| Glutinous rice is rich in amylopectin.In Asian countries such as China and South Korea,cereal products made from fermented glutinous rice are fresh and unique in flavor,and the texture is sticky.However,the fermenting ability of glutinous rice sourdough is weak and the fermentation cycle is long,which is not conducive to the industrial production of glutinous rice sourdough,and because of the limited protease secreted by endogenous grain proteases and microorganisms,it cannot effectively degrade the protein in sourdough to improve Gel structure of glutinous rice sourdough.This study explored the improvement of fermentability and quality characteristics of sourdough by casein hydrolysate on the physical and chemical properties of Glutinous rice sourdough,and explore the effect of casein hydrolysate on the bacterial flora in Glutinous rice sourdough.Finally this study explore the relationship between the physical and chemical properties of sourdough and the bacterial flora.First of all,the natural fermentation method and the yeast fermentation method were used to prepare glutinous rice sourdough,and casein hydrolysate was added.The fermentability of the sourdough was analyzed by measuring the p H,total titration acidity value(TTA)and fermentability of the sourdough;the content of lactic acid and acetic acid were analyzed;and the degradation of protein and starch was analyzed by analyzing the changes of reducing sugar,protein content and protease activity.(1)After adding casein hydrolysate,the rate of acidification and fermentability of natural fermented sourdough and yeast fermented sourdough are improved,and the natural fermentation time is advanced at least 3 h,which greatly shortens the natural fermentation time.(2)After adding casein hydrolysate,the content of lactic acid bacteria in naturally fermented sourdough significantly increased(p<0.05),and content of lactic acid and acetic acid also increased significantly(p<0.05).(3)In naturally fermented sourdough and yeast fermented sourdough,compared with sourdough without added casein hydrolysate,the content of reducing sugar increased by 4.5% and 15.4%,respectively.Casein hydrolysate increased the enzyme activity in naturally fermented sourdough and yeast fermented sourdough by 23.5% and 26.3%,respectively,and the degradation degree of protein was also significantly improved.Casein promotes the production of proteases by microbes such as lactic acid bacteria in sourdough,which improves protein degradation and has a greater impact on yeast fermented sourdough.Secondly,through scanning electron microscopy pictures and TPA analysis of sourdough,the effect of adding casein hydrolysate on the quality characteristics of sourdough was studied.After adding casein hydrolysate,the hardness of naturally fermented and yeast fermented sourdough was significantly reduced,and the viscosity,elasticity and adhesiveness were significantly increased.After adding casein hydrolysate into the naturally fermented sourdough,the granules became smoother,the gaps between the starches became smaller,and the gel structure became tight.In the yeast fermented sourdough,the granules became spherical and the starch gel structure became loose.Therefore,adding casein hydrolysate to naturally fermented sourdough has better microstructure of sourdough.Again,high-throughput sequencing technology was used to analyze the changes in bacterial flora structure in sourdough.After adding casein hydrolysate to naturally fermented sourdough,the relative abundance of Firmicutes in sourdough increased to become the dominant bacteria(76%).After adding casein hydrolysate to yeast fermented sourdough,the relative abundance of thick-walled phylum and Proteobacteria increased significantly,while the relative abundance of cyanobacteria decreased,but it was still the dominant phylum.At the genus level,after adding casein hydrolysate,Pediococcus in the naturally fermented sourdough became the absolute dominant genus)The relative abundance also increased significantly;and in yeast fermented sourdough Pediococcus,Enterococcus,Lactobacillus,Streptococcus,Enterobacter and Pantoea all were increased.Therefore,the addition of casein hydrolysate can promote the growth of lactic acid bacteria in sourdough,and the effect is more significant in naturally fermented sourdough.In addition,the relative abundance of Paenibacillus,Brevibacillus,and other contaminants in the sourdough added with casein hydrolysate was significantly higher than that of the control group.The addition of casein hydrolysate increased the diversity of bacterial flora in naturally fermented sourdough.Finally,the Spearman correlation analysis was used to study the relationship between the fermentability of sourdough and the metabolite of lactic acid bacteria and the bacterial flora.The results showed that Pediococcus,Enterococcus,and Paenibacillus were significantly positively correlated with reducing sugar content,protease activity,fermentability,lactic acid content,and TTA in sourdough,and showed a negative correlation with p H.This result indicates that Pediococcus,Enterococcus and Bacillus play an active role in the fermentation of sourdough;Brevibacillus is significantly positively correlated with lactic acid content and is related to the increase of acidity,there is a significant negative correlation between Bacillus and p H.In addition,Brevibacillus and Devosia are adapted to grow in an acidic environment,and the acidic environment with high acidity can inhibit the growth of Pantoea.In summary,the addition of casein hydrolysate changes the structure of bacterial flora in sourdough,increases the relative abundance of lactic acid bacteria,improves the metabolic activity of lactic acid bacteria,increases the fermentation capacity of the system,accelerates the acidification of sourdough,and allows the fermentation time shorten it by at least 3 h.At the same time,the addition of casein hydrolysate also improve the texture and microstructure of the sourdough,which has a positive impact on the quality of sourdough. |
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