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 (25–658C) 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 � 1029 to 8.04 � 1029 and
2.94 � 1029 to 5.53 � 1029 m2 s21, 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.