Lactic acid bacteria in Baijiu fermentation: Diversity, ecological filtering, metabolic functions, and targeted...

Lactic acid bacteria (LAB) are widespread across the interconnected ecological niches of Baijiu production and frequently become enriched during middle and late fermentation, yet their high relative abundance should not be equated with realized metabolic contribution. This review integrates species-resolved evidence from Jiuqu starters, fermented grains, pit mud, and major Baijiu flavor types to examine LAB diversity, succession, ecological filtering, metabolic functions, and targeted modulation. LAB succession follows process-specific trajectories rather than a universal pattern. Sauce-flavor and some strong-flavor fermentations may converge toward Acetilactobacillus jinshanensis, whereas Lactobacillus acetotolerans, Lactobacillus helveticus, and several Limosilactobacillus, Pediococcus, and Weissella species show distinct temporal and niche-dependent distributions in other systems. Progressive acidification is a major selective event whose effects are modified by ethanol, oxygen depletion, temperature, water availability, substrate turnover, and microbial interactions. Intracellular pH homeostasis, membrane remodeling, proteostasis, and amino acid-dependent buffering support acid adaptation. LuxS/autoinducer-2 (AI-2)-associated responses represent an underexplored regulatory layer, although their causal contribution to Baijiu community convergence remains unverified. Functionally, LAB redistribute carbohydrate-derived carbon through lactic and acetic acid production, supplying precursors for esterification and cross-niche anaerobic chain elongation. Evidence for effects on other flavor compounds, higher alcohols, and sulfur compounds is more context-dependent. Ecological dominance does not necessarily correspond to high lactate productivity or large in situ metabolic flux. Future studies should integrate strain-resolved genetic perturbation, absolute abundance, in situ activity, stable-isotope tracing, and defined-community experiments to quantify LAB contributions and enable predictable strain addition and microbial-community design.