Cellar age reshapes Huangshui micro-ecosystem and metabolism to drive flavor formation in strong-aroma baijiu.

Huangshui, a slurry-like liquid exuded during strong-aroma Baijiu fermentation, serves as the core medium for material exchange between pit mud and fermented grains. However, how its micro-ecosystem evolves with cellar age and drives flavor formation remains unclear. Using Huangshui as a dynamic window, this study integrated metagenomics, metabolomics, and flavoromics to compare its temporal dynamics in new and old cellars over a complete fermentation cycle, systematically characterizing how cellar age is associated with the restructuring of the microbial community and metabolic functions of Huangshui, and how these changes are consistent with the flavor profiles observed in the final base liquor. The results showed that Huangshui from old cellars harbored a more diverse and stable microbial community, forming a syntrophic consortium of caproic acid-producing bacteria (Caproicibacterium, Caproiciproducens), syntrophic bacteria (Syntrophomonas), and methanogenic archaea (Methanosarcina), whereas new cellars were dominated by lactic acid bacteria (Acetilactobacillus). Metabolically, the old-cellar community exhibited a clear phase-dependent division. During the acid-producing phase, the TCA cycle, arginine biosynthesis, and pyruvate metabolism were preferentially activated to generate core precursors; during esterification, butanoate metabolism and acyl-CoA supply pathways were enhanced. This orderly shift was associated with higher concentrations of ethyl caproate and ethyl octanoate in old-cellar base liquor. Functional gene analysis revealed coordinated upregulation of chain-elongation, methanogenic, and acetate-activating pathways in old cellars. Network analysis revealed a tightly coupled caproic acid-producing co-occurrence module in old cellars, which was not observed in new ones. Together, these findings suggest that Huangshui may serve as a rapid proxy for assessing both the fermentation status and the maturity level of the cellar. This work identifies potential bioaugmentation targets to accelerate flavor development in new cellars and provides a theoretical basis for the precise micro-ecological management of strong-aroma Baijiu quality.