Fermentation dynamics of yerba-mate and coffee-husk kombuchas: bioactive compounds, organic acids, volatile metabolites, and microbial succession

Introduction Although yerba-mate and coffee-husk have individually been explored as alternative kombucha substrates, it remains unclear how these chemically distinct matrices differentially shape metabolite evolution and microbial succession when fermented under equivalent conditions. This study directly compared yerba-mate kombucha (YMK) and coffee-husk kombucha (CHK) to investigate substrate-dependent chemical and microbial fermentation trajectories. Methods YMK and CHK were produced using the same parental SCOBY and fermentation protocol. Infusions containing 10 g/L substrate and 60 g/L sucrose were fermented statically at 30 °C for 9 days and subsequently stored at 4 °C for 4 days. Phenolic and flavonoid contents, antioxidant responses, organic-acid profiles, volatile compounds, and bacterial and fungal community succession were evaluated. Results The two matrices generated distinct fermentation trajectories. YMK maintained consistently higher phenolic content and antioxidant-related responses, reaching 467.97 ± 43.09 μg GAE/mL at the end of refrigerated storage, and exhibited a broader and more persistent distribution of volatile chemical classes. CHK showed lower but comparatively stable antioxidant-related measurements and a more restricted volatile profile. Organic acids varied non-monotonically in both formulations, although YMK showed a higher final malic acid concentration. Komagataeibacter remained dominant throughout YMK fermentation, whereas CHK underwent a stronger temporal shift toward Acetobacter. Fungal communities were comparatively stable in both substrates and were dominated by Starmerella and Dekkera . Discussion These findings demonstrate that substrate composition influenced both beverage chemistry and the temporal organization of the kombucha microbiota. By directly comparing a phytochemical-rich botanical infusion and an agro-industrial coffee by-product under standardized fermentation conditions, this study extends previous single-substrate investigations and highlights the potential of coffee-husk as a plant-derived ingredient for sustainable fermented beverage development.