The vacuum precooling technology is based on the phenomenon that as the vapour pressure on a liquid reduces, its boiling point reduces. This method is defined as the removal of the latent heat of the water in the cooled product with rapid decrease temperature as a consequence.
With the constant decrease in the pressure of the cooling system, the continuous evaporation of the product takes place. During vacuum cooling, free water evaporates at the temperature corresponding to the boiling (flash) point (Dostal & Petera, 2004).
When the vacuum precooling starts, with the effect of pressure, the water available in the interior of the product migrates to the surface of the product through boiling and evaporates. Therefore, cooling time becomes considerably shorter (Houska et al., 2003; McDonald & Sun, 2000, 2001a, b; McDonald et al., 2000, 2001, 2002; Sun & Wang, 2004; Wang & Sun, 2001, 2002a, b, 2004; Rodrigues et al., 2012).
For most leafy vegetables, even after packaging before pre-cooling, the temperature can be reduced to 4-5 degrees in 20 ~ 30min.
Due to the balanced pressure in the vacuum chamber, the moisture in the fruits and vegetables can evaporate even, follows to absorb the stored field heat, the temperature of the cooling produce, both externally and inside, can be cooled uniformly.
Vacuum precooling vegetables such as lettuces, leafy greens, cabbage, celery, spinach has proven to be very successful to keep fresh and extend the product shelf life in the cold chains. This precooling process is the removal of field heat immediately following harvest, where field heat accelerates the deterioration and senescence processes.
Vacuum precooling, nowadays, is as well applicate to rapid cooling the high temperature cooked food, like packed meal, to room temperature (≤15℃) before MAP packaging process.