DRYING KINETICS, ENERGY REQUIREMENT, BIOACTIVE COMPOSITION AND MATHEMATICAL MODELING OF ALLIUM CEPA SLICES

Abstract

The drying kinetics, specific energy consumed (SEC), effective moisture diffusivity (EMD), flavonoid, phenolic and vitamin C contents of onion slices dried under convective oven drying (COD) were compared with microwave drying (MD). Drying was performed with onion slice thicknesses of 2, 4, 6 and 8 mm; air drying temperatures of 60, 80, and 100°C for COD and microwave power of 450 W for MD. A decrease in slice thickness and increase in drying air temperature led to a drop in the drying time. As thickness increased from 2 – 8 mm, EMD rose from 1.1– 4.35 ´10-8 at 60oC, 1.1 – 5.6 8 10´ - at 80oC and 1.25 – 6.12 8 10´ - at 100°C with MD treatments yielding the highest mean value (6.65X 10^-8 m2 s-1) at 8 mm. Maximum SEC for onion slices in COD was 238.27 kWh/kg H2O (2 mm thickness) and the minimum was 39.4 kWh/kg H2O (8 mm thickness) whereas maximum during MD was 25.33 kWh/kg H2O (8 mm thickness) and minimum, 18.7 kWh/kg H2O (2 mm thickness). MD treatment gave a significant (p£0.05) increase in the flavonoid (39.42 – 64.4%), phenolic (38.0 – 46.84%) and vitamin C (3.7 – 4.23 mg 100 g-1) contents while COD treatment at 60°C and 100°C had positive effects on only vit. C and phenolic contents respectively. In comparison, the Weibull model gave the overall best fit (highest R2=0.999; lowest SSE=0.0002, RSME=0.0123 and χ2= 0.0004) when drying 2 mm onion slices at 100°C.