In order to prolong the shelf life of fruits and vegetables, reduce their dry consumption and various losses in circulation, and enable consumers to obtain fruits and vegetables with high freshness and cleanliness, a series of post-harvest processing treatments must be carried out in the factory, such as selection, pedicle removal, washing and pre-cooling, water filtration, packaging, etc., so the pre-cooling process is even more essential. Before refrigeration or freezing, the food is quickly cooled from the original temperature (such as the temperature of fruits and vegetables within 24 hours after harvesting or the temperature after slaughter of livestock and poultry) to a predetermined temperature, which can inhibit the biochemical reaction of the food itself and the reproduction of microorganisms in time. Common cooling methods are:

① Air cooling method, ventilate cold blow to the tunnel where the food is placed, or use a pressure difference of 1300~ 2000Pa to make cold air enter and pass through the food packaging box with air windows to accelerate the cooling of the product. The temperature, relative humidity and flow rate of cold air are determined according to the type of food, generally do not freeze the food, and the relative humidity is usually 90~ 95%.

② Water cooling method, using water as the heat conduction medium, uses a large-capacity water pump to spray cold water close to 0 ° C on the product. Because the heat capacity of water is much larger than that of air and the heat transfer efficiency is high, most foods can be cooled within 10 to 15 minutes. When cooling fish with seawater, the speed is fast, the temperature is uniform, and the storage period can be extended.