What is the role of liquid ammonia in food freezing technology?

If you are not an expert in the industry, you probably won’t understand what application liquid ammonia has in refrigeration technology. In fact, it is simple. When we go to the supermarket, we open the refrigerator and see very cool air. This is liquid ammonia. Liquid nitrogen is a colorless, odorless, low-viscosity transparent liquid with stable chemical properties. The boiling point of liquid nitrogen at normal pressure is -195.8℃. When it comes into contact with frozen food, it can absorb 198.9kJ/kg of latent heat of evaporation; if the nitrogen vapor is heated to -20℃, the average specific heat is 1.047kJ/(kg·K), which can absorb 184.1kJ/kg. The total of the two is 383.0kJ/kg, which is an ideal refrigerant. The use of liquid nitrogen to freeze food started in the United States. The United States began research in this area in the 1950s, and it was officially used in quick-frozen food in 1960. It began to be rapidly promoted in production in 1964.

The background of liquid nitrogen quick freezing technology is:

1. In the late 1950s, the development of space technology and the need for large amounts of liquid oxygen as rocket fuel prompted the rapid development of the air liquefaction separation industry. The production of liquid nitrogen made it possible to produce large amounts of liquefied nitrogen, which is 78% of the nitrogen in the air, thus opening up new applications for the frozen food industry.

2. In the early 1960s, Midea's frozen food industry faced a new turning point. At that time, frozen food developed in three directions:

(1) Frozen food developed in the direction of "individual quick freezing" (IQF);

(2) It was required to increase the production of frozen food through continuous quick freezing equipment;

(3) It was required that frozen food develop into high-quality quick-frozen fresh-keeping food. As these requirements prompted the freezing method to be technically updated, liquid nitrogen quick freezing technology came into being and was quickly widely used.

Liquid nitrogen quick freezing has the following advantages:

(1) Liquid nitrogen is non-toxic and inert to food ingredients. Moreover, since it replaces the air coming out of the food, it can minimize oxidation changes during freezing and packaging storage.

(2) Liquid nitrogen can be in close contact with all parts of irregularly shaped food, minimizing heat transfer resistance.

(3) The dry loss of frozen food is low. The dry loss rate of food frozen by ordinary freezing equipment is between 3% and 6%, while the dry loss rate of food frozen by liquid nitrogen freezing equipment is between 0.6% and 1%. Therefore, it is suitable for freezing some foods with high water content, such as bayberry, tomato, crab meat, etc.

(4) It occupies a small area, has low initial investment, and has a high installation rate.

(5) The quality of frozen food is high. Since liquid nitrogen is in direct contact with food, a strong heat exchange is carried out with a temperature difference of more than 200K, so the freezing speed is fast, and the temperature can be reduced by 7 to 15K per minute. The ice crystals in the food are small and uniform, and the quality of the food after thawing is high. 

With the improvement and development of liquid nitrogen quick freezing technology, it has been rapidly and widely used in the freezing of fish, shrimp, crab, chicken, duck, meat (beef, sheep, etc.), fruit (bayberry, lychee, etc.), vegetables, and various pre-prepared foods (steak, sashimi, meatballs, fried shrimp, meat patties, hamburgers, pizza, egg products, soups, etc.). In 1981, liquid nitrogen quick-frozen foods accounted for 10% of frozen foods in the UK, and the amount of liquid nitrogen used exceeded 100kt. One-third of the liquid nitrogen production in Western Europe is used for food quick freezing and refrigerated transportation.

Liquid nitrogen quick-frozen foods in Japan account for about 40-50% of frozen foods. At present, hundreds of food processing companies in the United States use liquid nitrogen quick freezing, and only one quick-frozen steak factory uses 700t of liquid nitrogen every day. In the early 1970s, Beijing and Shanghai had developed a liquid nitrogen quick-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, and fresh aquatic products such as whitebait, lake crabs and prawns were successfully quick-frozen with liquid nitrogen. Since 0.8 to 1 kg of liquid nitrogen is required for each kilogram of quick-frozen food, affected by the high price of liquid nitrogen, in 1991, among the 250 various quick-freezing devices I manufactured myself, few used liquid nitrogen, resulting in a very low proportion of liquid nitrogen quick-frozen food in frozen food, which has not been widely popularized. 

In recent years, with the deepening of reform and opening up, major foreign multinational gas companies have competed to establish joint ventures in China, bringing advanced air separation equipment, technology and management, which has greatly increased the output of low-temperature liquids, continuously expanded the supply area and scope, and significantly reduced the price (the selling price of liquid nitrogen has dropped from about 2 yuan/kg to less than 1 yuan/kg), which has greatly promoted the application of liquid nitrogen.

There are many advantages of using liquid nitrogen to quickly freeze food, but there are also the following problems in its current application, which need to be solved urgently:

(1) Liquid nitrogen becomes low-temperature nitrogen gas after evaporation, which has the ability to absorb a large amount of sensible heat. Making full use of this part of cold capacity is the basis for improving the economic efficiency of liquid nitrogen freezing equipment, but the current utilization rate is not high and measures need to be strengthened. 

(2) Due to the fast freezing speed, there will be a large instantaneous temperature difference between the surface and the center of the food, and the expansion pressure is large, causing low-temperature fracture, destroying the organizational structure of the food, and bringing adverse effects on food quality.

Liquid nitrogen food quick freezing device There are three types of liquid nitrogen freezing methods: immersion, spray, and cold air circulation.

Immersion freezing is to immerse the food completely in liquid nitrogen. It can achieve the desired rapid freezing. The food occupies a small space, and the amplitude of the capacity change is also significant, but the liquid nitrogen consumption is large because only the latent heat of liquid nitrogen is used.

Spray freezing has three freezing zones: pre-cooling zone, freezing zone, and uniform temperature zone. Liquid nitrogen is misted through the nozzle to exchange heat with the food. Liquid nitrogen absorbs heat and evaporates into nitrogen, which is used to pre-cool the newly entered food. In this way, both the latent heat of liquid nitrogen and the sensible heat of liquid nitrogen are utilized, so that the cold capacity is fully utilized.

Cold air circulation freezing, which uses liquid nitrogen to cool the circulating cold air and uses air as a coolant to freeze food, can abandon huge refrigeration equipment and reduce initial investment.

Specific freezing devices can be divided into: liquid nitrogen cabinet freezing device, tunnel freezing device, immersion freezing device, rotary freezing device, etc.

The theory and device of fluidized food quick freezing The fluidization phenomenon has long been known to people. It was first used in chemical engineering, and then gradually applied in energy, metallurgy and food engineering. In 1959, Swedish Frigoscandia first used this method to freeze food, and successfully developed the world's first experimental fluidized freezing device in 1962. Since then, the United States, France, Bulgaria, the former Soviet Union, Japan and other countries have attached great importance to the application and theoretical research of fluidization, especially the development of frozen food in the past 20 years, which has promoted the development of fluidized freezing devices. At present, this freezing device has been widely used in various frozen food factories, especially vegetable processing plants.

The basic principle of fluidized food quick freezing Fluidized quick freezing is a process in which granular, flake or block food placed on a screen or slot plate forms a boiling state under the action of low-temperature air at a constant flow rate from bottom to top, moves like a fluid, and is quickly frozen during movement.

The main points of fluidized quick freezing of food are:

(1) Fast freezing speed. The fluidized freezing process has a strong heat exchange property. Compared with the traditional air forced circulation freezing device, the heat exchange intensity is increased by 30 to 40 times. This is because: the thermal resistance of food during suspension freezing is reduced by 15 to 18 times, the heat release coefficient between the product surface and the cold air is increased by 4 to 6 times, and the heat exchange area is increased by 3.5 to 10 times. Therefore, the freezing speed of the fluidized freezing device is dozens of times higher than that of ordinary freezing equipment. Due to the fast freezing speed, fluidized freezing can maintain the original nutritional components and freshness of the food to the maximum extent. 

(2) Realize single-body rapid freezing. Since the food is suspended during the freezing process, it will not stick together after freezing, thus achieving IQF freezing. It is not only of good quality, but also easy to package and consume for consumers. 

(3) Low food dry loss. Each quick-frozen food has a very thin ice film on its surface, which is beneficial for maintaining the freshness of the food and preventing oxidation, and also has low dry loss. A comparative test conducted by Swedish scholars on mushrooms, strawberries, etc. showed that the dry loss of fluidized freezing is almost only about half of that of forced air tunnel freezing. This is particularly important for higher-priced foods. 

(4) It is easy to achieve mechanized and automated continuous production, with high productivity. Workers can operate at room temperature, which improves working conditions. 

Of course, food fluidized freezing also has limitations. It is only suitable for granular foods, and its size is generally within 50mm and must not exceed 100mm. At present, the main types of food that can be frozen externally by fluidization are: vegetables such as green peas, beans, corn, green beans, fried or boiled potatoes, carrot cubes or slices, whole or sliced mushrooms, cauliflower, peppers, tomatoes, cabbage, and various vegetables cut into blocks, slices, and strips.