Agriculture Reference
In-Depth Information
membrane responsible for osmotic transport is no ideal selectivity and other dissolved
substances present in the cell may also exceed the osmotic solution (Lerici et al., 1985). The
diffusion rate of water from the material composed of such a tissue depends on several factors
such as temperature and osmotic solution concentration, size and shape of the material, the
weight ratio of the solution and the material and many other factors. Mass transfer takes place
through a semipermeable membrane that represents the dominant resistance to the process of
osmotic dehydration. Condition of the cell membrane can be changed from partial to full
permeability due to changes in its structure. Loss of contact between the outer membrane of
the cell and cell wall is the most likely cause of damage to the cells and is triggered by the
loss of water during the osmotic treatment (Rastogi et al., 2000). It is assumed that during the
osmotic dehydration formation of a surface layer of 2-3 mm in a product has a great influence
on the mass transfer by favoring water loss, while limiting the diffusion of the solute in the
tissue and loss of dissolved cell substances into the osmotic solution (Raoult-Wack et al.,
1991).
Explanation of differences between the mechanism of osmotic dewatering of the
homogeneous (non-biological) and biological material is suggested by Rastogi et al. (2000).
In a homogeneous material it is assumed that constant rate of diffusion occurs due to a
uniform gradient of humidity. Yet this is not entirely true especially after the initial steps of
the process when it comes to change in the physical structure of the material during the
progression of osmotic dehydration. In the case of biological materials is assumed that
dehydration front progressively advance towards the center of the material (Rastogi et al.,
2000), resulting in the disintegration of the cell membrane in a dehydrated region.
Characteristic of water transport is the existence of the three regions as follows:
diffusion from the center of the material to the front of dehydration,
diffusion through the front and
diffusion through osmotic treated material into the surrounding solution.
Process of water diffusion from the outer layers of the sample into an osmotic medium, in
the early stages of the osmotic dehydration, leads to an increase in osmotic pressure at the
surface. When the osmotic pressure reaches a certain critical value cell membrane disrupts
and cell shrinks, which results in the reduction of the number of unchanged cells inside of
plant tissue. As the osmotic dehydration continues, dehydration front moves toward the center
of the product. On this front the cells are in the process of disintegration and hence the rate of
mass transfer increases rapidly. In this section a relatively large amount of water diffuses out
of the cells, while the cells in the center of the material remained unaltered, the diffusion of
water from the center is much smaller compared to the front of dehydration. Part of osmotic
solution does not enter the cells but is retained in the intercellular space of the tissue. This
effect the so called impregnation of plant tissue is essential, as it can alter the sensory quality
of the sample. The essential difference of osmotic dehydration as compared to other processes
of dehydration lies precisely in said impregnation food osmotic solution. To a certain extent
impregnation allows changing nutritional, functional and sensory properties of food that
undergoes the process of osmotic dehydration.
