Rehydration characteristics of osmotic pretreated and dried onion

The effect of osmotic pretreatments (10% sodium chloride for 1 h or 50% sucrose for 3 h treatment) on rehydration kinetics of dried onion was studied over a range of temperatures (25-65degreesC) and compared with an untreated (control) sample. The effective diffusion coefficients for water and solute were determined by considering the rehydration process to be governed by Fickian diffusion. The diffusion coefficients for water absorption and solute infusion were found to be in the range 1.96 x 10(-9) to 8.04 x 10(-9) and 2.94 x 10(-9) to 5.53 x 10(-9) m(2) s(- 1), respectively. Osmotic pretreatment resulted in a decrease in the diffusion coefficient of water as well as an increase in the diffusion coefficient of solute during rehydration. Decrease in the diffusion coefficient of water was due to an increase in the proportion of ruptured and shrunken cells caused by osmotic treatments, which in turn resulted in reduced ability of dried onion tissue to absorb water. The increase in diffusion coefficient for solid during rehydration was higher in the case of osmotic pre-treated material, some of the solids absorbed during osmotic dehydration were not retained in the cell matrix and dissolve faster than the constitutive onion dry matter.