Solar drying of indigenous vegetables using enclosed convectional solar driers

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University of Dar es Salaam
Four indigenous vegetables were solar dried in three enclosed driers of similar design and dimensions that differed in secondary features which were intended to test the shading and black coating effects on the drier performance. The driers’ performance with respect to drying rate and retention of ascorbic acid and total carotene was compared to drying in the open sunshine. The moisture content in the external air, drying air and out-going air were read from Moillier diagram using wet bulb and dry bulb temperature data. The average temperatures at the driers’ outlets were higher than the temperatures of the drying air. Shading significantly depressed the temperature of the drying air by 8.36-+2.05Oc indicating that the temperature of the drying air was to a large extent dependent on the insolation arriving at the driers base. Drying efficiency was relatively higher in the driers without shade than in the shaded drier, although there were no significant differences in the moisture content of the drying air and the mean wet bulb depressions at the driers’ outlets. Drying efficiency in the enclosed driers was greater than in the open sunshine. Black paint did not significantly increase the temperature of the drying air. The optimum blanching times for fresh Ameranthus sp. And Ipomea batatus leaves were 0.50 and 0.80 minutes respectively. Direct immersion of Vigna unguiculate leaves ib boiling water for up to 12 minutes did not inhibit peroxidase whereas Manihot dulcis leaves have been revealed to require no blanching prior to dehydration. Blanched Amaranthus sp. And Ipomea leaves had mean ascorbic acid content ranging from 5.05 – 11.67 and 18.93 – 22.53mg per 100g dry matter respectively whereas in non-blanched Nanihot dulcis and Vigna unguiculate the means ranged from 180.08 – 415.85 and 93.14 – 179.04mg per 100g dry matter respectively; in all cases the highest mean represented amounts retained in vegetables dried in the drying system with shade. Drying under shade also promoted the highest carotene retention. Although drying under shade had a sparing effect on carotene and ascorbic acid, the retained quantities represented relatively low percentages of the content in fresh leaves. Sulphating of non-blanched Manihot dulcis was effective at 400ppm so2 in increasing the ascorbic acid available lyaine by 1.25 and 35.98 per cent respectively while the presence of sulphite ion had a negative effect on carotene retention. The analysis of the amino acid content in fresh and dried vegetables indicated losses in lysine methionine and hictidine. The quantity of retained lysine was however greater than the lysine content in the FAO reference protein.
Available in print form, East Africana Collection, Dr. Wilbert Chagula Library, Class mark (THS EAF TX609.M3)
Solar driers, Vegetables, Evaporation
Maeda, E.E. (1977) Solar drying of indigenous vegetables using enclosed convectional solar driers, Master dissertation, University of Dar es Salaam