Vacuum cooling
Vacuum cooling is known to be the most rapid cooling technique for any porous product which has free water and works on the principle of evaporative cooling .Vacuum cooling is generally used for cooling food products having a high water content and large porosities, due to its efficacy in losing water from both within and outside the products.This is the most widely used technique for rapid cooling of food product which has been proven to be one of the most efficient and economical method of cooling and storage of vegetables, fruits, flowers & more. This cooling technology not only strongly improves the product quality, but also increases the shelf life of product and at the same time it reduce the cooling costs compared to the conventional cooling method available.
Principle
The technology is based on the phenomenon that as the vapour pressure on a liquid reduces, its boiling point reduces.The boiling point of a liquid is defined as the temperature at which the vapour pressure of the liquid is equal to the external pressure. When the pressure above a liquid is reduced, the vapour pressure needed to induce boiling is also reduced, and the boiling point of the liquid decreases.Therefore, by reducing pressure we can even boil off water at lower temperatures . This rapid evaporation of moisture from the surface and within the products due to the low surrounding pressure, absorbs the necessary latent heat for phase change from the product itself.This latent heat required for evaporation is obtained mostly from the sensible heat of the product and as a consequence of this evaporation the temperature of the product falls and therefore the product can be cooled down to its desired storage temperature.
Vacuum cooling process
An airtight chamber is maintained by removing air from the inside of the chamber using a vacuum pump.The products to be cooled are kept in that airtight chamber.As the pressure is reduced the boiling point of water reduces and water starts to evaporate, taking the heat from the product . As a consequence of this evaporation the product temperature begins to decrease. This Cooling process of the products continues until it reaches the desired product temperature. For maintaining steady cooling process, it is necessary to evacuate the chamber continuously.
Other factors that determine the cooling process are surface area of product, available for heat transfer and also the product sensitivity during losing water.
Advantage
As the product is cooled uniformly throughout the body without any temperature gradient in the body the shelf life of the product increases.
Disadvantage
Sometimes excess moisture loss during the cooling process will deteriorate the product quality and therefore there is a limit to cooling process .This problem is to be taken care of by maintaining the required pressure, temperature and time of cooling .
References
- H.M. Ozturk, H.K. Ozturk, Effect of pressure on the vacuum cooling of iceberg lettuce Int. J. Refrig. Revue Int. Du. Froid, 32 (3) (2009), pp. 402–410
- http://dx.doi.org/10.1080/10408398.2011.594186
- http://issuu.com/coldmax/docs/coldmax_web_uk/1
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