1、 Development background of liquid nitrogen quick freezing technology:

1. In the late 1950s, due to the development of space technology and the need for a large amount of liquid oxygen produced as rocket fuel, the air liquefaction and separation industry developed by leaps and bounds. The production of liquid nitrogen makes it possible to liquefy 78% of nitrogen in the air, thus opening up a new way for the application of frozen food industry.

2. In the early 1960s, the frozen food industry in the United States faced a new turning point. At that time, frozen food was developing in three directions: (1) frozen food was developing in the direction of "rapid monomer freezing" (IQF); (2) It is required to increase the production of frozen food through continuous quick freezing devices; (3) It is required that frozen food should be developed into high-quality quick frozen fresh food. Because of these requirements, the freezing method must be updated in technology, so the liquid nitrogen quick freezing technology came into being and was rapidly widely used.

2、 Liquid nitrogen quick freezing has the following advantages

1. Liquid nitrogen can be in close contact with all parts of food with irregular shape to reduce the heat transfer resistance to a small extent.

2. Liquid nitrogen is non-toxic and inert to food ingredients. Moreover, it replaces the air from food, so it can reduce the oxidation change to a small limit during freezing and packaging storage.

3. The quality of frozen food is high. As the liquid nitrogen is in direct contact with food, the temperature difference of more than 200K is used for strong heat exchange, so the freezing speed is extremely fast, and the temperature can drop by 7-15K per minute. The ice crystals in the food are fine and uniform, and the food quality is high after thawing.

4. The dry consumption of frozen food is small. The dry consumption rate of food frozen with general freezing device is between 3% and 6%, while that of food frozen with liquid nitrogen freezing device is between 0.6% and 1%. Therefore, it is suitable to freeze some foods with high moisture content, such as waxberry, tomato, crab meat, etc.

5. Small floor area, low initial investment and high device efficiency.

With the further improvement and development of liquid nitrogen quick freezing technology, it is rapidly and widely used in the freezing of fish, shrimp, crabs, chickens, ducks, meat (cattle, sheep, etc.), fruits (bayberry, lychee, etc.), vegetables, and various prefabricated foods (steak, sashimi, meatballs, fried shrimp, meat cakes, hamburgers, pizza, egg products, soup, etc.). In 1981, the liquid nitrogen quick-frozen food accounted for 10% of the frozen food in Britain, and the amount of liquid nitrogen used exceeded 100kt. One third of the liquid nitrogen output in Western Europe is used for quick freezing and refrigerated transportation of food. Liquid nitrogen quick-frozen food in Japan accounts for 40~50% of frozen food. At present, hundreds of food processing enterprises in the United States use liquid nitrogen for quick freezing, and only one quick freezing steak factory uses 700t liquid nitrogen every day. In the early 1970s, Beijing and Shanghai had developed liquid nitrogen quick freezing food production lines. In the 1980s, various quick freezing devices (including liquid nitrogen quick freezing) were introduced from Japan, Sweden, France, the United States and Denmark, and liquid nitrogen quick freezing of fresh aquatic products such as whitebait, lake crab and prawn was successful. As each kilogram of quick-frozen food requires 0.8~1kg of liquid nitrogen, affected by the high price of liquid nitrogen, in 1991, few of the 250 quick-frozen devices made in China used liquid nitrogen, resulting in a very low proportion of liquid nitrogen quick-frozen food in frozen food in China, which is not widely used.

Liquid nitrogen is a colorless, tasteless, low viscosity transparent liquid with stable chemical properties. The boiling point of liquid nitrogen under normal pressure is - 195.8 ℃. When it contacts with frozen food, the latent heat of evaporation that can be absorbed is 198.9kJ/kg; Then let the nitrogen vapor rise to - 20 ℃, and the average specific heat is 1.047kJ/(kg · K), then 184.1kJ/kg can be absorbed. The total of the two items is 383.0kJ/kg, which is an ideal refrigerant Frozen food with liquid nitrogen started in the United States. The United States began research in this field in the 1950s, and it was officially used for quick frozen food in 1960. In 1964, it began to be rapidly popularized in production.

In recent years, with the deepening of reform and opening up, major foreign multinational gas companies compete to establish joint ventures in China, bringing advanced air separation equipment, technology and management, which has greatly increased the output of cryogenic liquids in China, expanded the supply area and scope, and significantly reduced the price, greatly promoting 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. Because of the extremely fast freezing speed, a large instantaneous temperature difference will be generated between the food surface and the center, and the large expansion pressure will cause low-temperature fracture, damage the organizational structure of the food, and bring adverse effects to the food quality.

2. After evaporation, liquid nitrogen becomes low-temperature nitrogen, which has the ability to absorb a large amount of sensible heat. Making full use of this cooling capacity is fundamental to improve the economy of liquid nitrogen refrigeration equipment, but the current utilization rate is not high.

3、 Liquid nitrogen food quick freezing device

There are three types of liquid nitrogen freezing: immersion type, spray type and cold air circulation type.

Immersion freezing is to completely immerse food in liquid nitrogen, which can achieve the desired rapid freezing. The space occupied by food is small, and the range of production capacity change is also significant. However, the consumption of liquid nitrogen is large, because only the latent heat of liquid nitrogen is used.

The spray refrigeration is equipped with three refrigeration areas: pre cooling area, freezing area and temperature equalization area. The liquid nitrogen is vaporized into nitrogen through the nozzle in the form of mist for heat exchange with food. Nitrogen is also used to pre cool new food. In this way, both the latent heat of liquid nitrogen and the sensible heat of liquid nitrogen are used to make full use of the cooling capacity.

Cold air circulating refrigeration: the cold air circulating by liquid nitrogen is used as the refrigerant to freeze food, which can discard huge refrigeration equipment and reduce initial investment.

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

4、 Theory and Device of Fluidized Food Quick Freezing

Fluidization has been recognized for a long time. It was first used in chemical engineering, and then gradually applied in energy, metallurgy, food engineering and other fields. In 1959, Frigoscandia Company in Sweden first used this method to freeze food, and in 1962, it successfully developed * * * experimental fluidized freezing devices in the world. 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 two decades, which has promoted the development of fluidization freezing devices. At present, this kind of freezing device has been widely used in frozen food plants in various countries, especially in vegetable processing plants.

Basic Principles of Fluidized Food Quick Freezing

Fluidized quick freezing is a process in which granular, flaky or blocky food placed on the screen or trough plate can form a similar boiling state under the action of low temperature air at a certain flow rate from bottom to top, move like a fluid, and be quickly frozen during the movement.

When the cold air flows through the food layer from bottom to top and the flow rate is low, the food particles are in a static state, which is called fixed bed A. With the increase of air flow velocity, the air flow pressure drop on both sides of the food bed will also increase, and the food bed will begin to loosen B. When the air flow velocity reaches a certain value, the food particles no longer remain static, and some particles suspend upward, causing the bed to expand and the void ratio to increase, which means that the food particles begin to enter the fluidization state. This state is the dividing point between fixed bed and fluidized bed, which is called critical state. The corresponding * * * large pressure drop is called critical pressure, and the corresponding wind speed is called critical wind speed. Critical pressure and critical velocity are the necessary conditions for fluidization C. When the gas velocity continues to increase, the bed will continue to expand, and the bed voidage will also increase. However, the actual gas flow velocity in the bed remains unchanged, and the pressure drop of the fluid is only consumed by the weight of the solid particles lifted, that is, the pressure drop of the bed is independent of the gas flow velocity and always maintains a constant value D. At this time, the strong cold air flow interacts with the food particles, making the food particles move irregularly from top to bottom. Therefore, the mass and heat transfer in the food layer is very fast, so that the food monomer can be quickly frozen. If the gas flow velocity further increases, the particles will be taken away by the fluid, the particles in the bed will decrease, the voidage will increase, and the pressure drop in the bed will decrease, and the fluidized bed will become the transport bed E. The production and experiment of fluidized bed quick freezing are carried out in front of the conveying bed, and the curve AD is the standard fluidization curve.

The main characteristics of food fluidization quick freezing are:

The freezing speed is fast. Fluidized freezing process has strong heat transfer characteristics. Compared with the traditional forced air circulation freezing device, the heat transfer intensity is increased by 30~40 times. This is because: the thermal resistance of food during suspension freezing is reduced by 15~18 times, the heat release coefficient between the product surface and cold air is increased by 4~6 times, and the effective heat exchange area is increased by 3.5~10 times. Therefore, the freezing speed of fluidization freezing equipment is dozens of times higher than that of ordinary freezing equipment. As the freezing speed is fast, the fluidization freezing can greatly maintain the original nutrition and freshness of food.

To achieve rapid freezing of individual buildings. As the food is suspended in the freezing process, it will not stick together after freezing. IQF freezing is realized, which is not only of good quality, but also convenient for packaging and consumer consumption.

Less dry food consumption. There is a thin ice film on the surface of each quick frozen food, which is not only conducive to maintaining the freshness of food and preventing oxidation, but also has less dry consumption. The comparative tests conducted by Swedish scholars on mushrooms and strawberries show that the dry consumption of fluidized freezing is almost half of that of forced air supply tunnel freezing. This is particularly important for higher priced foods.

It is easy to realize mechanized and automatic continuous production, with high production efficiency. Workers operate under normal temperature, improving the working conditions.

Of course, the fluidization freezing of food also has limitations. It is only applicable to granular food. Generally, its characteristic size is within 50 mm, and the size of * * * cannot exceed 100 mm. At present, the types of food frozen by fluidization in foreign countries mainly include:

Meat: diced meat, fried meatballs, fish fillets, fish sticks, shrimp, shrimp, etc.

Vegetables: green peas, beans, corn, green beans, fried or boiled potatoes, diced carrots or slices, whole or sliced mushrooms, cauliflower, peppers, tomatoes, cabbage, and various vegetables cut into pieces, slices, and strips.

Fruits: apple slices, pineapple slices, strawberries, blackberries, cherries, marlins, plums, apricots, peanuts, purple berries, grapes, lychees, longans, etc.