Comparative analyses of milk and water kefir: Fermentation temperature, physicochemical properties, sensory qualities, and metagenomic composition

Milk and water kefir have been attractive to consumers as health-promoter fermented beverages. Although both are called “kefir”, they differ in physicochemical and microbial properties. In this study, we aimed to compare milk and water kefir in terms of biomass increase, physicochemical properties, sensory qualities, and microbial loads incubated at different incubation temperatures (20 °C, 25 °C, and 30 °C) to mimic approximate room temperatures in various seasons. Additionally, the microbial diversity of the grains was compared through metagenomic analysis. The results showed that the biomass increase, physicochemical properties, and sensory analysis of milk kefir grains depended on the fermentation temperature (p < 0.05); while in general, water kefir was not affected by the fermentation temperature. The highest total dry matter, protein, and fat content, and the least acidity were recorded at 20 °C in milk kefir drinks (p < 0.05). Around 8 and 6 log CFU/ml microorganism loads (Lactobacillus spp., Lactococcus spp., yeasts, total aerobe, and anaerobe bacteria) were recorded for milk and water kefir drinks, respectively, regardless of the temperature. Bifidobacterium load was recorded as the lowest for both kefir drinks. The dominant species of bacteria and yeasts were determined as Lactobacillus parakefiri (21.28%) and Geotricum silvicola (65.10%) in milk kefir grain, and Lactobacillus nagelii (95.69%) and Dekkera bruxellensis (95.24%) in water kefir grain, respectively. The study showed that milk and water kefir differ in terms of physicochemical, sensory, and microbial aspects. Despite its low protein content, water kefir would be a probiotic substitute for milk kefir due to its probiotic features for vegans or people with lactose intolerance.