Unveiling the Acidic Backbone of Baijiu: A Comprehensive Review on Composition, Metabolic Pathways, and Quality Implications

Acids constitute fundamental molecular determinants that shape the sensory profile, typicity, and overall quality of Baijiu, a traditional Chinese distilled spirit. In this review, the latest advances in the studies of acidic compounds in Baijiu are systematically summarized within the framework of the "composition-function-microbe interaction" chain, with particular emphasis placed on the relationships among acid metabolism, flavor formation, microbial ecology, and product quality stability. We first catalog the diverse acid profiles in Baijiu, highlighting the four major acids (acetic, lactic, butyric, and caproic acids), and their dynamic evolution during product aging. In addition, the roles of acids in sensory perception, aroma regulation, and quality evaluation are comprehensively discussed, while their potential health-related implications are briefly and critically appraised. Crucially, we elucidate the microbial origins of these acids, detailing key metabolic pathways including glycolysis, amino acid catabolism, and reverse β-oxidation. Current studies have demonstrated that microbial community interactions play critical roles in shaping acid metabolism and acid-ester balance, although the underlying mechanisms remain insufficiently resolved. This review concludes by identifying critical knowledge gaps regarding in situ metabolic flux quantification and predictive modeling of microbial interaction networks governing acid-flavor compound dynamics. Future studies integrating multiomics approaches, ecological modeling, and targeted metabolic regulation are expected to facilitate the transition of Baijiu fermentation from descriptive characterization toward predictive and controllable bioprocesses.