| Sourdough is a complex biochemical system,in which protein degradation plays an important role in the quality of sourdough made from the grain(wheat flour).There are many reports in the literature,but the change of glutenin macropolymer(GMP)that is the most complex structure and composition of protein during the fermentation of sourdough has not been reported.This paper mainly studied the degradation of GMP that is made by wheat flour during the fermentation of sourdough and its structure change rule.The sourdough was fermented with Lactobacillus plantarum and compared with the blank sourdough and the weak acid sourdough.Research indicates:First,SE-HPLC was used to optimize the separation conditions of GMP in sourdough.The effects of extraction method,liquid-to-feed ratio,SDS concentration and DTT concentration on the separation of GMP by SE-HPLC were studied.Based on the results,three factors including the ratio of liquid to solid,SDS concentration and DTT concentration were selected as response surface analysis.The DTT extraction method is 14.25% higher than the GMP extraction rate of the ultrasonic extraction method,and the DTT extraction method is the optimal extraction method.The results of response surface analysis showed that the best conditions for separation of GMP by SE-HPLC were liquid-to-feed ratio,SDS concentration and DTT concentration were 17.72:1,1.59% and 1.69% respectively.Under these conditions,the theoretical extraction rate of GMP was 67.85%.Under this optimized condition,and the validated test that the extraction rate of GMP was 65.71%,which was 2.84 times of the initial condition.The content of glutenin in the elution curve of SE-HPLC was reduced by 20.81%,while that of gliadin was increased by 12.59%.The contents of albumin and globulin were basically unchanged.The content of GMP in the early stage of fermentation(0-8 h)in weak acid sourdough decreased rapidly,and the content of GMP in the Lactobacillus plantarum sourdough decreased by 6.5%,and the content of gliadin increased by 3.44%.However,there was no significant change in each contents of the late stage of fermentation(10-20 h).The content of GMP during fermentation of blank sourdough dough was almost unchanged.According to the analysis of the SDS-PAGE electrophoretogram,it was found that the content of HMW-GS in the Lactobacillus plantarum sourdough decreased from 17.4% to 9.7%,and that of LMW-GS increased from 24.9% to 35.9%.During the fermentation of sourdough,the HMW-GS in the weak acid sourdough decreased by 7.8%,while the content of LMW-GS increased by 14%,and the content of subunits of GMP during the initial stage of fermentation was changed sharply.There was no significant change in the late stage of fermentation;there was no significant change in the content of subunits of GMP during the fermentation of the blank sourdough.The analysis results presented by capillary electrophoresis were consistent with the SDS-PAGE electrophoresis pattern.The particle size distribution of GMP during the fermentation of sourdough dough is as follows: During the fermentation of Lactobacillus plantarum sourdough and weak acid sourdough,the large particle GMP(40-100 μm)decreases with the increase of small particle GMP(10-40 μm),but the weak acid sourdough was no significant change in the protein size of each particle in the late stage of fermentation.Finally,using CLSM to observe the microstructure of GMP during the fermentation of sourdough,it was found that the Lactobacillus plantarum sourdough and the weak acid sourdough all have the phenomenon of thinning of protein film,discontinuous phase and fracture,loose structure of protein network and increase of void area to a certain extent.However,the early stage of fermentation in weak acid sourdough changes drastically,but the change of the stage of fermentation is not obvious.These results show that the fermentation of sourdough can lead to the degradation of GMP and change the structure of GMP. |
