Integrated study of microbial dynamics, functional properties, and bioaccessibility in artisanal green and black kombucha teas: impact of backslopping and in vitro digestion

Kombucha, a fermented tea beverage known for its health-promoting effects, is believed to have originated in Northeast China (Manchuria) over 2,000 years ago. Over time, it spread globally, gaining popularity for its distinctive flavor and potential health benefits. By the present day, kombucha has become widely consumed for its rich composition of bioactive compounds and it is often produced at home using a Symbiotic Culture Of Bacteria and Yeast (SCOBY). This production process typically involves a backslopping technique, where a small portion of SCOBY, commonly shared among households, is used as a starter culture for a new batch of kombucha. This method results in an artisanal, handcrafted fermented beverage that reflected both tradition and individuality. The fermentation process in kombucha is driven by a complex interplay of microorganisms, including acetic acid bacteria (AAB), lactic acid bacteria (LAB), and yeasts. These microbial groups work synergistically to convert tea and sugar into a nutritionally enriched beverage. Specifically, yeasts such as Saccharomyces, Zygosaccharomyces, and Brettanomyces initiated fermentation by metabolizing sucrose into ethanol and CO2. Subsequently, AAB, including species like Acetobacter, Gluconobacter, and Komagataeibacter, converted ethanol into acetic acid. This acid production lowered the pH of the beverage, contributing to kombucha characteristic sour taste and enhancing its safety by inhibiting the growth of harmful pathogens. In addition to organic acids, the fermentation process produces various other bioactive compounds, including polyphenols, glucuronic acid, and vitamins, among others. These metabolites are associated with several health benefits, such as antioxidant activity, antimicrobial properties, and improved gut health. This thesis focused on the fermentation of green and black kombucha teas over a one-year period using backslopping cycles. The aim was to understand how repeated production influenced the microbial consortium and the resulting functional properties of the beverages, with the goal of optimizing the backslopping technique for enhanced health benefits. In addition, simulated in vitro digestion was conducted to evaluate the bioaccessibility of key metabolites in kombucha, shedding light on how digestion impacted the beverage health-promoting potential, such as antioxidant and antimicrobial activities. The results indicated that both green and black kombucha teas retained significant bioactive properties after digestion, with potential implications for gut health. Moreover, simulated in vitro digestion was used to assess the survival of microbial groups, providing a preliminary evaluation of kombucha microbiota after gastrointestinal digestion. Overall, this thesis underscored the importance of microbial dynamics in shaping the functional properties of kombucha, improving its status as a health-promoting beverage. The findings offered important insights that could influence the future development of kombucha as a functional food, with broader applications in creating fermented beverages aimed at promoting overall health and wellness.