Synergistic Bioactive Potential of Combined Fermented Kombucha and Water Kefir

The rising interest in functional fermented beverages, such as kombucha and water kefir, has stimulated research into their health benefits. This study aimed to investigate the combined bioactive potential of kombucha and water kefir by fermenting a novel medium prepared by mixing them in a 1:1 v/v ratio. The fermentation process involved using both SCOBY and water kefir grains (WKGs) separately, as well as co-cultivation, to explore the bioactive properties of the three fermented beverages. Samples were analyzed at 24, 48, and 72 h for changes in pH, microorganism growth, and concentrations of flavonoids and phenolics. Antioxidant activity was assessed using DPPH, ABTS, and FRAP tests, alongside colorimetric analysis and enzyme inhibition assays against α-amylase, α-glucosidase, and lipase. The results demonstrated that longer fermentation times increased both bioactive compound content and antioxidant capacity. The highest phenolic concentration was found in the WKG-fermented mixture (47.58 ± 2.13 mg GAE/100 mL), while the highest iron-reducing capacity was observed in the product fermented with both WKGs and the co-culture of SCOBY-WKGs. Additionally, SCOBY fermentation showed significant inhibitory activity (over 70%) against digestive enzymes. These findings suggest that co-fermenting kombucha and water kefir represents a promising alternative to traditional water kefir, with improved bioactive compound profiles.