Drying and rehydration characteristics of green papaya.

The drying characteristics of green papaya, normally sold at the local market, have been investigated using a laboratory forced-convection dryer. The effects of air temperature (40, 50 and 60 deg C), air velocity (1.02 and 2.03 m/s), thickness of material (0.50, 0.75 and 1.0 cm.) and blanching on the drying rate of green papaya were evaluated. Increasing the air temperature or reducing the thickness of papaya strips resulted in significant increase in the drying rate throughout the drying process. A significant increase in the drying rate was also affected by blanching the material or increasing air velocity, however, the relative effect being dependent on the moisture regions under consideration. Drying-rate curves were constructed and used for the determination of the critical moisture contents and effective diffusivities of moisture at different drying conditions. Straight lines were obtained in moisture region below 3.0 (db) when the non-dimensional moisture contents (W-We)/(Wc-We) were plotted against time in a semi-log paper. The result suggested that rate of moisture removal was a function of the free water content. The moisture movement in this region, however, can be described by molecular diffusion. The diffusivity of moisture in green papaya was found to increase with temperature and the drying time being proportional to the square of the thickness of papaya strips. The desorption isotherm of green papaya, however, fitted the Halsey, Kuhn and Smith equations. On the other hand, the rehydration characteristics of dehydrated green papaya were evaluated at room temperature. The differences on the percent rehydration of dehydrated green papaya obtained from different drying conditions were found significant. Regardless of the thickness of the material, the highest percentage rehydration were all obtained from the same drying conditions (air velocity, 1.02 m/s and air temperature, 60 deg C).