Effect of Microbial Community in Artificial Pit Mud on the Formation of Flavor Metabolites during the Fermentation of Nongxiangxing Baijiu

In this study, a small-scale simulated fermentation system of fermented grains (Jiupei in Chinese) without pit mud (PM) was used as a control, and the differences in the microbial community and metabolite components between the PM and control groups were compared. The results indicated that PM significantly enhanced the contents of major flavor compounds in Jiupei including butyric acid, caproic acid, ethyl acetate, ethyl butyrate, and ethyl caproate, and reduced the contents of lactic acid and ethyl lactate. For both groups, the relative abundance of Lactobacillus, one of the dominant bacteria in the upper and bottom layers of Jiupei, significantly increased at the initial stage, and showed a slight difference between them at the end of fermentation. The decay period of Lactobacillus in the PM group was about 15 days earlier than that in the control group. At the same time, the relative abundance of Rhodococcus in the control group, and the relative abundance of Kroppenstedtia, Clostridium_sensu_stricto_12, and Acetobacter in the PM group increased. The abundance of most of the enzymes in the Embden-Meyerhof-Parnas (EMP) pathway, as well as that of caproic acid synthase (EC 1.3.1.38, EC 2.3.1.16, EC 6.2.1.1) and butyric acid synthase (EC 2.3.1.9, EC 2.8.3.8) increased during fermentation, and the abundance of these enzymes was significantly higher in the bottom layer of Jiupei than in the upper layer. The abundance of lactate dehydrogenase (EC 1.1.1.27), which uses lactic acid as a substrate, increased in the PM group. These findings reveal the contribution of PM to the microbial communities and metabolite components of Jiupei.