Gradient-Regulated Acid–Alcohol–Ester Metabolism in Stress-Tolerant Functional Units During Chinese Baijiu Solid-State Fermentation

Chinese Baijiu solid-state fermentation operates as a heterogeneous reaction bed in which heat, moisture, O2, acidity, ethanol, and mass-transfer conditions vary across space and time. Because aroma-related microorganisms respond differently to these local stresses, abundance data alone cannot identify which populations remain active or contribute to ester formation at a particular layer, stage, or batch. This review therefore focuses on stress-tolerant functional units that sustain acid–alcohol–ester conversion and argues for spatially resolved sampling combined with activity-, enzyme-, and metabolite-based evidence. As a conceptual verification platform, rather than a reported experimental apparatus or a replacement for traditional pits, a gradient solid-state bioreactor could reproduce selected temperature, O2, moisture, and interface gradients under controlled conditions. Coupling multipoint temperature, CO2/O2, moisture, and acidification-related signals with spectral fingerprints and volatile measurements, together with offline biological assays, could help define functional windows and improve the mechanistic reproducibility of Baijiu fermentation.