The Content of Biologically Active Substances and Total Antioxidant Activity of Black Tea Drinks and Decoctions of Medicinal Plants Fermented with Kombucha

Kombucha is a healthy-fermented beverage traditionally made by fermentation of sweetened tea with a symbiotic culture of different yeasts and acetic acid bacteria, named also SCOBY – a symbiotic culture of bacteria and yeast. During the fermentation of glucose, yeast produces ethanol, which is used by acetic acid bacteria to produce acetic and other acids. The resulting ethanol and acids, as well as fermentation by-products, provide the original flavor and aroma of the beverage. In this study, we compared the growth and chemical composition of beverages when growing tea mushroom on a typical substrate, black tea, and decoctions of two medicinal plants, rose hips and yellow gentian rhizomes. To obtain the fermented drinks, the “tea mushroom" was grown on 1.3% (w/v) Ahmad black tea, and plant decoctions from rose hips and gentian roots with the addition of different concentrations of sucrose (0.5%, 1%, 5%, 10% and 20%) for 14 days. At day 0, 7 and 14 of cultivation, we determined body mass of the Kombucha and characteristics of fermented beverages (pH, total acid levels, ethanol content, sucrose concentration, total polyphenols and flavonoids, total antioxidant capacity). The results indicate that Kombucha grew well on both, its usual substrate -black tea drink and decoctions of medicinal herbs, rose hips and roots of yellow gentian. According to the growth rate of SCOBY on the tested media, the samples can be ranked in the following order: rose hips > yellow gentian roots > Ahmad black tea. The optimal concentration of sucrose in the medium for the development of Kombucha was 5%. During cultivation, sucrose was gradually fermented by the Kombucha yeast to ethanol, which in turn was used by acetic acid bacteria to synthesize acetic acid. As a result, an acidification of the pH of the culture medium was observed. The content of polyphenolic substances in the infusions increased slightly during cultivation, indicating that Kombucha has a limited ability to synthesize these compounds. The total antioxidant activity of the infusions decreased during the cultivation on the tea made of rose hips, but it increased on the decoctions of the roots of yellow gentian on the 14th day of the growth. Thus, not only phenolic substances determine the antioxidant properties of fermented kombucha beverages.