Microbial diversity and its links to retinol pathways and aroma compounds in ethnic fermented rice beverages of Assam

Traditional fermented rice beverages are produced through complex microbial fermentation processes that influence their physicochemical characteristics and metabolite composition. In this study, metagenomic sequencing and GC-MS/MS-based metabolomics were integrated to characterize four indigenous rice beverages: Black Rohi Modh (BR), Rohi Modh (RH), Jou Bidwi (JOU), and Sai Mod (SM). All beverages were mildly acidic, with pH values ranging from 4.1 to 4.5 and titratable acidity between 0.58 and 0.72% lactic acid. Ethanol content varied among samples, with BR showing the highest concentration (8.13% v/v), followed by JOU and RH (approximately 5.5% v/v), while SM exhibited the lowest level (4.28% v/v). Antioxidant activity differed across beverages, with RH and BR demonstrating higher DPPH radical scavenging activity and SM showing the highest ferric reducing antioxidant power (96.93 µmol/mL). Metagenomic analysis generated 57.69 Mb of assembled sequences, identifying 48 microbial phyla and 1,785 species, with Eukarya accounting for 66.12% of the total community. Ascomycota predominated in BR and JOU, whereas Bacillota was more abundant in RH. The genus Saccharomyces was consistently dominant across samples. Functional annotation indicated enrichment in metabolic pathways related to carbohydrate and amino acid metabolism, as well as genes associated with ethanol biosynthesis and retinol metabolism pathways, reflecting microbial metabolic potential rather than direct vitamin production. Metabolomic profiling identified 113-167 metabolites per beverage, with 93 compounds shared among all samples. Correlation analysis revealed significant associations between Saccharomyces cerevisiae and short-chain fatty acids (ρ = 0.62-0.71, FDR < 0.05), indicating a strong positive relationship between microbial abundance and metabolite production.