Cell osmotic dehydration

Freezing of tissues (vegetable or animal) generally causes some damage. At temperatures above — 10°C, ice crystals only form outside the cells. This causes freeze concentration of the extracellular liquid, hence to an osmotic pressure difference between intra- and extracellular liquid and hence to osmotic dehydration of the cells (plasmolysis). At lower temperatures, ice crystals tend to penetrate the cells, whereby intracellular crystallization occurs. This reduces plasmolysis but tends to increase mechanical damage, possibly leading to a soft texture of the tissue after thawing. 

Osmotic dehydration is a water removal process that consists of placing foods, such as pieces of fruits or vegetables, in a hypertonic solution. As this solution has higher osmotic pressure and hence lower water activity, a driving force for water removal arises between solution and food, whereas the natural cell wall acts as a semipermeable membrane. As the membrane is only partially selective, there is always some diffusion of solute from the solution into the food and vice versa. Direct osmotic dehydration is therefore a simultaneous water and solute diffusion process [55]. Up to a 50%... 

Water, on the other hand, is removed by osmosis and the cell sap is concentrated without a phase transition of the solvent. This makes the process favorable from the energetic point of view. The flux of water is much larger than the countercurrent flux of osmoactive substance. For this reason the process is called osmotic dehydration or osmotic dewatering. 

Osmotic dehydration occurs on a piece of material and not on a single cell. Hence, it should be assumed that the piece exists in all kinds of plant tissue. As a rule, a skin is removed... 

The models based on the irreversible process thermodynamics show that the cell membrane (plasma lemma) represents the major resistance to mass transfer. This is contradicted by findings of Raoult-Wack et al. [46-48], who showed that membranes are not necessary for osmotic dehydration and merely diffusive properties of the material are responsible for high water flux with only marginal sugar penetration. These authors suggest the following mechanism. 

High-intensity electric field pulses accelerated osmotic dehydration of carrot [18], A Fickian diffusion coefficient for water and solute increased exponentially with electric field strength. This effect was attributed to increased cell wall permeability, which was also manifested by the softening of product. 

The sequence presented above also suggests the kinetics of the osmotic dehydration process. As long as the mass transfer processes are not strongly dependent on the symplasmic pathway, the water flux will predominate over the osmoactive substance flux. This is due to osmotic pressure flow, which will reduce the countercurrent diffusion of osmoactive substance but it will not strongly affect the diffusive flux of water as the self-diffusion of water in a solution is of the same order of magnitude as that for solute. When plasmolysis occurs and the hypertonic solution fills in the volume between cell walls and plasmalemma, the... 

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