Comparison of drying kinetics and product quality from convective heat pump and solar drying of Roselle calyx

This study investigated the effect of temperature (40, 50, and 60 degrees C) and sample mass (100, 200, and 300 g) on the drying characteristics and product quality of Roselle calyx using heat pump dryer. It was observed that higher drying temperature and lower sample mass led to a shorter drying time (faster drying rate). Higher temperature supplied more heat to the Roselle sample, which increased the activity of water molecules and moisture diffusion. This can be seen where the effective moisture diffusivity increased from 7.87 x 10(-10) to 2.05 x 10(-9) m(2)/s when air temperature of heat pump dryer was increased from 40 degrees C to 60 degrees C. The drying rates for all samples shared the same drying pattern: warming-up period where the drying rate started to increase due to the breakdown of the waxy layer on Roselle and falling rate period where it decreased gradually as the drying progressed. The Roselle sample dried with heat pump dryer at 40 degrees C (100 g) possessed the least degradation of quality, maintaining 86.8%, 63.2%, 98%, and 63.3% for color, catechin, protocatechuic acid, and vitamin C, respectively. Overall, heat pump dryer outperformed the sun drying method by recording a much shorter drying period without compromising the product quality. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.