Defined Microbial Communities Modulate Polyphenol Transformation and Quality of Kombucha Across Different Tea Substrates

Kombucha quality is largely governed by polyphenol transformation during fermentation. However, interaction between substrate composition and microbial communities regulating phenolic transformation and quality formation remains unclear. In this study, six tea substrates (white, green, yellow, black, oolong, and mint tea) were fermented using three defined microbial communities (SMC1-SMC3) and a traditional symbiotic culture of bacteria and yeast (SCOBY) to evaluate carbon metabolism, phenolic transformation, antioxidant activity, and sensory quality. After 10 d of fermentation, SMC2 and SMC3, containing acetic acid bacteria, showed stronger acidification (pH 2.2–2.5) and lower ethanol (0.34–0.52 mg/mL) than SMC1 (13.09–15.88 mg/mL). Phenolic transformation was substrate-dependent: total phenolics and flavonoids decreased in green tea, both increased in white tea, while flavonoids increased in oolong and black tea. Meanwhile, rutin decreased in white and green tea, whereas gallic acid accumulated in yellow, black, and oolong teas and was positively correlated with antioxidant activity. Sensory evaluation showed SMC3 achieved higher overall acceptability in most substrates, whereas SCOBY performed best in mint tea. These findings indicate substrate-microbiota interactions play a key role in phenolic transformation and quality formation in kombucha. Rational matching of tea substrates with defined microbial communities enables coordinated optimization of antioxidant activity, ethanol control, and sensory quality.