Dissecting the Relationship between Microbial Community Succession and Pyrazine Metabolite Synthesis in Nongxiangxing Daqu

The correlation between the stages of pyrazine synthesis and the microbial flora during the making of Nongxiang Daqu was analyzed, and the effect of microbial interactions on the synthesis of pyrazine during Daqu making was explored. Headspace solid phase microextraction (HS-SPME) combined with gas chromatography-mass spectrometry (GC-MS) and high-throughput sequencing were used for the analysis of the changes in pyrazine contents and the microbial community structure during Daqu making, respectively. Principal component analysis (PCA) and linear discriminant analysis (LDA) were used for finding out differential microorganisms. The correlation between differential microorganisms and pyrazine metabolites was evaluated using Spearman’s correlation coefficient, and a correlation network between these pyrazine metabolism-related microorganisms and the microbial flora of Nongxiangxing Daqu was constructed. The results showed that nine pyrazine metabolites were detected during Nongxiangxing Daqu fermentation, and there were significant differences in the number and types of pyrazine metabolites between the stages of 1–5 and 7–28 days. The microbial community succession could be divided into two stages: A (1–5 days) and B (7–28 days). At stage A, the differential fungi were mainly Kodamae and Candida, and the differential bacteria were mainly Kosakonia, Pantoea and Methylophilus; at stage B, the differential fungi were mainly Thermoascus and Thermomyces, and the differential bacteria were mainly Thermoactinomyces and Saccharopolyspora. The synthesis of pyrazine metabolites during Nongxiangxing Daqu fermentation was positively correlated Cladosporium and Lichtheimia, but negatively correlated with Thermomyces and Fusarium. These pyrazine metabolism-related microorganisms were positively correlated with most other Nongxiangxing Daqu microorganisms, but showed negative correlation with only a few microorganisms, such as Thermomyces. The results of this study provide a theoretical basis for the analysis of the mechanism of microbial synthesis of pyrazine metabolites in Nongxiangxing Daqu.