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.
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:
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
Application of Osmotic dehydration
Some online reference
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