Co-Fermentation with Cider By-Products for Enhanced Beverage Quality: Kinetics, Functionality, and Sensory Impact

This study evaluated a new co-valorization strategy using apple pomace and a nitrogen-rich yeast lees extract to correct low concentrations of amino acids and phenolic compounds that can impair fermentation kinetics, aroma formation, and antioxidant activity, thereby improve cider quality and support the principles of the circular economy. Apple must (Gala) was supplemented with nitrogen extract (80 mg/L), dehydrated apple pomace (47.3 g/L), or both, and fermented with Saccharomyces cerevisiae. Fermentation kinetics, physicochemical parameters, individual phenolic profile (HPLC), antioxidant activity (DPPH, ABTS, FRAP, and CUPRAC), color (CIELAB), and sensory attributes were assessed. Nitrogen supplementation accelerated fermentation, increasing the maximum rate from 9 to 16 g CO2/L/day when combined with apple pomace. While nitrogen extract alone reduced phenolic levels, its combination with pomace resulted in a balanced phenolic profile and superior fruity aroma. Apple pomace addition increased titratable acidity (1.6 times) and ethanol content (>1% v/v) and enhanced total phenolics (up to 20%) and antioxidant activity (up to 46% by FRAP). Sensory analysis indicated that the combined treatment achieved the highest overall acceptance. The integrated use of apple pomace and yeast lees extract represents a sustainable new strategy to enhance cider bioactivity, fermentation performance, and sensory quality while valorizing agro-industrial residues.