Aldehyde metabolism in Maotai-flavor Baijiu: insights from integrated metagenomic and metaproteomic analyses.

Acetaldehyde and acetal are crucial to the flavor of Maotai-flavor Baijiu, affecting aroma release and sauce-aroma traits. Targeted control of acetaldehyde metabolic flux is critical for stabilizing base liquor quality. However, the diversity of acetaldehyde-metabolizing enzymes and their microbial drivers remain uncharacterized, critically impeding precision control. This study selected the third production round to systematically investigate acetaldehyde accumulation dynamics and metabolic mechanisms. Time-resolved profiling showed that acetaldehyde and acetal concentrations display an initial rise, followed by a decline and stabilization. By integrated metagenomic and metaproteomic analyses, we confirmed that the metabolic network comprised two biosynthetic and three conversion pathways. The dominant biosynthetic pathway featured pyruvate decarboxylase (PDC)-catalyzed decarboxylation, while ethanolamine ammonia-lyase contributed minimally. Among conversion pathways, NADH-dependent alcohol dehydrogenase (NADH-ADH) reduction to ethanol predominated, with lower-flux oxidation to acetate and acetyl-CoA. Dynamic enzyme profiling revealed that PDC activity preceded the activation of NADH-ADH during stacking fermentation, resulting in a temporal mismatch between aldehyde production and consumption, which led to metabolic retention. During pit fermentation, acetaldehyde was more efficiently converted to ethanol through upregulation of NADH-ADH and NADH/NAD+ ratio. As main contributors of PDC and NADH-ADH, Schizosaccharomyces pombe and Saccharomyces cerevisiae exhibited dual regulatory roles in acetaldehyde metabolism, with their metabolic mode shifts governed by dissolved oxygen and the NADH/NAD+ ratio. Microbial interaction analysis and simulated fermentation confirmed that Pichia kudriavzevii synergized with Saccharomyces cerevisiae and Schizosaccharomyces pombe, driving rapid acetaldehyde accumulation during the stacking fermentation. These findings establish a theoretical framework for optimizing brewing processes and enhancing base liquor quality.