Comparative Study between Open-direct Sun and Solar Drying of Onion Slices (Allium cepa L.)

Abstract

Thin layer drying of onion slices (Allium et al.) was experimentally investigated using a free convective solar dryer and open-direct sun. The objective of this study is to compare the two methods of drying onions. The data of sample weight for open-direct sun and solar drying experiments were hourly recorded, drying rate (g water/100 g DM) and moisture contents (%) in dry basis (d,b) and wet basis (w,b) were calculated. Quality analysis for protein, fats, fiber, ash, and carbohydrates contents was also analyzed and evaluated. According to the results, direct open sun drying of onion slices took (19 hours) with a final moisture content of 16% and 13.5% dry basis and wet basis, respectively. At the same time, solar drying took (32 hours) with a final moisture content of 10% and 9.08% dry basis and wet basis, respectively. Solar drying showed a high drying rate as compared with direct sun drying. However, the two drying methods have a high drying rate at noon. The drying experiments showed no constant rate period but a falling rate period. For quality assessments or evaluation of onion slices, onion dried with the direct open sun was very dirty and contaminated with some foreign materials and microorganisms, while onion dried with solar showed no contamination. For quality evaluation analysis of protein, ash, fat, fiber, and carbohydrates contents, these constituents showed an increase for boss solar and direct open sun drying of onion. This is because drying reduces the moisture content of the product significantly; therefore, the dry matter was increased as a percentage.