to the production of traditional steamed bread is the development direction of staple food industry.However,the quality of steamed bread decreased seriously when the frozen dough technology was used,which hindered its development.At present,the studies of frozen dough steamed bread mainly focused on the change of single component separated from frozen dough,the improvement effect of additives,and the laboratory transformation and screening of freeze-resistant yeast.There is a lack of targeted solutions to the practical problems encountered by frozen dough in production,the interaction between yeast and dough,and the poor freezing resistance of commercial yeast.Therefore,this topic explores the deterioration of frozen dough steamed bread,and analyzes its main influencing factors.On this basis,it establishes a solution to the problems such as the integrity of gluten protein network after freezing and the freeze-thaw tolerance of commercial yeast,then explores the improvement mechanism of freeze-thaw tolerance of yeast through metabolomics.Firstly,the deterioration of frozen dough steamed bread during frozen storage at-18?and freeze-thaw cycles at-18??30?were compared and analyzed.The results showed that the specific volume,hardness and brightness of frozen dough steamed bread decreased significantly with the extension of frozen storage time to 25 days or freeze-thaw cycle once(P<0.05).Large pores gradually appeared in the steamed bread,and the pores were unevenly distributed;The fermentation ability of dough decreased significantly(P<0.05).In the process of 5d?50 d frozen storage and 1?4 freeze-thaw cycles,the elastic modulus decreases and the viscous modulus increases.The relaxation time of T₂ in the relaxation curve of low field nuclear magnetic resonance(LF-NMR)shifted to the right and the fluidity of water increased.Endothermic enthalpy and frozen water content increased significantly.The effects of freeze-thaw cycle for 1 times and frozen storage for 25 days on the quality of steamed bread and the physical and chemical properties of dough were similar.Secondly,the main influencing factors leading to the deterioration of steamed bread quality in the process of frozen storage and freeze-thaw cycle were analyzed.The effects of gluten protein and yeast on the fermentation ability of frozen dough,the changes of gluten protein network structure and yeast cells during frozen storage and freeze-thaw cycle were studied.The results showed that the effect of yeast/dough frozen alone on the fermentation ability of dough was limited,while the interaction of yeast/dough mixing in the frozen storage was the main reason for the significant decrease of dough fermentation ability(P<0.05).The further analysis results of gluten protein showed that the wet weight of glutenin macropolymer(GMP)in yeast-free dough was higher than that in yeast dough.After frozen,holes appeared in the continuous gluten network wrapped with starch particles.With the increase of freeze-thaw cycles,the protein network in the dough gradually changed from compact rules to lose fragments.The protein network analysis(PNA)diagram also showed that the total area and length of the protein network decreased.The results of yeast cell analysis showed that the survival rate of yeast decreased significantly(P<0.05),and the cell morphology showed depression and shrinkage.During frozen storage,the intracellular sucrose content decreased while glucose fructose increased,and the yeast was in a lag period.The change of dough osmotic pressure and ice crystal were the main reasons for the inactivation of yeast.During the freeze-thaw cycle,the content of sucrose,glucose and fructose in cells decreased,and the cells crossed the log growth period and reached the stable period/decay period,which is the main reason for their inactivation.Thirdly,aiming at the problem that the network structure of gluten protein is difficult to maintain during frozen storage and freeze-thaw.NaHCO₃ is used to strengthen gluten protein network,and the improvement mechanism of different NaHCO₃ addition on the quality of frozen dough steamed bread is analyzed.After 4 times of freeze-thaw cycles,the specific volume of the dough with 0.4%NaHCO₃ increased significantly and the hardness decreased significantly(P<0.05).The improvement mechanism of NaHCO₃ was explored from two aspects:the water state in dough and the aggregation characteristics of gluten protein.The results showed that NaHCO₃ enhanced the water-solid interaction in the dough and decreased the frozen water content and water fluidity during the freeze-thaw cycle.Compared with the control group,after freeze-thaw cycle,the proportion of extractable protein and the content of free sulfhydryl group of sodium dodecyl sulfonate(SDS)added with NaHCO₃ decreased significantly(P<0.05).At the same time,although NaHCO₃ enhanced the electrostatic repulsion of gluten,the gluten network increased with the addition of NaHCO₃,indicating that electrostatic interaction is the secondary factor affecting the gluten network of alkaline frozen dough.Based on the observation of dough microstructure and corresponding PNA results by laser confocal microscope(CLSM),it can be seen that the addition of NaHCO₃ enhances the alkali/protein-protein interaction,improves the gluten protein network area and total network length,and forms a dense gluten protein network structure.In addition,aiming at the problem of maintaining the freeze-thaw stability of yeast,the method of using?-poly-L-lysine(?-PL)mild stress treatment yeast to improve the quality of frozen dough steamed bread during freeze-thaw cycles and its improvement mechanism were analyzed.The results showed that the frozen dough steamed bread with 0.3 mg/m L and 0.6mg/m L PTY were still maintained high specific volume(2.5?2.6),low hardness(600?900)and uniform internal texture after four freeze-thaw cycles.During the freeze-thaw cycles,the CO₂ yield of dough prepared in PTY was stable.?-PL acted on yeast to improve the quality of frozen dough steamed bread by changing the growth characteristics of yeast,rather than directly acting on flour.The improvement mechanism was further analyzed by analyzing the aggregation characteristics of gluten protein during freeze-thaw process and steam heating process.SDS extractable protein,protein subunit distribution and chemical analysis results show that PTY instead of yeast can reduce the acidification of dough and the depolymerization of gluten protein during freeze-thaw cycle,and reduce the acid hydrolysis and deamidation reaction during heating.Heating induced gluten network aggregation to maintain the gluten network structure and gas holding capacity of frozen dough,so as to improve the quality of frozen dough steamed bread.Finally,focus on the changes of yeast cells to explore the effects of?-PL on yeast growth curve,aggregation characteristics,microstructure and survival rate,and to further explore the molecular biological mechanism of?-PL to improve yeast freeze-thaw tolerance based on mass spectrometry yeast metabolomics.The results show that the lag period of yeast was prolonged and hydrophilic layer was adsorbed on the cell surface after?-PL treatment.Compared with the untreated group,the cell survival rate and glycerol accumulation increased significantly during freeze-thaw cycle(P<0.05).Metabonomic analysis based on gas chromatography-mass spectrometry(GC-MS)was carried out the effect of?-PL treatment on intracellular metabolites of yeast during freeze-thaw cycles was analyzed by multivariate statistics and differential metabolite heat map.The changes of metabolite abundance were mapped to the metabolic network enhancement mechanism of?-PL treatment on freeze-thaw stability of yeast.?-PL promoted the fatty acid synthesis pathway,accumulated membrane structure related fatty acids,and enhanced the tolerance of yeast in the early stage of freeze-thaw.After 2 and 4 freeze-thaw cycles,the significant increase of trehalose and glycerol levels can protect yeast cells during the freeze-thaw cycle.?-PL inhibited the tricarboxylic acid cycle(TCA)of yeast,controlled the cell cycle,promoted the accumulation of protective metabolites in the lag period,and inhibited the rapid leap of log growth period in the process of freeze-thaw cycles.