Apple Pomace as a Source of Valuable Phenolics: From Drying Kinetics to Optimization of Ultrasound-Assisted Extraction Using Conventional and Alternative Solvents

Industrial processing of apple to obtain products like juice or cider generates a significant amount of pomace, which represents 25-30% of the fresh fruit mass. Different technologies are needed to valorize apple pomace (AP), considering its significant amount of high-value compounds, such as fiber, vitamins, and polyphenols. Hot-air convection (CA) and infrared (IR) drying are widely used methods for preserving polyphenols from by-products, such as apple pomace (AP), while also extending their shelf life. This study aimed to evaluate the influence of CA and IR drying on drying kinetics, color parameters, and the preservation of polyphenolic compounds, as well as to identify a sustainable extraction approach. Both drying methods significantly affected the color characteristics and content of polyphenols with high antioxidant activity. A significant impact was noticed at higher temperatures, which may be associated with the partial inactivation of browning enzymes. IR drying resulted in a shorter drying time and lower specific energy consumption compared to CA. Furthermore, the assessment of solvent efficiency in ultrasound-assisted extraction (UAE) indicated that the natural deep eutectic solvent (NaDES) composed of choline chloride and glycerol (1:1 molar ratio) provided superior recovery of phenolic compounds with high antioxidant activity compared to conventional solvents and the other NaDES analyzed. Optimization of UAE conditions using this polyol-based NaDES allowed for achieving an extract characterized by a polyphenolic profile dominated by flavan-3-ols (catechin and epigallocatechin), followed by phenolic acids, mainly chlorogenic acid. These results confirm the potential of AP as a valuable source of bioactive compounds and of polyol-based NaDESs as a sustainable and efficient alternative for their recovery.