With the further improvement and development of liquid nitrogen quick freezing technology, it has been rapidly and widely used in fish, shrimp, crab, chicken, duck, meat (bovine, sheep, etc.), fruits (yangmei, lychee, etc.), vegetables, and various Pre-made foods (steak, sashimi, meatballs, fried shrimp, patties, hamburgers, pizza, egg products, soups, etc.). In 1981, liquid nitrogen quick-frozen foods accounted for 10% of frozen foods, and liquid nitrogen used exceeded 100kt. One-third of the liquid nitrogen production in Western Europe was used for food freezing and refrigerated transport. Japanese liquid nitrogen frozen food took 40-50% of the total of frozen food. At present, there are hundreds of food processing enterprises in the United States are using liquid nitrogen to freeze, and even one frozen steak factory uses 700 tons of liquid nitrogen per day.
In the early 1970s, Beijing, China developed a liquid nitrogen fast-frozen food production line. In the 1980s, various quick-freezing devices (including liquid nitrogen quick-freezing) were introduced from Japan, Sweden, France, the United States, and Denmark. The fresh aquatic products such as lake crab and prawns have been successfully frozen in liquid nitrogen. Since the liquid nitrogen needs to be used 0.8-1 kg per kilogram of frozen food, affected by the high price of liquid nitrogen, in the 250 kinds of quick-freezing devices manufactured by China in 1991, the use of liquid nitrogen was very rare. The proportion of frozen foods was very low and had not been widely used.
Liquid nitrogen quick freezing has the following advantages:
(1) Liquid nitrogen can be in intimate contact with all parts of an irregularly shaped food to minimize heat transfer resistance.
(2) Liquid nitrogen is non-toxic and inert to food ingredients. Further, since it replaces the air coming out of the food, the oxidation change can be minimized during freezing and packaging storage.
(3) The quality of frozen food is high. Due to the direct contact between liquid nitrogen and food, the heat exchange is carried out with a temperature difference of more than 200K, so the freezing speed is extremely fast, and the temperature can be lowered by 7 to 15K per minute. The ice crystals in the food are fine and uniform, and the quality of the food after thawing is high.
(4) The dry consumption of frozen food is small. The foods frozen by the general freezing device have a dry consumption rate of 3% to 6%, and are frozen by a liquid nitrogen freezing device, and the dry consumption rate is between 0.6% and 1%. Therefore, it is suitable for freezing some foods with high water content, such as bayberry, tomato, crab meat and etc.
(5) The floor space is small, the initial investment is low, and the device efficiency is high.
Liquid nitrogen quick freezing has the following disadvantages:
The advantages of using liquid nitrogen to quickly freeze foods are numerous, but it also has the following problems in its current application, which needs to be solved urgently:
(1) Because the freezing speed is extremely fast, a large instantaneous temperature difference is generated between the surface and the center of the food, and the expansion pressure is large, causing low temperature fracture, destroying the structure of the food, and adversely affecting the food quality.
(2) Liquid nitrogen evaporates into low-temperature nitrogen, which has the ability to absorb a large amount of sensible heat. Making full use of this part of the cooling capacity is the fundamental to improve the economics of liquid nitrogen refrigeration equipment, but the current utilization rate is not high, and measures need to be strengthened.
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