Study On The Microbial Community Structure And Flavor Quality Of Mixed Fermented Bean Curd

As a kind of fermented food with unique flavor,Sufu with many types is a soy product produced by synergistic fermentation of a variety of microorganisms.The selection of raw materials and fermentation strains determines the fermentation process and nutritional quality of Sufu.In this study,two high-protease strains were screened and identified from commercially available Sufu.Used black beans(BS)and chickpeas(CS)to compound with soybeans as ingredient respectively,inoculated the isolated high-yield protease strains and Actinomucor elegans,three strains to produce Sufu(Sufu products made by mixing two or more different beans),and optimized the pre-fermentation conditions of the two Sufu with mix beans,Carry out the systemic production of Sufu under optimal conditions.On this basis,the physical and chemical properties,texture characteristics and sensory characteristics of the finished Sufu with mix beans were measured at different fermentation stages of the two kinds of Sufu,and compared with the commercially available Sufu,the advantages of the Sufu in terms of nutritional quality were determined.Finally,the microbial diversity of different fermentation stages and the content of flavor substances in the post-fermentation stage were determined,and their correlation was analyzed.This research lays a theoretical foundation for the development of a new type of Sufu and the utilization of beans.The main findings are as follows:(1)Isolation and identification of high-protease-producing strains.Used commercially available Sufu as a sample,through microscopic examination of morphological identification,protease activity determination,and sequence comparison,a high-yield protease strain for fermentation was finally screened out.Rhizopus microsporum SE-3,was finally determined.The protease activity was 45.73±5.74 U/m L,the Rhizopus oryzae CD-1 protease activity was 31.83±6.01 U/m L.Combined with Actinomucor elegans,the three strains were combined for fermentation.(2)Research on the pre-fermentation technology of Sufu with mix beans.Used single factor test and response surface test,with protease as a reference indicator,the best pre-fermentation conditions for the three strains to ferment two kinds of Sufu were determined.BS Sufu: fermentation time 5 d,inoculation amount 11%,fermentation temperature28 ?;CS Sufu: fermentation time 5 d,inoculation amount 10%,fermentation temperature28 ?,the protease activities at this time were 189.41 U/g and 182.65 U/g.We set the ratio of the three strains to 1:1:1 and combined with Actinomucor elegans,the three strains were combined for fermentation.(3)Physical and chemical properties of Sufu with mix beansThe protein content of the two kinds of Sufu with mix beans gradually decreased from the beginning of fermentation,while the total acid and amino acid nitrogen content were hoisted.The amino acid nitrogen content of BS Sufu was significantly higher than CS Sufu,which proved that the post-fermented microorganisms catabolism is more complete in BS Sufu.The p H of the finished Sufu was slightly lower than that of commercially available Sufu,which proves that the organic acid content in miscellaneous Sufu was relatively high.The hardness of the mixed tofu fermented for 60 days was slightly lower than that of the commercially available Sufu,indicated that the structure of Sufu with mix beans was different from that of the commercially available Sufu.The chewiness of the two kinds of Sufu showed an upward trend and then a downward trend.The overall characteristics of the texture are somewhat different from those of commercially available Sufu.Through sensory evaluation,the aroma of BS Sufu is very prominent,while CS Sufu has more advantages in taste and color.(4)Flavor ingredients of Sufu with mix beansThe flavor compounds of BS Sufu were extracted by HS-SPME,and 78 volatile components were obtained by GC-MS analysis,mainly including 9 alcohols,31 esters,3aldehydes,7 ketones,2 acids and 8 olefins,7 alkanes,3 ethers and 8 other organic compounds.A total of 105 volatile compounds have been identified in different stages of CS Sufu,including13 alcohols,48 esters,7 aldehydes,4 ketones,4 acids,8 olefins,3 ethers,9 alkanes and 9 other compounds.The content of ester compounds is the highest of the two sufu,followed by alcohol compounds.17 kinds of free amino acids were detected in the two kinds of Sufu,and the content of sweet and umami amino acids was the highest.(5)Analysis on the microbial diversity of SufuThrough the Alpha diversity analysis,the bacterial diversity of the two kinds of Sufu is higher than the diversity of fungi,the main phyla which are Firmicutes,Proteobacteria and Mucor of Sufu.The more abundant genus-level species in BS Sufu mainly include Enterobacter,Enterococcus,Actinomucor and Rhizopus.However,the abundance of CS Sufu,Enterobacter,Acinetobacter,Pseudomonas,and Mucor is higher.Through the correlation network analysis,the correlation between the two Sufu bacteria genera is more uniform,and the fungal correlation is more staged.(6)Correlation analysisBased on CCA analysis,the content of amino acid nitrogen(Amino)has a stark positive correlation with Enterobacter;Monascus has a significant positive correlation with water,ash,and reducing sugar content.Rhizopus and Actinomucor were significantly and positively correlated with protein content.The correlation heatmap shows that all amino acids are positively correlated with remarkably with Monascus,Saccharomyces,Wallemia,Millerozyma and Xeromyces.There is also a significant correlation between volatile compounds and microorganisms.