Abstract: This study aimed to explore the impact of pit mud (PM) on caproic acid biosynthesis by the microbiota in Huangshui (HS), a by-product of solid-state fermentation in baijiu production. PM and HS from the late fermentation stage of nongxiangxing baijiu were used to design a PM-HS micro-fermentation system and an HS micro-fermentation system which were used to carry out four consecutive rounds of fermentation. A medium with lactic acid as the sole carbon source was used to select the core functional microorganisms degrading lactic acid. Then, 16S rRNA gene amplicon high-throughput sequencing, physicochemical analysis, redundancy analysis (RDA), and functional prediction using Picrust2 were conducted to compare the enrichment of dominant caproic acid-producing procaryotes and their potential caproic acid synthesis functions between the two fermentation systems. The results showed that the mixed fermentation system was beneficial to the enrichment of Caproiciproducens, with Caproicibacterium lactatifermentans LBM19010 being the dominant caproic acid producer. This consortium showed a strong ability to degrade lactic acid to produce caproic acid in the presence of glucose. On the other hand, the HS system was beneficial to the enrichment of the potential butyric acid-producing genus Clostridium_sensu_stricto_12, with Clostridium tyrobutyricum ATCC 25755 being the predominant strain. This strain showed a strong ability to degrade lactic acid into butyric acid in the presence of glucose. Both fermentation systems enriched strains able to degrade lactic acid into butyric acid. In the fermentation system of nongxiangxing baijiu, the microbial genera producing caproic acid were not the same as those degrading lactic acid. This study provides theoretical support for further research on synergistic metabolism of caproic acid-producing and butyric acid-producing microorganisms and also provides a scientific basis for the targeted enrichment of caproic acid-producing and butyric acid-producing microorganisms.

Key words: nongxiangxing baijiu; pit mud-Huangshui micro-fermentation system; Huangshui micro-fermentation system; prokaryotic community; caproic acid production; lactic acid degradation