After the fruits and vegetables are received, they are still absorbing oxygen, expelling carbon dioxide and water. In the process of metabolism, a large amount of respiratory heat is generated, which makes the nutrients continue to be consumed and a large amount of water is lost. As a result, fruits and vegetables are discolored and softened, vitamins are reduced, and even deteriorated, losing commercial value. Therefore, experienced people should open the package of fresh vegetables immediately after buying them, and put them in the refrigerator as soon as possible. Otherwise fresh vegetables will spoil quickly. The effective way to reduce the respiration intensity of fruits and vegetables is to rapidly reduce the temperature of fruits and vegetables to reduce the respiration heat. Low temperature can not only inhibit the growth and reproduction of microorganisms, but also maintain the nutritional value and color, aroma, and quality of fruits and vegetables.

The basic principle of vacuum pre-cooling is to place the fruits and vegetables in a vacuum container, seal and vacuumize, when the pressure reaches the saturated water vapor pressure of the surface temperature of the fruits and vegetables, the water on the surface of the fruits and vegetables begins to evaporate, and the water evaporation increases with the pressure drop, and the water evaporates. The evaporative heat required in the process is provided by the fruits and vegetables. After the fruits and vegetables lose a certain amount of heat, they are cooled. Usually, it can be cooled to below 3°C in about 20min. At this time, the water loss of fruits and vegetables may only be 1%-3% of the mass, and only the water on the surface of fruits and vegetables.

Advantages of vacuum precooling:
Fruits and vegetables are cooled quickly, usually only 20-30 minutes, if they are cooled in a refrigerator, it will take several hours.
The treatment time is short and the water loss is low.
The freshness of fruits and vegetables pre-cooled by vacuum can be maintained for a long time, which is suitable for long-term storage and long-distance transportation.

The vapor pressure of saturated water vapor at different temperatures:
(Pressure unit: hpa)

Temperature ℃	Steam pressure	Temperature ℃	Steam pressure
-60	0.01	25	31.66
-55	0.02097	30	42.46
-50	0.04	35	56.22
-45	0.07232	40	73.75
-40	0.1289	45	95.82
-35	0.2244	50	123.40
-30	0.3816	55	157.41
-25	0.6351	60	199.20
-20	1.00	65	250.09
-15	1.656	70	311.60
-10	2.60	75	385.49
-5	4.02	80	473.70
0	6.10	85	578.06
5	8.72	90	701.00
10	12.28	95	845.26
15	17.04	100	1013.25
20	23.39	105	1208.00