Process Optimization for Development of a Novel Water Kefir Drink with High Antioxidant Activity and Potential Probiotic Properties from Russian Olive Fruit ( Elaeagnus angustifolia )

Kefir is a dairy-based probiotic beverage with high antioxidant activity, among other health benefits. To extend kefir’s beneficial health effects to non-dairy consumers, studies on kefir fermentation using alternative matrices (referred to as water kefir) are needed. As such, the purpose of this study was to formulate a novel water kefir beverage using Russian olive, as a non-dairy product with high antioxidant activity and potential probiotic properties. To this end, the Russian olive kefir water (RWK) fermentation process was optimized to maximize the total phenolic content, antioxidant activity, and microbial viability of this product. The experimental design was set using a rotatable central composite design with response surface methodology (RSM). The optimized independent variables included the substrate concentration (20–30% of Russian olive juice), fermentation time (24–48 h), and incubation temperature (20–32 °C). The optimal fermentation conditions were observed to be 31.2 °C incubation temperature, 24 h incubation time, and 30% Russian olive juice concentration. Under these conditions, the values for FRAP antioxidant activity, DPPH radical scavenging, and TPC in RWK were 0.22 (μmol FSHE/mL), 0.096 (μmol Trolox Eq/mL), and 98.32 (μg GAE/mL), and the microbial viability (of AAB, LAB, and yeasts) was 7.20, 7.06, and 7.17 log10 CFU/mL, respectively.