Metagenomic and physicochemical insights into ultrasound-assisted fermentation: Reprogramming Komagataeibacter intermedius metabolic efficiency for astringency control.

Oral astringency remains a major challenge limiting consumer acceptance of functional fermented beverages. Conventional mitigation strategies often reduce astringency at the expense of bioactive polyphenols, highlighting the need for non-destructive quality improvement approaches. In this study, low-power ultrasound-assisted fermentation (20 kHz, 240 W, 16.68 min/day) was applied to Kombucha, and the underlying mechanisms of astringency alleviation were investigated through integrated physicochemical characterization, metagenomic analysis, and temporal data modeling. Ultrasound significantly reduced sensory astringency and saliva-protein precipitation (OD650, p < 0.05) by Day 7 while preserving monomeric catechins, thereby maintaining antioxidant potential. Small-angle X-ray scattering (SAXS) and spectroscopic analyses revealed that ultrasound promoted the accumulation of water-soluble polysaccharides (1.69 mg/mL on Day 7, p < 0.01), which acted as macromolecular steric barriers and modified the aggregation behavior of polyphenol-salivary protein complexes. Consequently, the radius of gyration (Rg) increased to 608.67 nm, suppressing the formation of large aggregates associated with astringency perception. Furthermore, temporal machine-learning and time-lagged correlation analyses indicated that the reduction in astringency was associated not with increased abundance of Komagataeibacter intermedius, but with upregulated specific metabolic activity during the early fermentation stage. This early-stage metabolic adaptation was strongly associated with subsequent polysaccharide accumulation and sensory improvement. The findings suggest that low-power ultrasound can modulate microbial metabolic activity without altering microbial community structure, thereby influencing polysaccharide biosynthesis and colloidal interactions during fermentation. These results provide new insights into the biological and physicochemical mechanisms underlying ultrasound-assisted fermentation.