Untargeted metabolomic analyses of fermented unpolished black rice with melanogenesis inhibition activity

Fermentation of rice can enhance the release of bioactive ingredients and generate diverse microbial metabolites contributing to various functional properties. Previous studies have demonstrated that the mixture of selected microorganisms called "De-E11 starter," comprised of Rhizopus oryzae, Saccharomyces cerevisiae, Saccharomycopsis fibuligera and Pediococcus pentosaceus yields fermented unpolished black rice sap (FUBRS) with a melanogenesis inhibition activity. To further understand this fermentation process, we characterized FUBRS and profiled its metabolite composition in comparison to unfermented unpolished black rice (Un-FR), recognizing the substantial enzymatic activity of FUBRS microorganisms and their potential for extensive metabolite production. The results indicated that fermentation decreased the pH, increased total acid content and elevated reducing sugar content. Moreover, significant alterations in phytochemical profiles were observed in FUBRS. In terms of biological activity, fermentation significantly enhanced antioxidant and tyrosinase/melanogenesis inhibitory activities. Untargeted metabolomic analysis utilizing orthogonal projections to latent structures discriminant analysis (OPLS-DA) revealed a clear differentiation in metabolite profiles between FUBRS and Un-FR. Volcano plot analysis (≥2-fold change) indicated a general increase in metabolites, including sugars, phenolic acids, organic acids, and fatty acids, after fermentation. Quantitative analysis confirmed the accumulation of p-hydroxybenzoic acid, lactic acid, acetic acid, and succinic acid, that are all known melanogenesis inhibitors. This study provides valuable insights into the characteristics and metabolite profile of FUBRS, and informing strategies for optimizing the fermentation processes to enhance the production of melanogenesis and tyrosinase inhibitory compounds, and identifying key metabolites as critical biomarkers for monitoring and controlling these processes. Together, they will facilitate the efficient and reproducible generation of high-efficacy ingredients for the cosmetic, nutraceutical, and potentially pharmaceutical industries.