Optimization of Two‐Stage Variable‐Temperature Air‐Impingement Drying of Apple Cubes by CCD ‐ RSM and Quality Evaluation

ABSTRACT A two‐stage variable‐temperature air‐impingement drying process for apple cubes was investigated, optimized, and evaluated. Drying stages were identified by monitoring changes in internal sample temperature and moisture content during drying, and the preliminary process conditions were determined according to the heat‐ and mass‐transfer characteristics of the material at different stages. High‐temperature stage drying time (HTST), low‐temperature stage drying temperature (LTST), and sample thickness were selected as the independent variables, while drying time, color lightness ( L *), vitamin C ( VC ) content, total phenolic content ( TPC ), rehydration rate ( RR ), and crispness were used as the response indicators. Single‐factor experiments and a central composite design ( CCD ) coupled with response surface methodology ( RSM ) were employed to optimize the process. The optimized conditions were a sample size of 10 mm × 10 mm × 6 mm, drying at 100°C for 44 min in the first stage, and subsequent drying at 69°C in the second stage until the target moisture content was reached. Under these conditions, the drying time was 153 min, L * was 82.87, VC content was 2.18 mg/100 g, TPC was 4.21 mg/g, crispness was 578.42 mg, and RR was 3.21. These results indicate that two‐stage variable‐temperature air‐impingement drying can simultaneously shorten drying time and maintain desirable product quality in dried apple cubes.