Drying characteristics of onion shreds under different drying methods

Abstract

Experiments were conducted to study drying characteristics of onion shreds in heat pump drying at 35, 40, 45 and 50ºC, microwave assisted convective drying at 50ºC convective air temperature at four microwave power levels, i.e. 120, 240, 360 and 480 W and convective hot air drying at 50ºC (46±4% R.H.), 55ºC (35±4% R.H.), 60ºC (28±4% R.H.) and 65ºC (20±4% R.H.). Drying rate was found to increase with increase in drying air temperature in all drying methods and microwave power level in microwave assisted convective drying. The drying times observed for bringing down the moisture content of onion shreds from 675.8 to 6±2.5 % (d.b.) were 480 min (maximum) for hot air drying (50ºC, 46±4% R.H.), 360 min for heat pump drying (50ºC, 15±2% R.H.) and 75 min (minimum) for microwave assisted convective drying (240 W power and 50ºC). The Page model can be used to predict moisture content of onion shreds at any time during drying process under different conditions with an acceptable accuracy.