Developing a two-stage strategy in the dehydration treatment of fresh apple slices: Combined freeze and radio frequency-vacuum drying

Freeze drying (FD) usually provides the best final quality but results in the high drying energy consumption, and the slow drying speed. To improve the FD efficiency, a two-stage dehydration strategy using a radio frequencyvacuum assisted freeze-drying process (FD + RFVD) was explored by taking apple slices as the representative samples. The drying rate changes during the FD treatment process of apple slices were experimentally determined. The best transition point of the two drying stages was also clearly obtained based on the changes of drying speed and product quality. The best processing parameters would be the FD drying time of 10.5 h followed by the RF-vacuum treatment at the electrode gap of 100 mm and vacuum pressure of 0.02 MPa. The optimized FD + RFVD process required only 81.25% of the FD drying time to achieve the same target moisture content, and similar or better product quality as compared to FD alone. This study may provide a technical guidance for developing effective and practical FD + RFVD technology to improve singe FD processes.