Characterising Thin Layer Drying of High Moisture Content Vegetables in a Solar Tunnel Dryer

Abstract

The post-harvest losses experienced by high moisture content vegetables form the bulk of most of the postharvest losses, estimated to be 30-40% in the developing countries in the tropics and subtropics. This study involved thin layer drying of five high moisture content vegetables (kales, cabbage, cowpeas, amaranth and stinging nettle) in a solar tunnel dryer and the open sun, while monitoring the temperature, relative humidity and solar insolation on the solar tunnel dryer and then evaluating drying characteristics, effect of drying on colour, ascorbic acid and beta carotene retention of the dried vegetables. The vegetables dried from an initial moisture content of 1288.9, 669.2, 474.7, 566.7, and 566.7% dry basis for cabbages, kales, cowpeas, amaranth and stinging nettle, respectively, to stable moisture content of less than 12% dry basis within 8 hours except cabbages which took 30 hours. Hue angles for the dried vegetables (127.9, 81.92, 107.01, and 109.370) for kales, cabbage, amaranth and stinging nettle, respectively, were higher than those corresponding to open sun drying (118.7, 77.25, 96.61, and 102.020). Ascorbic level dropped significantly for both drying methods compared to the fresh ones. However, beta carotene retention in all the vegetables were higher for the solar tunnel drying than the open sun. Results of temperature and relative humidity analysis showed that there was significant difference in temperature and relative humidity in the collector, dryer and the open sun (P<0.05). The Page model best described the drying of kales, cabbage, cowpeas and amaranth with R2 values of 0.985-0.989, χ2 = 0.00145-0.00220, RMSE = 0.03850-0.04559 and ε(%) = 7.9±11.9-27.3±34.6, respectively, in the solar tunnel dryer. In addition, the Verma et al model was best in characterising thin layer drying of stinging xix nettle (R2 = 0.994, χ2 = 0.00112 and RMSE = 0.02899 and ε(%) =11.9±19.1) in the solar tunnel dryer. The results therefore demonstrate that the solar tunnel dryer can be utilized in drying high moisture content vegetables without adversely affecting the colour and nutritive properties of the vegetables. The solar dryer can therefore provide easy and applicable solution to the post-harvest losses of high moisture content vegetables especially to small scale farmers in rural areas while utilising environmentally friendly and abundantly available free solar energy.