Osmotic dehydration is a non destructive technology to reduce the water content, as well as to improve the quality of the final product. This process is being used in industry to dehydrate fruits, vegetables, meat, and fish, but the industrial application is still limited.

 

Basic principle

Principle of osmosis (movement of water from a low concentrated solution to a high concentration solution via a semi-permeable membrane) is used. In this method of dehydration the cut foods in immersed in concentrated solutions of sugars and salts. A flux of water out of the food and of other solutes into the foodstuff develops due to the difference in osmotic pressure. The product thus loses some water to the external solution.  Rate of removal of water can be enhanced by increasing the concentration of the osmotic solution or the temperature.

 

To understand the principle of osmosis

 


Some experimental evidences related to osmotic dehydration

In an experiment by Rogachev and Kislenko (1972) it was noticed that coupled alternate heating and cooling with acoustic fields to increase osmosis. In another experiment, Shi and Maupoey (1993) applied vacuum (100 mb) treatment to increase water transport during osmosis in 65 Brix sugar solutions for dehydration of pineapple and apricot cubes. They claimed higher dehydration rates at lower temperatures, thus improving the quality of dehydrated fruit products. Grabowski and Mujumdar (1992) have proposed a novel scheme for solar-assisted osmotic dehydration of fruits. Solar energy is used to dry the osmotically dehydrated slices of fruits and also to concentrate the osmoticum.
 

In the light of the published literature, some general rules can be noted:

  • water loss and solid gain are mainly controlled by the raw material characteristics and are certainly influenced by the possible pre-treatments;
  • it is usually not worthwhile to use osmotic dehydration for more than a 50% weight reduction because of the decrease in the osmosis rate over time. Water loss mainly occurs during the first 2 hr and the maximum solid gain within 30 min.;
  • the rate of mass exchanges increases with temperature but above 45 ° C enzymatic browning and flavour deterioration begin to take place. High temperatures, i.e. over 60° C, modify the tissue characteristics so favouring impregnation phenomena and thus the solid gain;
  • the best processing temperature depends on the food; mass exchanges are favoured by using high concentration solutions;
  • phenomena which modify the tissue permeability, such as over-ripeness, pre-treatments with chemicals (SO2), blanching or freezing, favour the solid gain compared to water loss because impregnation phenomena are enhanced;
  • the kind of sugars utilised as osmotic substances strongly affects the kinetics of water removal, the solid gain and the equilibrium water content. Low molar mass saccharides (glucose, fructose, sorbitol, etc.) favour the sugar uptake;
  • addition of NaCl to osmotic solutions increases the driving force for drying.
  • Synergistic effects between sugar and salt have also been observed.

 

 

Osmotic medium

The osmotic medium is commonly a sugar solution with a fruit/syrup ratio of 1/3–5 at a temperature between 20 to 50 C and slightly agitated. The duration of the process and syrup concentration are dependent on the type of fruit being processed. After osmotic dehydration has been completed (reducing up to 50% of the initial water content), the fruits are drained and placed into a hot air dryer to achieve the final desired moisture content. Syrup solution can be recuperated and after the sugar concentration has been adjusted, it could be used again for osmotic dehydration.

  



An example of osmotic dehydration process

Source: www.fao.org/docrep/V5030E/V5030E0j.htm

  

Application of Osmotic dehydration

 

 

 

Some online reference

 

QUALITY AND SENSORIAL CHARACTERISTICS OF OSMOTICALLY DEHYDRATED MAN...

 

 

 

References

Rastogi,N.K., K.S.M.S.Raghavarao and K.Niranjan(2005). Developments in Osmotic Dehydration. Emerging technologies for food processing.ISBN 0-12-676757-2.

 

Processing Fuits Science and Technology 2nd Edition l 2005 l Diane M. Barrett, Laszlo Somogyi, Hosahalli Ramaswamy l CRC Press

 


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Comment by deepa sonawane on December 11, 2011 at 7:37pm

nice! It helped alot. thanks sir!

Comment by Subham Kumar on November 8, 2011 at 4:00pm
very good
Comment by Nitin Rana on October 18, 2011 at 5:53pm
My college days practical...nice file sir.
Comment by Anusha Ramani on October 16, 2011 at 9:34pm
superb one

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