Application of kombucha fermentation to improve the stability of bioactive metabolites in purple corn (Zea mays L.) cob extract

The cob of purple corn ( Zea mays L.), hereafter CPC, is rich in bioactive compounds, especially anthocyanins, yet is discarded as an agricultural by-product. Anthocyanins are increasingly extracted for food applications, although formulation development is needed to address their environmental instability. This study aimed to evaluate the influence of CPC extract supplementation on the physicochemical properties, antioxidant activity, and metabolite profiles of kombucha during fermentation. Kombucha was prepared by fermentation with CPC extract at concentrations of 0, 2.5, 5, 7.5, and 10 g/L for 16 days. CPC extract supplementation increased total sugar content, thereby promoting organic acid production (gluconic, acetic, citric, and succinic acids), which lowered the pH, creating an acidic environment that contributed to anthocyanin stabilization and enhanced redness during fermentation. The abundant phenolic compounds and anthocyanins in CPC contributed to increased total phenolic and flavonoid contents and antioxidant activity. Notably, microbial-mediated biotransformation during fermentation induced the degradation of acylated anthocyanins and promoted the formation of anthocyanin glucosides. These findings suggest the strong potential of CPC extract as an additive to improve consumer preference (color) and the bioactivity of kombucha beverages. • Kombucha was prepared by fermentation with the cob of purple corn extract (CPCE) • CPCE supplementation increased sugar content, stimulating organic acid production. • Accumulated organic acids lowered pH, stabilizing anthocyanins during fermentation. • CPCE increased phenolic, flavonoid contents, and antioxidant activity in kombucha. • Fermentation degraded acylated forms and enriched non-acylated anthocyanins.