An Experimental Study on the Thin Layer Drying Behavior of Onion Slices Under Various Air Temperature Conditions

Abstract

High postharvest losses during onion storage are mainly attributed to high moisture content and inadequate drying control. In the present work, thin-layer drying characteristics of onion slices were experimentally examined to understand the combined influence of drying air temperature, relative humidity, and slice thickness under convective drying conditions. Drying experiments were carried out at a constant air velocity of 0.5 m/s, with drying temperatures varied between 50 to 80 °C and relative humidity maintained in the range of 10–25%. Onion slices of different thicknesses were investigated to evaluate moisture transport behaviour. The initial moisture content of approximately 85% (wet basis) was reduced to about 5% through controlled drying. A substantial reduction in drying time was observed with decreasing slice thickness, where a 65% reduction in thickness resulted in nearly 62.5% faster drying. Lower relative humidity promoted enhanced moisture removal by sustaining a higher vapor pressure gradient. The water activity of dried samples remained below 0.6, indicating improved microbial stability. Thin-layer drying models were fitted to the experimental data to obtain reliable kinetic parameters. The results provide practical guidance for selecting suitable operating conditions in convective dryers and demonstrate the potential of controlled drying strategies for improving process efficiency and product stability in high-moisture agricultural materials.