Mass transfer parameters and quality characteristics of aonla slices under refractance window drying.

This study investigates the effects of refractance window (RW) drying process parameters on mass transfer and quality characteristics of dried aonla slices. RW drying of aonla slices was carried out at three levels of water temperature (75, 82 and 90 °C) and slice thickness (2, 4 and 6 mm). In terms of quality characterization, the total phenolic content, ascorbic acid content, and browning index were determined. Higher retention of ascorbic acid (64.49 ± 0.34%) and phenolic content (37.84 ± 0.08 mg GAE/g dry matter) was found at 90 and 82 °C water temperature, respectively with inconsequential variation in browning index. Mass transfer parameters such as Biot number, effective moisture diffusivity and mass transfer coefficient were estimated using Dincer and Dost model. Effective moisture diffusivity during drying varied from 4.27 × 10⁻¹⁰ to 1.09 × 10⁻⁰⁹ m²s⁻¹ and Biot number was observed to be in the range of 0.268 to 8.666 for different drying conditions. XRD pattern suggested that RW dried aonla slices had semi-crystalline structure. The changes in crystalline structure to amorphous was pronounced more at high water temperature. It was also revealed by the disintegration of cell wall/membrane and connecting surfaces, which resulted into smooth and flaky microstructures with sharp edges. The gamut of crystallite size was obtained as 49.12 ± 0.26 to 99.12 ± 1.19 nm. The presence of bound water after completion of drying was represented by FTIR peaks centered around 3577 cm⁻¹. The infrared spectroscopy of RW dried aonla postulated that peak intensity of absorption bands negligibly changed with varying processing conditions, but minor peak shift was observed. This study elucidates the suitability of RW drying for retention of heat-sensitive compounds in food produce such as aonla with better quality retention and morphological characteristics. [ABSTRACT FROM AUTHOR]