Microbial Community Analyses Of Naturally Fermented Sufu And The Application And Screening Of Fermentation Strains

Sufu is a kind of fermented soybean food,which originated in China.Traditional sufu belongs to natural fermentation,which has complex microbial composition and is easy to be influenced by environment,personal experience and other factors,therefore,the flavor quality stability of sufu is poor.Most of the modern sufu manufacturers use artificial inoculation to ferment sufu,but because the enzyme system is relatively single,compared with the traditional sufu,the fermented beancurd produced by artificial inoculation pure mucor is plain in taste and not good in aroma,and it is increasingly unable to meet the dietary needs of modern people.In this paper,the representative sufu with excellent flavor in Baixi area was studied,and the microbial composition and changes in the fermentation process of Baixi sufu were comprehensively analyzed by high-throughput sequencing technology and culture-based methods.In addition,the related strains were used in the fermentation of sufu.After the screening in pre-fermentation and the verification of the fermentation performance in post-fermentaton,a group of optimal mixed fermentation strains whf-2020 was developed to improve the flavor of sufu.The main research results are as follows:(1)The microorganisms in naturally fermented sufu were analyzed based on the culture method.The dominant bacteria in naturally fermented sufu were LAB(lactic acid bacteria)and Bacillus,and the dominant fungi were mucor and Geotrichum candidum,and the total number of bacteria was higher than that of fungi.The content of coliforms in the early stage of fermentation was very high,especially in the mucor pehtze up to 8.24 lg CFU/g.The number of spore-forming bacteria has been maintained at 10~5?10~6 CFU/g,most of which belongs to Bacillus cereus group,suggesting potential safety risks of traditional Baixi sufu.(2)The relationship between microorganisms and sufu was analyzed based on high-throughput sequencing and correlation analyses.A total of 283 bacteria and 464 fungi were identified by Illumina Miseq sequencing.The microbial richness and diversity were far higher than those based on traditional culture methods.Among them,the diversity of bacteria was higher in the pre-fermentation stage,and the fungal composition and diversity were different except in the white stage The correlation analyses showed that Enterococcus and Bacillus were negatively correlated,the fungal network showed several independent structures,and Geotrichum candidum had significant positive correlated with amino nitrogen(P<0.05).(3)Multiple combinations of strains for sufu fermentation were screened based on the activity of protease and lipase.A total of 3 high-protease-producing strains BM4(M2),MD(M3),and ZM3(M4)were obtained through screening(M1 is Actinomucor elegans).Compared with single mold fermentation,the sufu with double molds could significantly increase the lipase activity of pre-fermentation,especially M1M3 and M2M3.The protease activity of strains M1M3Y and M2M3Y combined with Geotrichum candidum was no less than that of the double mold,and the lipase activity was higher.(4)The fermentation performance of the mixed fermentation strains was verified based on sensory sense and physicochemical characteristics.The content of amino nitrogen of sufu inoculated with mixed strains was significantly higher than that with single mucor.Moreover,the amino nitrogen content of sufu inoculated with M2M3Y was nearly one times higher than that of the control group.Mixed fermentation of multiple strains can accelerate the fermentation of sufu and shorten the fermentation time.In the late stage of fermentation,the sufu inoculated with M1M3Y had a heavy sour taste and did not form a typical flavor.Strain combination M2M3Y fermented sufu has high amino nitrogen content,good flavor,low total acid content and fine texture,so it is selected as the optimal mixed fermentation strain combination.(5)The molecular weight distribution of sufu by the optimal mixed fermentation strains was analyzed.There was a certain similarity in the peptides composition between the sufu inoculated with M2M3Y and the control group(fermented by Actinomucor elegans).Peptides with molecular masses of approximately 447 Da,657 Da and 569 Da are present in the two sufu samples.The molecular weight of peptides in the sufu inoculated with M2M3Y was mainly below 1000Da,while the molecular weight distribution in the control was higher.The strain combination M2M3Y was renamed whf-2020.