Substituting Cow’s Milk with Goat’s Milk Changed the Nutritional, Rheological, and Volatile Profiles of Kefir Produced by Kefir Grains

Kefir grains, originating from the Caucasus, are irregularly shaped, semi-solid granules resembling popcorn or cauliflower (0.3 to 3.5 cm), composed of bacteria and fungi embedded in extracellular polysaccharides such as kefiran. In this study, kefir samples were produced using different ratios of goat’s and cow’s milk (100:0, 75:25, 50:50, 25:75, and 0:100) to examine microbial populations, physicochemical and volatile properties, rheological behaviour, antioxidant capacity, and organic acid content. The type of milk used significantly affected the chemical composition and pH (p < 0.05), although titratable acidity remained stable during storage. Increasing the proportion of goat’s milk decreased viscosity but enhanced the total free amino acid (FAA) content. Goat milk kefir exhibited stronger antioxidant activity than cow milk kefir due to the formation of bioactive peptides and FAAs through proteolysis, with the highest values observed in samples with a higher proportion of goat’s milk and the lowest in 100% cow’s milk kefir. Organic acid levels increased during fermentation and, in 100% goat milk kefir, lactic, acetic, propionic, hippuric, butanoic, and citric acids reached their highest concentrations. The diversity and intensity of volatile compounds also increased proportionally with the goat milk ratio. In conclusion, considering antioxidant activity, volatile aroma components, organic acid content, sensory properties, and viscosity values, sample D, which is 75% goat milk kefir, is recommended for consumers.