Moisture transfer-driven quality enhancement in solid-state fermented Daqu: Synergistic effects of microbial...

Solid-state fermented Daqu exhibits a typical heterogeneous structure, where moisture regulates the microbial activity by driving gas diffusion in the pores and nutrient enrichment, playing a crucial role in the quality of the final product. However, there is a lack of clarity regarding how moisture transfer affects Daqu microbial assembly and metabolic flux. This study pioneered a real-time controllable fermentation platform, employing stoichiometry, nuclear magnetic resonance, and metagenomics to investigate microbial saccharifying metabolic functions under moisture transfer regulation. Comparing representative low (LM: 34%, 36%) and high (HM: 38%, 40%) moisture groups, we found that porosity exhibited a strong positive correlation with water activity (coefficient > 0.9, p < 0.01), serving as the primary physicochemical contributor governing moisture transfer priority. Furthermore, steady-state mass transfer in the HM group (≥ 38%) enhanced the transfer rate from free water (T23) to capillary water (T22: 10-100 ms), shaping a saccharifying functional microbial community dominated by Rhizopus and Bacillus. Weighted network and functional gene predictions indicated that this process strengthened the substrate preference of core microorganisms toward starch, significantly reinforcing the metabolic synergy between glucoamylase and α-amylase. Conversely, transient mass transfer in the LM group (< 38%) triggered microbial functional differentiation, promoting the redistribution of non-starch polysaccharide hydrolases. Our research revealed the effects of moisture transfer on nutrient availability, microbial adaptation, and metabolic functions in stack-fermented Daqu. This work ensures Daqu stability and presents novel strategies to optimize solid-state fermentation efficiency through moisture-driven microbial metabolic trade-offs.