Azarpazhooh, Ramaswamy - Osmotic Dehydrationreas plasticity comes from pectin and hemicelluloses (Lewicki and Pawlak, 2003). Mid-
dle lamella has two thin semi-permeable membranes: the tonoplast and the plasma-
lemma. Protoplast is separated by the plasmalemma from the cell wall, and cytosol solu-
tion. The osmotic phenomenon is largely controlled by the plasmalemma (Nobel, 1999).
The cytosol is the major component of protoplast which contains different organelles
such as the chloroplasts, mitochondria, peroxisomes, ribosomes, and proteins. These
macromolecules and structures can affect the thermodynamic properties of water. The
vacuole is a large central space inside the protoplast filled with water and surrounded
by tonoplast. A vacuole has an osmotic pressure that pushes protoplasm and plasma-
lemma toward the cell wall. This osmotic pressure is called turgor pressure which is the
difference between the osmotic pressure in the cell and its surroundings. When the cell
and the surroundings have the same osmotic pressure, the turgor pressure is zero and
the system is in equilibrium. If the osmotic pressure of the surroundings is higher than
the cell, the water transfers into the cell and the cell swells. During osmotic dehydration,
the plant cell is placed in a hypertonic solution with the osmotic pressure higher than
that of the cell; as a result, the cell loses its water and decreases its volume. Consequent-
ly, plasmalemma is detached from the cell wall. This process is called plamolysis.
A mass transfer phenomenon is a complex mechanism occurring in plant tissue dur-
ing osmotic dehydration. Water is transferred from the inner tissue to the outside,
through the porous tissue structure, and then through the outside boundary layers.
There are three important pathways during osmotic dehydration; symplastic (the trans-
port within the intracellular volume), free-space transport (the transport within the
extracellular volume) and apoplastic (water passing through plasma membranes) (Shi
and LeMaguer, 2002). The transport of water between cells along the symplastic rout is
mediated by plasmodesmata, whereas in the transcelluar path water has to cross plasma
membranes. Furthermore, water moves across a tissue by crossing two membranes per
cell layer and the apoplast (Steudle and Frensch, 1996). The removal of water during the
osmotic process is mainly by diffusion and capillary flow, whereas solute uptake or
leaching is only by diffusion.
4.2.3. Osmotic dehydration mass transport phenomena
In fruits or vegetables, the cell wall membranes are living biological units which can
stretch and expand under the influence of growth and turgor pressure generated inside
the cells. The Semi-permeable membranes present in biological materials are the domi-
nant resistance to mass transfer during osmotic dehydration. The cell membrane can
change from being partially to totally permeable, leading to significant changes in tissue
architecture (Rastogi et al., 2002). When plant cells are placed in a hypertonic solution,
water removal starts from the surface that is in contact with the osmotic solution, result-
ing in cell disintegration (Rastogi et al., 2000b). It is reported that sugars penetrate to a
depth of 2-3 mm into the plant tissue while changes in water content are observed up to
5 mm (Bolin et al., 1983; Lenart and Flink, 1984b). Water leaves the cell surface by os-
mosis; therefore, the vacuole and the rest of the protoplasm will shrink, and plosmolysis
occurs. However, the interior surface of the material can remain in full turgor pressure.
A turgor pressure gradient results in detaching of plasma membrane and the middle la-
mella due to the degradation or denaturation of the components of the middle lamella.