Drying Kinetics of Onion Under Different Drying Conditions

Abstract

The dehydrated onion products have the potential to become an important product because of relatively inexpensive, easily and quickly cookable and reach in several nutrients which are essential for human health. The proper drying techniques are the most important aspect of fruits and vegetables preservation. A greenhouse type solar dryer was used and the experiments were conducted to develop dehydrated onion slices so as to enhance the availability of onion slices during off season. In the present study, fresh onion slices were pretreated in two ways (i) KMS treated samples (ii) NaCI treated samples (ratio of water 1 :4 w/w) and untreated onion slices were also dried as control samples. The slices were dried at different thicknesses of 2mm, 4mm, and 6mm under greenhouse type solar dryer and in open sun. Physico-chemical analysis i.e. moisture content, moisture ratio, drying rate, ascorbic acid, acidity, total sugar, reducing sugar, colour, Ill and pungency were evaluated during the experiment. Experiments were also conducted to study the effect of drying conditions on sensory quality and rehydration characteristics. It was found that total drying time decreased in drying air temperature at 54.48 °C under greenhouse type solar drying condition. Major drying took place in falling rate period. The average drying rate increased with increase in temperature and decrease with time and thicknesses. Chemically treated samples dried under greenhouse type solar dryer took lesser time than untreated samples. It was observed that total moisture loss increased with increase in drying temperature. The rehydration ratio for dehydrated onion increased under greenhouse type solar dryer as compared to open sun drying. The product quality was found to be more acceptable in case of 2mm thickness, KMS treated samples dried under greenhouse type solar dryer.