Evaluation of Ultrasound- and Microwave-Assisted Convective Drying of Kale (Brassica Oleracea L.) in Regard to Process Kinetics and Quality

Abstract

This study analyses the hybrid drying of kale leaves by convection assisted with microwaves and airborne ultrasound at two different temperatures of 50 and 70 °C. The research assessment includes parameters of drying kinetics and specific energy consumption, as well as selected quality indicators such as relative color change, browning index, and the ascorbic acid, phenolic, carotenoid and chlorophyll retention. The results revealed that the drying rate increased with an increase in microwave/ultrasound power and air temperature, while improving the energy efficiency of hybrid drying, as compared to convective drying. The quality assessment proved that kale leaves dried by different methods were characterized by similar color, but the highest retention of vitamin C, phenolics and natural dyes was found in products dried at higher temperature and greater power of microwaves and/or ultrasound.

Keywords: hybrid drying, effective diffusion coefficient, color, vitamin C, polyphenols, natural dyes