Azarpazhooh, Ramaswamy - Osmotic Dehydrationison with other dehydration methods is the incorporation of solute into the food system,
to a certain extent, which can change the functional properties of the product; it is poss-
ible to achieve specific formulation properties without modifying its integrity
(Torreggiani, 1993). Research on osmotic dehydration of foods was pioneered by Pont-
ing et al. (1966), and since then a steady stream of publications have appeared. These in
general have dealt with various parameters, such as the mechanism of osmotic dehydra-
tion, the effect of operating variables on osmotic dehydration, modeling of water loss
and solid gain, and enhancement of mass transfer (Lenart and Lewicki, 1987; Torreggia-
ni, 1993, Raoult-Wack, 1994; Raoult-Wack et al., 1994; Azuara et al, 1992; Lenart, 1996;
LeMaguer, 1996; Rastogi et al., 1997; Lewiciki, 1998; Nsonzi and Ramaswamy, 1998a,b;
Khin et al., 2005; Mastrocola et al., 2005; Falade and Igbeka, 2007; Vadivambal and Jayas,
2007 ; Li and Ramaswamy 2006a,b,c).
Despite its well-recognized advantages and the large amount of research work that
has been published in this area, industrial application of osmotic dehydration is limited.
This chapter details the basic concepts and recent developments in osmotic dehydration
highlighting the effect of process variables, modeling of mass transfer, techniques devel-
oped to enhance mass transfer rates, factors affecting quality parameters as well as a
brief overview of the different techniques employed for the finish drying.
4.2. BASIC PRINCIPLES OF OSMOTIC DEHYDRATION
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 mate-
rials, thereby creating new products. Osmotic dehydration can be defined as a simulta-
neous counter-current mass transfer process in which biological materials (such as
fruits and vegetables) are immersed in a hypertonic aqueous solution for a selected pe-
riod. 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 as-
sociated with the simultaneous counter diffusion of solutes from the osmotic solution
into the tissue. 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 respon-
sible 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 com-
pared 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 es-
tablish general rules about them. However, osmotic pressure, plant tissue structure and
mass transport relationship, are the most important ones (Islam and Flink, 1982; Lerici
et al., 1985).