Osmotic dehydration can be defined as a ‘dewatering and impregnation soaking process’ (DISP) (Torreggiani, 1993; Raoult-Wack, 1994), a combination of dehydration and impregnation processes which can modify the functional properties of food materials, thereby creating new products. Osmotic dehydration can be defined as a simultaneous counter-current mass transfer process in which biological materials (such as fruits and vegetables) are immersed in a hypertonic aqueous solution for a selected period.
The driving force for the diffusion of water from the tissue into the solution is the higher osmotic pressure of osmotic solution and its lower water activity that results in the transfer of water from the product across the cell wall. The diffusion of water is associated with the simultaneous counter diffusion of solutes from the osmotic solution into the tissue.
Importance of Osmotic dehydration
This contributes to a net opposite flux of water and solutes that allow the tissue to become concentrated with a determined ratio solute gain/water loss (SG/WL) depending on process conditions (Chiralt and Fito, 2003). Since the membrane responsible for osmotic transport is not perfectly selective, other solutes (sugar, organic acids, minerals, vitamins) present in the cells can also leach into the osmotic solution (Lenart and Flink, 1984a; Torreggiani, 1993) in amounts that are quantitatively negligible compared with the other transfer;
However, they are important in terms of final product quality (Dixon and Jen, 1977). During osmotic dehydration, there are different variables that affect the rate of water diffusion from any materials; therefore, it is difficult to establish general rules about them. However, osmotic pressure, plant tissue structure and mass transport relationship, are the most important ones.
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