Functional enhancement of kombucha fermentation through the utilization of watermelon flesh and rind

Kombucha is a fermented tea produced via metabolic interactions within a symbiotic culture of bacteria and yeasts (SCOBY). This study investigated the effects of watermelon flesh and rind supplementation during secondary fermentation on the microbiological, physicochemical, and antioxidant characteristics of kombucha, emphasizing rind utilization as an underutilized resource. Primary fermentation was conducted for 10 days, followed by secondary fermentation with either watermelon flesh or rind for 5 days. Flesh-added kombucha showed enhanced yeast proliferation during secondary fermentation, whereas rind-added kombucha maintained high antioxidant activity (> 89% DPPH radical-scavenging activity) and relatively stable polyphenol content. Both treatments showed decreased soluble solids and pH during fermentation. Microbial community analysis revealed that the SCOBY consortium was predominantly composed of acetic acid bacteria and fermentative yeasts, particularly Komagataeibacter, Zygosaccharomyces, and Starmerella. These findings indicate that watermelon rind can serve as a functional low-sugar fermentation substrate while supporting stable microbial fermentation and antioxidant properties. The present study highlights the potential utilization of watermelon rind for the development of value-added and sustainable fermented beverages with ethnically relevant food applications.