The quick-freezing methods of fruits and vegetables are as follows:

(1) The blast freezing method generally adopts a tunnel-type blast freezer, and the product is carried through the tunnel with a mesh belt, which generally goes against the cold air. This freezing method usually adopts the cold air temperature of-18 ~-34℃ and the wind speed of 31 ~ 1166 meters per minute.

In addition, there is a separate freezing method, which can be used for small granular products such as green peas, sweet corn and all kinds of vegetables cut into small pieces. The granular products are pinned on a perforated mesh belt or a perforated plate with a thickness of 2.5-12.5 cm. When freezing, the cold air is strongly blown upward from under the mesh belt at a sufficient speed, and the products are blown but not taken away. This method increases the contact between the cold air and the materials.

(2) indirect contact freezing method This method is to contact and freeze the product with a metal plate cooled by refrigerant. The main device is to install a movable hollow metal plate in an insulated kitchen. The refrigerant flows in the hollow interior of the plate, and the product is placed in close contact between the upper and lower hollow plates for heat exchange. There are mainly the following three types: intermittent contact freezer, semi-automatic contact freezer and full-automatic contact freezer. The general freezing temperature is-45℃.

(3) The product of immersion freezing method is directly immersed in liquid refrigerant, which is a good conductor of heat, and has the largest contact area with the product and the fastest freezing speed in immersion freezing.

(4) Low-temperature freezing method This method is a method that the product can be quickly frozen under the condition that a refrigerant with a very low boiling point undergoes a phase change (liquid state changes to gas state). This is because the refrigerant needs to absorb a lot of heat in the process of boiling phase change, which is absorbed by the product to cool it down. The usual refrigerant is liquid nitrogen with a boiling point of-195.81℃, followed by carbon dioxide with a boiling point of -78.5℃. This method is faster than the previous ones and has good effect. The liquid nitrogen quick freezing device is shown in Figure 18.