Sustainable plant-based kombucha production: Investigating the antimicrobial properties following addition of lemon peel, banana peel, and Calendula officinalis flowers

The use of agro-waste in food production offers a sustainable solution to environmental challenges while fostering innovation in the development of functional food products. As an example, kombucha, a fermented beverage produced using a Symbiotic Culture Of Bacteria and Yeast (SCOBY), sugar, and tea, has gained significant global interest for its functional properties and growing role in health-focused diets. Typically fermented over 7–20 days, kombucha produces a range of bioactive compounds, including probiotics, polyphenols, and organic acids. These components contribute to its antioxidant, antimicrobial, and therapeutic potential, making kombucha an important topic in the development of functional foods. This thesis investigates the potential of banana and lemon peels as substrates for kombucha fermentation, aiming to transform these by-products into beverages with antimicrobial properties and enhanced nutritional value. Kombucha beverages were prepared using green tea, with one variant infused with 5% banana peel at 70°C and the other infused with 10% lemon peel at the same temperature. These specific conditions were selected based on comprehensive pH measurements and microbial colony count evaluations. The study employed three antimicrobial testing methods: disc diffusion, agar well-diffusion, and broth microdilution assays, targeting common foodborne pathogens Listeria innocua (DSM 20649), Escherichia coli (DH 1), and Pseudomonas fluorescens (DSM 50090). The results demonstrated that kombucha fermented with lemon peel exhibited significant antimicrobial activity against L. innocua. In contrast, banana peel kombucha showed less pronounced effects, suggesting that the fermentation process did not substantially enhance its inhibitory capacity. Neither substrate demonstrated significant antimicrobial activity against E. coli or P. fluorescens, likely due to the inherent resistance of these Gram-negative bacteria to acidic environments and antimicrobial compounds. This research underlines the importance of substrate selection in kombucha fermentation for maximizing antimicrobial efficacy. The use of banana and lemon peels not only contributes to the development of functional beverages with enhanced health properties but also supports sustainable practices by utilizing fruit waste that would otherwise be discarded.