Orange Juice as a Substrate for Milk and Water Kefir Grains: Production and Characterization of Novel Kefir-like Beverages

Orange juice was investigated as a substrate for the production of novel kefir-like beverages using milk kefir grains (MKG) and water kefir grains (WKG). Fermentation was monitored for 48 h to characterize changes in physicochemical parameters, substrate consumption, metabolite production, microbial populations, antibacterial activity, vitamin and mineral composition, and volatile compounds. Both kefir grain types supported the growth of lactic acid bacteria, acetic acid bacteria, and yeasts, reaching final populations of approximately 6–7, 5–6, and 8 log CFU/mL, respectively. WKG promoted greater early production of lactic acid and consistently higher levels of acetic acid, ethanol, and antibacterial activity, whereas MKG produced a slightly higher lactic acid concentration after 48 h and was associated with greater increases in potassium, phosphorus, magnesium, and calcium. Vitamins B1, B2, and C decreased during fermentation with both MKG and WKG. The volatile profile was substantially altered during fermentation, with the potential aroma contribution shifting from predominantly balsamic and citrus notes toward fruity and sweet characteristics, particularly after 48 h. Overall, kefir grain type and fermentation time strongly influenced the physicochemical, microbiological, nutritional, antibacterial, and volatile characteristics of fermented orange juice. These findings demonstrate the potential of both grains for developing non-dairy kefir-like beverages.