Azarpazhooh, Ramaswamy - Osmotic Dehydration
Consequently, the mechanical properties of the product will change and the structure
will deform. Lewicki and Porzecka-Pawlak (2005) reported cell debonding during os-
motic dehydration of apple. Consequently, the cell is damaged and reduces in size by the
loss of water and contact between the outer cell membrane and the cell wall (Rastogi et
al., 2000b; Rastogi et al., 2002). Extensive uptake of osmoactive substance results in the
development of a concentrated solids surface layer posing an additional resistance to
mass transfer (Lenart and Lewicki, 1987; Lenart, 1994).
Consequently, porosity of the product will increase, and the tissue shrinks because
the amount of water flowing out is generally greater than the solutes diffusing in. The
diffusing substances are assumed to consist of water and sucrose only (Marcotte et al.,
1991 ). Therefore, the weight of the foods will decrease, as will the water activity. It is
reported that up to a 50% reduction in the fresh weight of fruits or vegetables may be
brought about by osmosis (Rastogi et al., 1997; Kar and Gupta, 2001). All these mass ex-
changes may have an effect on the organoleptic and/or nutritional quality of the dehy-
drated product (Sablani et al., 2002). As a consequence of this exchange, the product
loses weight and shrinks. Cellular shrinkage during dehydration has been observed dur-
ing osmotic dehydration of apple (Lewicki and Porzecka-Pawlak, 2005 ).
4. 3. FACTORS AFFECTING OSMOTIC DEHYDRATION
The rate of diffusion of water from any material during osmotic dehydration is de-
pendent upon factors such as type of osmotic agent, concentration of the osmotic solu-
tion, temperature, the size and geometry of the material, the solution-to-material mass
ratio and the level of agitation of the solution. There are several publications which de-
scribe the influence of these variables on mass transfer rate (Lerici et al., 1985; Raoult-
Wack, 1989; Raoult-Wack, 1994; Rastogi et al., 1997; Rastogi and Niranjan, 1998; Rasto-
gi et al., 1999; Corzo and Gomez, 2004). However, the variables mentioned above can be
manipulated over a limited range; outside of these ranges, the quality was adversely af-
fected even though mass transfer rates may be enhanced (Rastogi et al., 2002). There
are also some techniques which can combine with osmotic dehydration, and have the
ability to alter membranes in order to enhance mass transfer rate. They include: ultra-
sound (Rodrigues and Fernandes, 2007) high-intensity electric field (Rastogi et al.,
1999 ), or high hydrostatic pressure (Akyol et al., 2006) and microwave (Li and Ramas-
wamy, 2006c; Azarpazhooh and Ramaswamy, 2010a,b). The choice of process condi-
tions depends on the expected water loss, soluble solids gain, and the sensory properties
of the food products.
4 .3.1. Influence of size and shape on the mass transfer
Some research has been done on the influence of size and shape on the mass transfer
kinetics. The surface area to volume ratio has been shown to be the influencing factor
with higher ratios favoring better osmotic dehydration rates. Islam and Flink (1982) re-
ported that the size and geometry of the food has some influence on the extent of final
solute concentration, especially during short dehydration times; at such times, dehydra-
tion was primarily a transport phenomenon related to surface area. Lerici et al. (1985)
compared osmotic drying of apple slices of four different shapes of (i.e slice, stick, ring
and cube) and reported that the solids content increased with a decreasing surface