Effects of water kefir fermentation on the nutritional and functional properties of black mulberry (Morus nigra L.) juice

Water kefir fermentation can modify the bioactive composition and nutritional properties of fruit juices. This study evaluated changes in the nutritional, physicochemical, microbiological and antioxidant properties of whole black mulberry (Morus nigra L.) juice after 48 hours of water kefir fermentation. Total phenolics and flavonoids, antioxidant capacity by DPPH and ABTS assays, physicochemical parameters, proximate composition and microbiological profile were determined. Results were compared descriptively and expressed as mean ± standard deviation of analytical triplicates (n = 3). Total phenolics increased from 2.76 ± 0.09 to 3.38 ± 0.12 mg gallic acid equivalents/mL (22.5%), and flavonoids from 0.92 ± 0.04 to 1.17 ± 0.05 mg quercetin equivalents/mL (27.2%). Antioxidant capacity increased from 18.45 ± 0.71 to 24.08 ± 0.86 µmol Trolox/mL by DPPH (30.5%) and from 25.72 ± 0.94 to 32.18 ± 1.10 µmol Trolox/mL by ABTS (25.1%). Soluble solids decreased from 10.15 ± 0.12 to 6.85 ± 0.14 °Brix, pH decreased from 3.45 ± 0.03 to 3.12 ± 0.02, and titratable acidity increased from 0.84 ± 0.04 to 1.18 ± 0.05% as citric acid. Carbohydrates decreased from 8.72 ± 0.20 to 7.38 ± 0.12%, and energy content from 40.2 ± 0.9 to 35.2 ± 0.8 kcal/100 g, corresponding to reductions of 15.4% and 12.4%, respectively. In the fermented juice, lactic acid bacteria and yeasts reached 7.69 ± 0.04 and 6.80 ± 0.03 log₁₀ CFU/mL, respectively. Salmonella spp. and Listeria spp. were not detected in 25 mL of either sample. These changes are consistent with fermentative metabolism and the modulation of bioactive compounds reported in other fruit matrices. Water kefir fermentation was therefore associated with acidification, lower carbohydrate and energy contents, and higher phenolic and flavonoid contents and in vitro antioxidant capacity, indicating technological potential for developing black mulberry beverages.