Comparatve Study of Greenhouse Type Solar Dryer and Open Sun Drying for Amaranth

Abstract

The dehydrated leafy vegetables have the potential to become an important product because of relatively inexpensive, easily and quickly cookable and rich in several nutrients which are essential for human health. The proper drying techniques are the most important aspect of leafy vegetable preservation. The use of solar dryer helps to reduce the losses and improves the quality of product. A Greenhouse type solar dryer was used and the experiments were conducted to develop dehydrated amaranth leaves so as to enhance the availability of amaranth leaves during off season. In the present study, fresh amaranth leaves were pretreated in two ways viz. (i) dipping in a solution (leaves to water 1:5 (w/w)) containing 0.1% Mgcl2 + 0.1% NaHC03 + 2% KMS in distilled water for 15 minute and (ii) blanching in boiling water containing 3% sodium bisulphate for 2 min. The leaves were dried at 1.5, 2.0 and 2.5 kg/m2 loading density, under greenhouse type solar dryer and in open sun. Untreated amaranth leaves were also dried as control samples. Physico-chemical analysis i.e. moisture content, moisture ratio drying rate, and drying time were evaluate during the experiment. Experiments were also conducted to study the effect of drying condition on sensory quality and rehydration characteristics. It was found that total drying time considerably reduced with the increase in drying air temperature from 30 °C under open sun drying to 44 °C under greenhouse type solar drying conditions. Major drying took place in falling rate period. The average drying rate increased with increase in temperature and decrease with time and loading density. Chemically treated samples dried under greenhouse type solar dryer took lesser time than blanched and untreated samples. It was observed that total moisture loss increased with increase in drying temperature and decreased with decrease in drying temperature. The rehydration ratio increased under greenhouse type solar dryer as compared to open sun drying. The product quality was found to be most acceptable of amaranth leaves treated in the solution of 0.1% Mgcl2 + 0.1% A: 2'/~MS, dried under GSD using 2.0 kg/m