| Dazu Dongcai is a specialty in Sichuan and Chongqing,China.It is rich in essential amino acids,proteins,vitamins and other nutrients required by the human body.At present,Dazu Dongcai is still produced by traditional manual production.Uncontrollable factors such as extensive management and long production cycle hinder the large-scale development of Dazu Dongcai industry.Therefore,it is urgent to systematically and deeply study the whole fermentation system of Dazu Dongcai.In this paper,the changes of microbial community structure in different fermentation periods of Dazu Dongcai were comprehensively analyzed by metagenomic sequencing technology,and the quality indexes were detected and analyzed by relevant national detection standards.Finally,the correlation analysis model was established by the abundance of microbial flora and quality indexes,and the main microbial genera affecting the quality of Dazu Dongcai were analyzed.The main research contents and results are as follows :1.Analysis of microbial flora structure of Dazu Dongcai(1)The results of microbial species annotation in samples by metagenomic sequencing technology are as follows:According to the results of species annotation,the base mass values greater than 20(Q20)and 30(Q30)were more than 97%,the proportion of GC was51.8% and 57.91%,and the effective sequence was greater than 59.53%,indicating that the detection data was reliable.Venn diagram analysis showed that F2017 contained the largest number of OTUs,with 2829 OTUs.The number of unique OTUs in different samples was 302,151,199,136 and 321,and the number of total OTUs was 502.(2)The results of metagenomic sequencing technology on the dominant flora and relative abundance in the samples are as follows:According to the relative abundance distribution of species,the dominant bacteria in Z2021 and O2020 were Cyanobacteria at the initial stage of fermentation,with relative abundance of 58.56% and 57.73%,respectively.In the middle stage of fermentation,T2019 was Firmicutes and Proteobacteria,with relative abundance of 27.71% and 37.88%.In the later stage of fermentation,TH2018 and F2017 were Firmicutes,with relative abundance of35.15% and 50.69%.2.Quality analysis of Dazu Dongcai(1)Nitrite is an important indicator of harmful substances in fermented food.The results showed that the nitrite content decreased during the fermentation of Dazu Dongcai.There was a slight decrease in the first three years of fermentation,and the F2017 decreased by 80% compared with the TH2018,and the nitrite content of F2017 was only 0.26 mg/kg.(2)Crude fiber and total sugar are the sources of energy provided by microbial flora.The results showed that the crude fiber content decreased,while the total sugar content increased.The crude fiber content and total sugar content in Z2021 reached 12.07% and 3.6%,respectively,and 10.47% and5.58% in F2017.(3)Protein and amino acids are important indicators of nutritional value of Dazu Dongcai.The results showed that the contents of protein,total amino acids and essential amino acids decreased.The protein in Z2021 reached5.66g/100 g,and that in F2017 was 3.98g/100 g,which was 30% lower than that in Z2021.However,the content of essential amino acids(EAAs)in total amino acids(TAAs)showed an upward trend,accounting for about 28%-30 % in the early stage of fermentation,and the proportion of EAAs/TAAs gradually increased to 33%-36% in the later stage of fermentation and remained stable.3.Microbial metagenomic analysis(1)In order to deeply analyze the microbial community diversity of Dazu Dongcai,Alpha Diversity and Beta Diversity were analyzed respectively.The results showed that the observed species,Chao 1 and Shannon index increased with the increase of fermentation time.The dilution curve tended to be stable,indicating that the sequencing depth was relatively sufficient to cover bacterial diversity.The span of hierarchical clustering curve gradually increased,indicating high species richness.PCA analysis could not separate samples at different fermentation stages.The overall distribution of PCoA was relatively diffuse,indicating that the community structure was different in different fermentation periods.In the UPGMA cluster tree,Z2021 and O2020 had strong community structure similarity,so did TH2018 and F2017,while T2019 had strong community structure similarity with the other four years.It can be inferred that the microbial flora in T2019 sample plays an important role in the whole fermentation process.(2)In order to deeply analyze the differences between the microbial communities of Dazu Dongcai,box plot analysis was performed.The results showed that the OTUs index,Chao1 index and Shannon index of Z2021 showed an upward trend,and the OTUs index of Z2021 was significantly different from that of the other four samples.Through LEfSe analysis,it was found that there were significant differences in 30 microorganisms.Z2021 contained 6microorganisms,T2019 contained 8 microorganisms,TH2018 contained 4microorganisms,and F2017 contained 12 microorganisms.4.Correlation analysis between microbial flora and qualityIn order to explore the effect of main microbial genera on the quality of Dazu Dongcai during fermentation,the correlation model between the relative abundance of microbial genera and quality indicators was established by Spearman correlation analysis.The results showed that Staphylococcus,Lactobacillus,Terrisporobacter,Blautia,Romboutsia,Bifidobacterium and Streptococcus were dominant in the whole fermentation process,and were strongly correlated with quality-related indicators. |
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