Characterization and Evaluation of Apple Pomace Extract as a Fermentable Substrate in Water Kefir

Water kefir is a fermented beverage produced by a consortium of diverse microorganisms that can adapt to different substrate sources, modifying the beverage’s physicochemical and functional properties. Apple pomace is an agro-industrial by-product with nutrients and compounds that can enhance microbial activity and influence metabolite production, leading to subsequent physicochemical and phytochemical changes. In this study, apple pomace extract served as the substrate for water kefir fermentation at 30 °C for 120 h. Changes in total sugar content, pH, total polyphenols, antioxidant activity, microbial growth, organic acids, and volatile compounds were evaluated. The results showed a significant difference between substrates during fermentation. The process with apple pomace extract decreased total sugars, pH, and total polyphenols and increased antioxidant activity and microbial growth. Using apple pomace extract, acids such as acetic, lactic, malic, and citric, along with volatile compounds such as ethanol, isobutanol, isoamyl alcohol, and ethyl acetate, were detected, highlighting the water kefir consortium’s metabolic activity during fermentation. Apple pomace proved to be a suitable substrate for water kefir fermentation, as it promoted microbial growth and metabolite production; thus, it could serve as an alternative for developing fermented beverages with potential functional properties.