A comparison of the drying kinetics, energy consumption and colour quality of drying medicinal leaves in direct-solar dryer with different colours of collector cover

Abstract

In some countries, Neem and Bitter leaves are often offered as dried medicinal leaves and one of the appealing aspects to consumers is that they retain their original colour after drying. Hence, the purpose of this study was to examine if collector cover colour variations can impact the quality and chlorophyll loss in solar-dried Neem and bitter leaves. To vary the colour spectrum, three distinct coloured polyethene materials with different colours were used as top window-cover for the dryer chamber and collector, which formed a single drying unit of a solar cabinet dryer. The results showed that the yellow-covered solar drying unit achieved 38.8% thermal efficiency, which declined by 1.43% and 10.41% for the blue and white-covered drying unit respectively. The yellow-cover dryer created higher internal temperature and drying rate, enabling it to dry the leaves faster. The average drying rate for Neem and bitter leaves was 0.003762–0.003849 kg/h, and 0.004348–0.004689 kg/h respectively. The specific energy consumption for drying the leaves ranged from 10.52 to 13.89 MJ/kg for all dryers. Colour analysis showed that the yellow-covered dryer dried bitter leaf near its natural colour, while the blue-covered dryer dried Neem near its natural colour. Lograthimic model predicted their moisture ratio better for drying the leaves.