Upcycling Discarded Apples into Cider: Yeast and Nutrient Strategies Shaping Fermentation and Sensory Quality

The increasing volume of discarded apples generated by commercial grading standards and postharvest losses represents both an environmental burden and an opportunity for sustainable valorization. Despite growing interest in circular economy strategies in the fruit-processing sector, a comprehensive review of the technological, microbiological, and nutritional factors influencing cider production from discarded apples remains limited. To address this gap, this review discusses key aspects of cider production from discarded apples, focusing on raw material characterization, nutrient management, yeast strategies, and fermentation technologies. The physicochemical and microbiological properties of discarded apples are examined, including soluble solids, acidity, phenolic composition, and microbial spoilage risks. Special attention is given to nutrient optimization, particularly yeast assimilable nitrogen (YAN), vitamins, and minerals, as deficiencies may cause sluggish fermentation and adversely affect volatile compound formation and stability. This review evaluates yeast selection, comparing Saccharomyces cerevisiae with non-Saccharomyces yeasts and mixed fermentations, highlighting their effects on chemical composition, aroma, and sensory quality. Innovative approaches such as yeast immobilization and repeated-batch fermentation are reviewed as tools to improve process performance. Key technical challenges, including variability in raw material quality, nutrient supplementation needs, contamination risks, and process scalability, are discussed alongside opportunities for valorization of cider pomace within a circular economy framework.