Thin Layer Drying Characteristics of Stinging Nettle (Urticadioica L.) in a Solar Tunnel Dryer

Author(s):  Kamwere MM, Kanali CL, Mutwiwa UN, Kituu GM

Publication date: 12 December 2015

Publication type:  Journal Article

Journal: International Journal of Engineering Research and Technology

Abstract: High moisture content vegetables form the bulk of most of the postharvest losses, estimated to be 30-40% in developing countries in the tropics and subtropics, due to poor post-harvest handling, transportation, processing and storage facilities. Various technologies have been used to minimise the losses and include cooling, traditional open sun drying, chilling and solar drying among others. In this study, thin layer solar drying characteristics of stinging nettle (Urticadioica L.) vegetables in a solar tunnel dryer were modelled against open sun drying. Fresh stinging nettle vegetables with average moisture content of 566.7% (dry basis) were dried for eight hours to final moisture content of 7.3 and 13.9 % (dry basis) for solar dryer and open sun, respectively. The colour of the samples was recorded at the start and end of the drying process. The data obtained from the drying tests was applied to nine (9) mathematical models namely Verma et al, Newton, Page, Modified Page, Henderson & Pabis, Logarithmic, Diffusion approximation, Two term and Two Term exponential. All the nine models investigated were found to be suitable for describing the drying characteristics of stinging nettle vegetables with R2 values greater than 0.96. However, Verma et al model satisfactorily predicted thin layer drying of stinging nettle vegetables better than the other models with the highest R2 of 0.994 and lowest χ2 of 0.00112 and RMSE of 0.02899 for the solar dryer. Hue angle, h*, values obtained for the solar drying and open sun were 108.17±4.320 and 103.37±5.900, respectively, which were lower than the values for fresh stinging nettle of 125.60±1.310. Although the hue angle of dried vegetables is lower than the fresh ones, solar dried vegetables have high hue angle as compared to sun dried hence better quality.

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