The direct freezing of food with liquid nitrogen as refrigerant began to develop in the 1960s and gradually matured in the 1980s. Now it has been widely used in all kinds of food except vegetables. The principle of liquid nitrogen freezing is to spray low-temperature liquid nitrogen directly on food, and make use of the extremely low temperature of vaporization at atmospheric pressure (-196℃) and the high heat transfer coefficient of direct vaporization on the surface of the material to make the food quickly and deeply frozen. For frozen 1kg food, the consumption of liquid nitrogen is between 0.7 and 2 kg, depending on the variety and size of the material. The content of nitrogen in the air is 75%. Liquid nitrogen is generally a by-product of oxygen production, which is reduced to the atmosphere after melting and has no pollution to the environment. The liquid nitrogen quick freezing machine is tunnel type, the insulation board is polyurethane foam sandwich, double-sided stainless steel panel, and the material transmission is stainless steel mesh belt, which is driven by speed regulating motor. The equipment can be raised and lowered, and it can also be raised during maintenance and cleaning. The tunnel consists of precooling section, spraying section and temperature equalization section.

The precooling section is mainly used for moderately precooling materials, so as to make full use of the sensible heat after vaporization of liquid nitrogen and avoid the tendency of cracking of thicker materials due to excessive temperature difference during deep freezing; In the spraying section, liquid droplets are directly sprayed to the surface of the material through the nozzle under the pressure of the liquid nitrogen tank for quick freezing, and in the temperature equalization section, the liquid droplets that are not completely vaporized on the surface of the material can be completely vaporized. The vaporized low-temperature nitrogen is pumped out by the exhaust fan, and the materials and low-temperature nitrogen are stirred by the fan and sucked by the exhaust fan to the stainless steel mesh belt for countercurrent heat exchange. Because of the short freezing time, liquid nitrogen directly vaporizes and transfers heat, and the dry consumption of frozen products is much smaller than that of air cooling. At present, liquid nitrogen freezing method has been widely used in seafood, whole fish, patties, meat pieces, pies, hamburgers, ice cream hardening and other fields abroad.