Thin-Layer Drying Kinetics and Quality Attributes of Apple Pomace Powders from Different Varieties

Apple pomace, a major by-product of juice processing, requires effective drying for valorization and preservation. This study investigated the thin-layer drying kinetics of apple pomace from three varieties (Starkrimson, Idared, and Jonagold) under forced convection at 57 °C, 63 °C, and 68 °C. Seven mathematical models were used to fit the experimental moisture ratio data and evaluated using statistical indicators (R2, RMSE, χ2, and MBE). In addition, the proximate composition, titratable acidity, water activity, color parameters, rehydration capacity, total phenolic content, antioxidant activity, and phenolic and organic acid profiles of the resulting apple pomace powders were assessed. Of the models tested, the Midilli–Kucuk model demonstrated superior performance with the highest R2 (>0.99) and lowest RMSE and χ2 values across all varieties and temperatures. The Page and Logarithmic models also showed good predictive capability. Significant differences in phenolic content and antioxidant activity were observed between powders from different apple varieties, with Starkrimson showing the highest values, followed by Jonagold and Idared. These findings provide essential kinetic parameters for optimizing apple pomace drying processes and support the development of value-added products from this abundant agro-industrial by-product.