Research On The Effect Of Frozen Storage On The Quality Of Steamed Bread And Its Mechanism

Quick-freezing technology is an important means to realize the industrial production of flour products.At present,the research on quick-frozen flour products mainly focuses on the changes of water,protein,starch and quality of frozen dough and steamed bread,and probes into its deterioration mechanism.However,there are few studies on the deterioration mechanism and component changes of quick-frozen steamed bread.The vacancy of this knowledge make it difficult to reveal the detailed deterioration of quick-frozen steamed bread quality essentially,and make the improvement of quick-frozen steamed bread lack more complete theoretical support.This paper focuses on the deterioration mechanism and improvement of steamed bread in the process of frozen storage.Firstly,the changes of water state and quality of steamed bread during frozen storage were discussed.Then,the effects of frozen storage on the aggregation characteristics of gluten protein and the aging characteristics of starch were further studied and the mechanism of quality deterioration of quick-frozen steamed bread was expounded from the perspective of composition.Finally,the effect of gluten protein on the improvement of quick-frozen steamed bread quality was studied,in order to improve the effectiveness of regulating the quality of quick-frozen steamed bread.The main conclusions were summarized as follows:Frozen storage weakened the combination of steamed bread and water,resulting in poor quality of steamed bread.Differential scanning calorimeter(DSC)and low field nuclear magnetic resonance(LF-NMR)were used to study the changes of freezable water content and water state of steamed bread during frozen storage and re-steamed,the results showed that the weakening of the combination of steamed bread and water could be mainly manifested by the decreased semi-bound water content,enhanced semi-bound water mobility and the increased freezable water content(the melting enthalpy increased by 6.32 J/g).Through the determination of the specific volume and texture characteristics of steamed bread,it was found that the quality of steamed bread deteriorates with increasing hardness and decreasing specific volume during frozen storage.The correlation analysis indicated that the water loss rate was positively correlated with the hardness and chewiness of frozen steamed bread,and the content of freezable water was positively correlated with the hardness and chewiness of re-steamed bread.In the process of frozen storage,the depolymerization of the high molecular weight proteins occurred in steamed bread system and protein system with different depolymerization degrees.The molecular weight of the protein was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis(SDS-PAGE)and size exclusion high performance liquid chromatography(SE-HPLC).It was found that the disulfide cross-linking of heat-induced gliadin to gluten be disrupted,the SDS soluble proteins were generated by depolymerization of high molecular weight proteins,and the content increment of soluble monomer protein was more significant than that of polymer protein.The changes in protein secondary structure of the two systems during frozen storage were discussed by fourier transform infrared spectroscopy(FTIR).The data showed that the conformation of the protein was unfolded and rearranged and the random coil structure mainly changed to?-sheet structure,indicating that frozen storage had a negative effect on both systems,making the secondary structure more rigid and the orientation of the peptide chain more orderly.The aging characteristics of starch during frozen storage were studied by X-ray diffraction(XRD)and differential scanning calorimeter(DSC),the results showed that frozen storage could not completely inhibited the retrogradation of starch,the crystallinity of starch increased and the gelatinization degree decreased.Morever,in the starch system,the intensity of diffraction peak near 17~o changes more obviously,and it is more prone to aging compared with the steamed bread system.Meanwhile,the microstructure of the steamed bread system,protein system and starch system were further observed by scanning electron microscope(SEM).With the increase of frozen storage time,the internal structure was gradually destroyed by the extrusion of ice crystals,and the pore size became larger and discontinuous.Furthermore,the water states of the three systems were compared by low field nuclear magnetic resonance(LF-NMR).Compared with starch,the protein was more closely bound to the water and importantly the water loss of steamed bread during frozen storage was mainly caused by the weakened binding force of protein and water.Adding an appropriate amount of gluten protein can significantly improve the quick-frozen steamed bread characteristics in terms of specific volume,texture and internal structure.That was contributed to that the gluten protein could strengthen gluten network structure and inhibit the dehydration of steamed bread,thus could hinder the growth of ice crystals,and delay the depolymerization of polymer proteins.Firstly,the changes of dough farinaceous,alveograph and tensile properties after protein addition were studied by rheological determination.Then,the changes of quick-frozen steamed bread quality,water state and protein molecular in quick-frozen steamed bread after gluten addition were further studied by differential scanning calorimeter(DSC)and size exclusion high performance liquid chromatography(SE-HPLC).The results showed that adding gluten protein increased the tensile resistance and elongation of the dough,and enhanced its elasticity,gas capacity and strength,which further reduced the freezable water content in steamed bread,inhibited the moisture migration and alleviated the recrystallization of ice to some extent.At the same time,the addition of gluten protein significantly reduced the content of SDS soluble protein in steamed bread,weakened the depolymerization brought by frozen storage,and thus improved the quality of steamed bread.However,when excessive amounts of gluten(more than 1.0%)were added,it will have a negative effect.The strength of the dough was too strong,resulting in easy shrinkage of the steamed bread,the specific volume decreased and the hardness increased.