Unraveling the dynamic flavor profile of Tongchuan Douchi: an integrated multi-omics and flavor characterization.

Tongchuan Douchi is a representative Mucor-type fermented soybean product, yet the dynamic evolution of its flavor during long-term fermentation remains insufficiently understood. In this study, an integrated strategy combining gas chromatography-ion mobility spectrometry (GC-IMS), electronic nose (E-nose), and untargeted metabolomics was used to characterize flavor changes in Tongchuan Douchi fermented for 6, 12, 18, and 36 months. The results showed pronounced stage-dependent changes in both volatile organic compounds (VOCs) and taste-related non-volatile metabolites. Prolonged fermentation enhanced the responses of T- and P-type E-nose sensors, indicating increased accumulation of aromatic compounds and alcohols in the later stages. Key late-stage volatiles included esters, pyrazines, and terpenes, especially 2,6-dimethylpyrazine, ethyl 2-methylbutanoate, 2,6,6-trimethylbicyclo[3.1.1]hept-2-ene, and 4-methyl-1-(propan-2-yl)bicyclo[3.1.0]hexan-3-one. Metabolomics analysis further revealed marked remodeling of carbohydrate metabolism, with galactose metabolism and starch/sucrose metabolism identified as the most relevant pathways. Raffinose and stachyose, two major soybean oligosaccharides, were inferred to act as important precursors that released D-galactose and supported subsequent flavor formation. Overall, this study demonstrates that long-term fermentation, particularly around 18 months, promoted the development of the characteristic mature flavor of Tongchuan Douchi and provides a multi-omics basis for understanding flavor formation in long-fermented Mucor-type Douchi.