Revealing the Functional Microbiota of Caproic Acid-Producing and Lactic Acid-Utilizing Bacteria in the Pit Muds for Chinese Nong-Xiang Baijiu Fermentation

Low ethyl caproate and high ethyl lactate contents pose a significant challenge in producing Chinese <i>nong-xiang baijiu</i>. The formation of these esters depends on the metabolism of their precursors-caproic acid and lactic acid-within the pit mud (PM) microbiome. However, the specific taxa driving the metabolic flux from lactate accumulation to caproate synthesis remain unclear. This study aimed to identify potential functional microbes capable of caproate biosynthesis and lactate utilization by systematically analyzing PM samples from the upper, middle, and lower layers of three different pit ages (0, 20, and 50 years). Results showed that 50-year-old PM exhibited significantly higher caproic acid and ammonium nitrogen levels, but lower lactic acid content, compared to the 0- and 20-year-old counterparts. Notably, <i>Petrimonas</i>, <i>Caproiciproducens</i>, and <i>Sedimentibacter</i> were significantly enriched in the 50-year-old PM. Their relative abundances correlated positively with caproic acid and negatively with lactic acid. Furthermore, PICRUSt2 analysis indicated higher abundances of genes associated with caproate synthesis and lactate utilization in the 50-year-old microenvironment. We propose that <i>Petrimonas</i>, <i>Caproiciproducens</i>, and <i>Sedimentibacter</i> are potential functional candidates for lactate degradation and caproate generation. These findings provide a scientific basis for modulating the microbiome for "increasing ethyl caproate and reducing ethyl lactate", thereby enhancing <i>baijiu</i> quality.