Osmotic Dehydration of Fruits and Vegetable

Osmotic treatment as a preparation step to further processing of plant and animal material allows to improve the overall quality of food products by modifying the composition and the structure of the material (substrate). In the study presented focus is given to the impact of the solution applied for the treatment on the composition of the processed substrates (dewatering, uptake of osmotic agent) and on the structural parameters (overall and cellular shrinkage, porosity); the process was carried out at moderate temperatures (25 – 45°C) and as substrate apples, carrots and potatoes were used. The impact of the concentration was studied using sucrose (0.5 – 5.3 m), that of the osmotic active components was analysed using binary (sucrose, glucose, sorbitol, glycerol, salt) and ternary (sucrose-salt) aqueous solutions with similar water activity (aw = 0.91 ± 0.01). Moisture diffusion coefficients were estimated from dewatering rates. It was observed that the dewatering effect is enhanced by increasing the solute concentration, the solute uptake however is limited because of a parallel increase of the viscosity of the solution which has to be considered as a restricting factor for the mass transfer by the substrate. An increase of the molecular weight of the osmotic active component of the solution enhanced the dewatering effectivity. The degree of permeability of the cell membranes for small molecules and ions affected considerably the dewatering effect and the solute gain. Furthermore the beneficial effects of an osmotic treatment as a pre-treatment to convective warm air drying was confirmed for dried carrot slices.