Osmosis Osmotic dehydration

The nature of the plant material subjected to osmotic dehydration is the key point for both modeling and optimizing the osmosis in itself and as a pretreatment to further processing. The same osmotic medium, applied to different raw materials, under identical process conditions causes substantially different rates of dehydration and solute uptake. Data on these findings were reviewed previously (Lazarides et al., 1999 Torreggiani, 1995) and have been confirmed by recent research. 

ERTEKiN F K and CAKALOZ T (1996), Osmotic dehydration of peas. 11 Influence of osmosis on drying behavior and product quality , J Food Proces Preserv, 20, 105-119. 

An empirical equation derived based on osmotic dehydration of apple slices could predict rate of osmosis F, that is, percentage of dehydration of any given fruit slices of specific size with time T, given the concentration of sugar (% B) and the temperature as follows [59] ... 

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. 

The osmotic dehydration does not reduce water activity sufficiently to hinder the proliferation of microorganisms. The process extends, to some degree, the shelf life of the material, but it does not preserve it. Hence, the application of other preservation methods, such as freezing, pasteurization, or drying is necessary. However, processing of osmotically dehydrated sani products is much less expensive and preserves most of the characteristics acquired during the osmosis. 

Concluding the effect of procedures and processing parameters on the rate and efficiency of osmotic dehydration it can be stated that all of them are equally important. The kind and concentration of the osmoactive substance, the weight ratio of the solution to food, the kind of osmosed material, its size and shape, temperatnre and pressure, and the pretreatment of the material prior to osmosis affect strongly not only the course of the process but also, first of all, influence the quality and organoleptic attributes of the final product. 

It has been shown that apples dried by osmosis and then frozen compared favorably with the conventional frozen fruits [144,179], Osmotic dehydration preceding freeze drying shortens the time of the process and yields fruits superior to those not treated by osmosis [15,79,97], Osmotic dehydration followed by vacuum drying gives products that are very stable upon storage [65],... 

The effects of solution concentration, osmosis time, and the osmosis temperature were studied in the osmotic dehydration of pineapple in sucrose solution [58]. The solute diffusion was analyzed by Magee s model. The effect of sucrose concentration C on rate parameter K was given by power law regression equation as A = 4.15 x 10 at 20 C. 

Membrane Pervaporation Since 1987, membrane pei vapora-tion has become widely accepted in the CPI as an effective means of separation and recovery of liquid-phase process streams. It is most commonly used to dehydrate hquid hydrocarbons to yield a high-purity ethanol, isopropanol, and ethylene glycol product. The method basically consists of a selec tively-permeable membrane layer separating a liquid feed stream and a gas phase permeate stream as shown in Fig. 25-19. The permeation rate and selectivity is governed bv the physicochemical composition of the membrane. Pei vaporation differs From reverse osmosis systems in that the permeate rate is not a function of osmotic pressure, since the permeate is maintained at saturation pressure (Ref. 24). 

Ethanol produced by fermentation is conventionally dehydrated by distillation, an inefficient process that consumes energy equivalent to a large fraction of the energy content of the product ethanol.(X) Reverse osmosis (RO) has been considered before for ethanol-water separation because of its inherent energy efficiency. However, a difficulty encountered in using RO is the high osmotic pressures associated with concentrated ethanol solutions. For example, the osmotic pressure of a 15-volX ethanol solution is about 960 psi, and that of a 50-volX solution is about 3700 psi.(2,) Because most... 

Membrane separations involve the selective solubility in a thin polymeric membrane of a component in a mixture and/or the selective diffusion of that component through the membrane. In reverse osmosis (3) applications, which entail recovery of a solvent from dissolved solutes such as in desalination of brackish or polluted water, pressures sufficient to overcome both osmotic pressure and pressure drop through the membrane must be applied. In permeation (4), osmotic pressure effects are negligible and the upstream side of the membrane can be a gas or liquid mixture. Sometimes a phase transition is involved as in the process for dehydration of isopropanol shown in Fig. 1.8. In addition, polymeric liquid surfactant and immobilized-solvent membranes have been used. 

Osmotic pretreatment before microwave-assisted air drying increase the final overall quality of the product [187]. Fruits and vegetables treated by osmosis can be further dehydrated in a convection dryer to lower the water activity to the level of 0.65-0.90. At those water activities, water content in the material is still high and the product presents such organoleptic attributes as chewiness, softness, elasticity, and plasticity [76,188-190], The product has a natural color, well-preserved flavor, and high retention of vitamins [132], Its shrinkage is much smaller when compared with that observed in convection-dried products at the same water activity. 

Osmosis Salt solution More concentrated salt solution membrane Two liquids Tendency to achieve uniform osmotic pressures removes water from more dilute-solution Suggested for food dehydration 18, 43... 

Partial dehydration of fruits and vegetables by osmosis using various osmotic agents has been employed... 

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