Refractance Window Drying of Apple Slices: Mass Transfer Phenomena and Quality Parameters

Abstract

The present study investigates the effect of Refractance Window (RW) drying at different temperatures (60 °C, 70 °C, 80 °C, and 90 °C) on drying characteristics and quality of apple slices and its comparison with hot air (HA) drying. Results showed that RW drying requires a shorter time (~ 25–37.5%) as compared to HA drying under similar conditions of drying. Also, RW drying at 90 °C resulted in higher retention of ascorbic acid (96%), without any significant change in color (ΔE = 5.5), compared to freeze drying. The microstructure analysis showed porous structure in RW-dried slices as compared to HA-dried ones. The influence of drying conditions on moisture diffusion was estimated using Fick’s, anomalous diffusion, and Dincer and Dost models. The values of moisture diffusivity (from 2.75 × 10−9 to 1.14 × 10−8 m2 s−1) obtained with the Dincer and Dost model were higher for RW as compared to HA and also were higher with respect to other models employed. Anomalous diffusion and Dincer and Dost models showed excellent agreement (R2 > 0.989) between experimental and predicted moisture ratios. This study showed that RW drying could effectively be used to dry thin layers of heat-sensitive fruits such as apple in a shorter time with better product quality as compared to HA drying.