The process of vacuum cooling is the extremely rapid evaporation of water from porous products. The boiling point of water is reduced with the vacuum. The moisture inside the product acts as a heat transfer unit; thus the temperature of the product is the boiling point of the water vapor. The temperature is thereby controlled by the degree of vacuum obtained and held during the cycle.

The evaporated water is then condensed by means of a refrigeration system, and discharged after each cycle. The rate at which a product can be refrigerated is directly related to the density of the product and the amount of temperature drop. The average moisture loss is 1% for each 12ºF temperature drop.

The products that can be easily vacuum cooled should dissipate moisture readily. They can range from fresh vegetables, cut flowers, tobacco, crisp head lettuce, cauliflower, Chinese cabbage, bok choy, cabbage, carrots, bell peppers, mushrooms and other leafy vegetables.

Hydro and Vac cooling (vacuum cooling with injection of water prior to vacuum cycle) is effective for celery, spinach, romaine, green onions, most Chinese vegetables and many other leafy vegetables.