Effect of Temperature on Drying Characteristics and Quality of Three Nigerian Onion Varieties Using a Fabricated Electrically Powered Dryer

Abstract

The research work is done to evaluate the effect of temperature on drying characteristics and quality of red, cream and white varieties Nigerian onion using a locally fabricated electrically powered dryer. Clean onion samples of 1.5 kg from each of the three varieties were sliced into 3 mm thickness. Fresh samples of the three onion varieties were analyzed to determine the initial values of protein, fat, ash, crude fibre, and moisture content on a dried basis (m.c.d.b.) using recommended AOAC methods.. Vitamin C and microbial loads content were determined before loading into a locally fabricated electrically-powered dryer. The onion samples were dried at varying pre-determined temperatures of 50, 60, and 70 oC. After drying, the proximate compositions, vitamin C and microbial loads were also determined. The protein, fat, ash and crude fibre of the fresh onion samples were 1.5, 0.5, 0.6 and 0.5%, respectively, while m.c.d.b, vitamin C content and microbial loads ranged from 85.1-88.2%, 8.1-8.3 mg/100 gm and 4.45-5.39 mg. Red onion samples were dried for 13:15h, 12:10h and 10:30h; white onion samples for 13:00h, 12:25h and 10:40h; while cream onion samples for 13:10h, 12:35h and 10:15h at 50, 60, and 70 oC, respectively. The protein, fat, ash, crude fibre, m.c.d.b, vitamin C content and microbial loads of the dried onion samples at all the drying temperatures ranged from 10.110.4, 1.1-1.5, 3.3-3.5, 5.5-5.8, 5.5-6.9%, 3.1-4.9 mg/100 gm, 1.314.17 mg, respectively. The drying temperature had a significant effect (p-value < 0.05) on the quality of all the onion samples. At 70 oC, protein, fibre, ash, fat and pungency retention were significantly (p< 0.05) high and within acceptable limits while microbial loads were low with the reduction in Vitamin C contents in all the onion varieties. Therefore, onion can best be dried at 70 oC to achieve the optimum quality and flavour required for its use as a spice.