Studies On The Microbial Community Structure And Physicochemical Properties Of Rice Spontaneous Fermentation Sourdough And The Baking Quality Of Sourdough Bread

In recent years,the demand for gluten-free foods has increased year by year with the rising incidence of gluten-related diseases.However,the lack of gluten makes it difficult to form a network structure when producing gluten-free bread,resulting in a smaller final product,firmness and poor organoleptic properties.The multiple metabolites produced during the fermentation of sourdough work in synergy to improve the quality and flavor of bread.In this study,indica rice flour,japonica rice flour and glutinous rice flour were used as raw materials to obtain rice sourdough through natural fermentation,and the variation of microbial diversity and physicochemical properties in the fermentation process of rice flour sourdough was explored.The sourdough at different fermentation times was added to the rice dough with different additions to make rice bread.And the effects of rice sourdough on the rheological characteristics of rice dough and the baking characteristics and flavor characteristics of rice bread were explored.The main findings are as follows:(1)Microbiome analysis showed that among the three kinds of rice sourdough,Proteobacteria was the dominant bacteria in the whole fermentation process,while Firmicutes gradually became the dominant bacteria during the fermentation process.And Leuconostoc was the dominant bacteria in the late fermentation stage of the three rice sourdoughs.Through the prediction of metabolic pathways based on sequencing results,it was found that the synthesis pathways of amino acids,carbohydrates and aromatic compounds,as well as the degradationrelated pathways of carbohydrates,were enriched,and the changes of these compounds had a positive effect on the texture and flavor characteristics of the final product.In terms of quantity,the number of up-regulated metabolic pathways in the samples of indica rice sourdough(IRS)and japonica rice sourdough(JRS)was greater than that of glutinous rice sourdough(WRS)samples,indicating that the metabolic activity of microorganisms during WRS fermentation was lower than that of the other two acid doughs.(2)The analysis of biochemical characteristics showed that during the natural fermentation process,the p H value of the three rice sourdoughs showed a downward trend,while the accumulation of organic acids increased,which was mainly attributed to the increase in the number of lactic acid bacteria in the sourdough system.And the amount of acetic acid produced in the three rice sourdoughs is always lower than lactic acid,which is conducive to obtaining a softer dough.The content of total polysaccharides showed a slow upward trend,and the content of reducing sugars increased significantly,indicating that microorganisms grew and metabolized vigorously in sourdough.The content of protein showed a downward trend,indicating that the protein was hydrolyzed and a large number of free amino acids were released.After 24 h of fermentation,the content of almost all free amino acids in IRS,JRS and WRS increased significantly,and the contents of glutamic acid,alanine,arginine and asparagine were significantly higher than those of other amino acids,which mainly showed sweetness and umami.In addition,proline,the main precursor of bread’s key aroma compound,accumulates significantly at the end of fermentation.(3)The rheological characteristics analysis of rice dough showed that JRD and WRD with72 h acid dough added had the lowest G’ and G’’,indicating that the fermentation of JRS and WRS reduced the viscoelasticity of the corresponding dough.However,the G’ of WRD is much lower than that of IRD and JRD,indicating that the rheological properties of WRD are more liquid-like and less formable.The analysis of baking characteristics showed that the specific volume of rice bread showed an upward trend with the increase of fermentation time of the added sourdough,and the higher the amount of sourdough added,the greater the specific volume of bread.However,adding 0-30% WRS to WRB found no significant difference in specific volume.Indica rice bread and japonica rice bread obtained by adding fermented 48 h sourdough had the best quality characteristics,the highest chewiness,cohesion,resilience and the lowest hardness.Glutinous rice bread has high viscosity and poor overall structure,making it not suitable for making gluten-free bread.(4)GC-MS detected a total of 81 volatile flavor substances,mainly alcohols,aldehydes and ester compounds.The fermentation of sourdough changes the composition of volatile compounds of rice bread,and the content rises with the increase of fermentation time of added sourdough.Among them,phenethyl ethanol,ethyl octanoate and ethyl laurate are considered to be key aroma components.In summary,microorganisms during the fermentation of sourdough positively affect the baking properties and flavor characteristics of gluten-free bread by producing different metabolites.Choosing the right sourdough fermentation time and addition amount is a key factor in maintaining the quality of gluten-free bread.