Azarpazhooh, Ramaswamy - Osmotic Dehydration
1996 ). The sugar uptake is owing to the protective action of the saccharides (Ponting et
al. 1966 ).
4.6.1.2. Impact of osmotic dehydration on texture
Texture is a significant quality attribute of fruits and vegetables. During osmotic de-
hydration, the rheological properties of osmo-dehydrated products are changed due to
physical and chemical modifications occurring in the cell structure (Lewicki, 1998).
Properties of the cell wall and middle lamella and the turgor pressure are the most im-
portant factors to determine the texture of plant tissue (Jackman and Stanley, 1995; Chi-
ralt et al., 2001b). Plant tissue is affected by size and shape of the cell, volume of the va-
cuole, intercellular spaces volume, presence of starch granules and chemical composi-
tion. The majority of foods have visco-elastic behavior; that is why, during osmotic de-
hydration, the viscous natures of fruits and vegetables increase while their elasticity de-
creases due to the sugar uptake (Mayor et al., 2007). Osmotic dehydration weakens the
texture of apples and makes apple tissues softer and more plastic than those of raw ap-
ple (Monsalve-GonaLez et al., 1993). Although there are numerous reports dealing with
the effect of some sugars on the structural properties of osmo-treated plant material
(Marcotte and LeMaguer, 1991; Maltini et al., 1993; Barat et al., 2001b), only a few re-
ports talk about the structural changes at the cellular level which are only accessible
through microscopic observations (Saurel et al., 1994; Martinez-Monzo et al., 1998).
Puncture force is usually used to measure the textural properties of dehydrated prod-
ucts which is the measure of the hardness of the product surface, and presents the ex-
tent of case hardening during drying (Lin et al., 1998). During osmotic treatments, the
main changes that affect the mechanical behavior of plant tissue are changes in the air
and liquid volume fractions in the sample, the size and shape of the sample (Fito, 1994),
loss of cell turgor, alteration of middle lamella (Alzamora et al., 1996), alteration of cell
wall resistance, establishment of water and solute concentration profiles and composi-
tional profiles in osmotically dehydrated samples (Salvatori, 1998). Differences in me-
chanical behavior of the dried samples must be related to the differences induced in the
composition of the soluble water phase and in the solid matrix during treatments. Con-
treras et al. (2007) reported that soluble pectin is increased during drying which is al-
tered cell bonding zone results in changing the solid matrix consistency. Osmotic dehy-
drated products have a softer texture due to leaching of calcium into the osmotic solu-
tion which in turn results in lowering the concentration of calcium content ions inside
the tissue (Prothon et al., 2001).
4.6.1.3. Impact of osmotic dehydration on rehydration properties
There is a need for understanding the rehydration process due to the wide variety of
dehydrated foods which are available today to consumers. Of particular concern are
meeting quality specifications and conserving energy. Dehydrated products are usually
rehydrated by immersion in water or other liquids, such as fruit juices, sucrose or glu-
cose solutions. Restoring the properties of the fresh product by immersing dehydrated
products in a liquid phase is an important aspect during rehydration. Rehydration can
reflect the physical and chemical changes that have occurred during osmotic dehydra-
tion, and can therefore be used as a quality index. In other words, any pretreatment to
which the products have been subjected are modified by the composition of the samples.