Physicochemical properties, antioxidant activity, sugar profile, volatile compounds, and sensory evaluation of water kefir fermented from pineapple juice and pomace at varying fermentation durations

Water kefir is a fermented, non-dairy beverage known for its probiotic content and associated health benefits. Recently, there has been growing interest in utilizing fruit by-products such as pomace in fermentation to enhance nutritional value and promote sustainability. This study aimed to investigate the effects of different fermentation substrates which is pineapple juice and pineapple pomace and fermentation durations (0, 24, and 72 hours at 4°C) on the physicochemical characteristics, antioxidant properties, sugar composition, volatile compound profiles, and sensory attributes of water kefir. Two water kefir formulations were prepared: one using 300 mL of diluted pineapple juice (F1), and the other using 300 mL of boiled and filtered pineapple pomace (F2). Both were supplemented with 20 g of sugar and inoculated with water kefir grains. Samples were analysed at specified fermentation intervals to assess sugar content, total phenolic content (TPC), total flavonoid content (TFC), antioxidant activity (DPPH and FRAP), pH, viscosity, Brix, volatile compounds, and sensory characteristics. Substrate type significantly influenced fermentation dynamics and kefir quality. In F1, enzymatic breakdown of complex sugars led to increased levels of glucose and fructose, promoting the formation of esters and alcohols and enhancing fruity aromas. In contrast, F2, with lower initial sugar content and higher fiber, fermented more slowly, producing more acids and esters, resulting in a tangier and more complex flavour profile. Antioxidant properties improved significantly in F2, especially after 72 hours of fermentation, with notable increases in TPC, TFC, and radical scavenging activity. F2 also showed a greater pH reduction and higher viscosity, while F1 maintained a relatively stable pH and moderate viscosity. Sensory evaluation indicated that F2 retained favourable qualities in taste, colour, and overall acceptability over time, while F1's sensory appeal declined during fermentation. Pineapple pomace serves as a valuable substrate in water kefir production, enhancing both its functional and sensory qualities while contributing to the sustainable reuse of fruit waste. Substrate selection and fermentation time are critical factors in optimizing the quality and nutritional value of functional fermented beverages.