Nowadays, consumer demand for functional foods with health benefits has grown significantly. In response to this trend, a variety of potentially probiotic foods have been developed—most notably kefir and kefir-like beverages, which are highly appreciated for their tangy flavor and health-promoting properties. Traditionally, kefir is made by fermenting cow’s milk with milk kefir grains, although milk from other animals—such as goats, ewes, buffalo, camels, and mares—is also used. Additionally, non-dairy versions are made by fermenting plant-based milks (such as coconut, almond, soy, rice, and oat) with the same type of grains, or by fermenting fruit and vegetable juices (e.g., apple, carrot, fennel, grape, tomato, prickly pear, onion, kiwifruit, strawberry, quince, pomegranate) with water kefir grains. Despite their popularity, many aspects of kefir production remain poorly understood. These include alternative production methods beyond traditional batch fermentation, kinetic studies of the process, and the influence of key cultivation variables—such as temperature, initial pH, and the type and concentration of nutrients—on biomass production and fermentation metabolites. A deeper understanding of the fermentation process can enable the production of kefir beverages tailored to meet diverse consumer preferences.

This study evaluated the fermentation kinetics and properties of kefir beverages produced from whole milk using milk kefir grains (MKGs) or water kefir grains (WKGs) over 48 h. MKGs showed higher initial microbial loads and promoted rapid acidification, with pH decreasing from 6.70 to 4.99 and significant production of lactic acid (10.58 g/L) and ethanol (5.17 g/L), compared with WKGs (final pH 6.20, <0.5 g/L lactic acid, and <0.3 g/L ethanol). However, the final concentration of acetic acid in WKG fermentation (1.93 g/L) was comparable to that in MKG fermentation (2.02 g/L). Microbial populations increased in both systems, exceeding 10⁶ CFU/mL—one of the requirements for conferring probiotic relevance to a beverage—with MKGs reaching higher counts. Lactose and protein consumption were greater in MKGs, suggesting more intense metabolic activity. Fermentation enhanced nutritional value by increasing vitamins B2, B3, B5, and pyridoxine, while vitamin D3 decreased. Mineral composition remained largely unchanged. Volatile analysis identified 31 compounds: MKGs favored fatty acids and lactones associated with creamy notes, whereas WKGs promoted ester formation and fruity aromas. Overall, both grain types produced microbiologically safe beverages with distinct biochemical and sensory profiles, demonstrating the feasibility of using WKGs for milk fermentation.

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.