Analysis of vacuum freeze-drying food processing technology

Vacuum freeze-dried food, also known as "freeze-dried food", is in a basic anaerobic state during processing, which can greatly avoid chemical reactions in food, thereby maintaining the color, aroma, taste, and shape of the food, and effectively preserving most of the nutritional components such as vitamins, proteins, and carbohydrates inside the food. Due to the low moisture content of freeze-dried food, usually 1% to 3%, it can be stored for a long time after being sealed and stored, making it convenient for long-term storage and transportation.

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Vacuum freeze-drying food processing

The biggest obstacle of vacuum freeze-drying technology is the high production and processing costs. How to comprehensively utilize enterprise resources and reduce production costs is an important issue faced by the food freeze-drying industry. Due to the wide variety of freeze-dried foods and the complex processing techniques for each type, only by selecting different vacuum freeze-drying methods based on different ingredients can the freezing effect of the food be guaranteed.

1、 Initial treatment process

The main process of this stage is to select, organize, clean, and cut the food. The quality of freeze-dried food directly depends on the quality of the food raw materials. Due to the high processing costs and selling prices of freeze-dried food, the quality requirements for food raw materials are also relatively strict, especially the attention to the freshness of food materials is higher.

There are various types of food, and their processing conditions, time, temperature, and other requirements are also different, which seriously limits the degree of automation in the initial processing stage. For example, in the initial processing stage of fruit and vegetable foods, it is usually necessary to process or refrigerate them within 2-10 hours after picking to maintain their vibrant color and avoid the loss of vitamins and nutrients.

If the room temperature is 20 ℃, fresh spinach will turn yellow within 6 hours after picking, and fresh lettuce will lose about 1/3 of its vitamins within 10 hours after picking. The sorted and cleaned food undergoes a cutting process, in which the cutting direction, shape, thickness, and size of the food need to be determined based on the final use of the product.

In general, vacuum freeze-dried food mainly comes in various forms such as strip, sheet, block, dice, and powder. Different forms result in differences in the time and efficiency of vacuum freezing. Therefore, it is generally required that the thickness of food when cut into blocks should not exceed 20 mm; When cutting into slices, it is advisable to use a thickness of 4-5 mm; The thickness of Ding shaped food is measured by 5 mm square.

It is worth noting that when cutting food into slices, if there is fibrous tissue in the food, it should be cut perpendicular to the fiber direction of the food to facilitate water vapor overflow, reduce energy consumption, and improve the heat transfer coefficient of the dried part during freeze-drying.

In addition, when slicing, the thickness of the food should be minimized as much as possible to accelerate the freezing time of the food. For example, when slicing beef, if the slice thickness is 0.1 m, the freezing time needs to be 10.9 hours, but if the slice thickness is 0.2 m, the freezing time needs to be 26.6 hours.

2、 Pre treatment process

Pre processing mainly refers to the steps of blanching, pre cooling, filtering, and loading food. The main purpose of blanching is to inactivate enzymes in food, in order to avoid enzymes affecting the flavor and color of the food.

During the processing, hot water and steam are usually used for blanching. Different foods will choose the appropriate blanching method based on their characteristics, and the blanching time needs to be reasonably controlled, ranging from a few seconds to several minutes. Among them, when using hot water for blanching, the temperature of the hot water is controlled at 90-100 ℃. At the same time, organic acids, sulfites, vitamin C and other substances can be added to the hot water to reduce nutrient loss and improve the color of the food.

But for vegetables and fruits, blanching can have a significant impact on the taste of the food, so 0.2% to 0.6% sulfite will be directly used to soak the food.

After blanching, food needs to be pre cooled immediately. On the one hand, the blanched food will be oxidized and turn yellow in the air. On the other hand, the initial temperature of the food will affect the average temperature at the end of freezing. Therefore, pre cooling is necessary in the pre-treatment process to promote the escape of moisture from the food and improve the freeze-drying efficiency of the food.

3、 Freezing process

For many large food freeze-drying equipment, it is usually necessary to freeze the food first, and then send it to the drying chamber to sublimate the remaining moisture in the food to achieve the purpose of drying.

The freezing process is an important part of vacuum freeze-drying food processing technology, and the rate and temperature of freezing directly affect the processing quality of food, the efficiency of water sublimation, and the consumption of processing energy.

In general, slow freezing is used to freeze food, with a freezing speed of 5-10 mm · h-1. However, during the slow freezing process, the amount of ice crystals formed by the water in the food is relatively small, and most of them are coarse and continuous network crystals. The water vapor sublimation rate in the food is relatively fast.

During rapid freezing, the amount and volume of ice crystals formed by the water in the food are large and small. These ice crystals cause less mechanical damage to the food and can greatly maintain its freshness and taste. However, the rapid freezing method produces many small and numerous ice crystals, which increases the resistance of water vapor escape, thereby delaying the drying speed of the food and increasing the energy consumption of the equipment. To solve this problem, increasing the volume of ice crystals is the primary choice.

In the process of food freeze-drying, as the moisture continues to freeze, the freezing point of the food will also decrease. When the temperature reaches -5 ℃, more than 80% of the moisture in the food has been frozen. In order to reduce processing costs, the pre-set temperature is generally retained for a period of time in this stage to promote the growth of ice crystals, which is also an effective method to reduce the cost of rapid freezing processing.

4、 Freeze drying process

Freeze drying is the most complex and crucial step in vacuum freeze-drying food processing technology. In this process, it mainly includes three processes: heating, vacuum, and refrigeration.

(1) Heating

In order to keep freeze-dried food at room temperature for a long time, it is necessary to first inhibit the growth of microorganisms in the food through heating. During the heating process, the water content in the food sublimates. When the water activity value in the food is below 0.5, microorganisms are difficult to survive.

At the same time, due to the fact that food needs to absorb a certain amount of heat during the sublimation process, in order to avoid a decrease in the temperature of the food itself, it is necessary to provide a certain amount of heat to frozen food, as well as increase the saturation vapor pressure and sublimation rate of ice crystals. In addition, to prevent excessive heating from causing frozen food to thaw, it is necessary to always control the temperature of the food at -10 to -30 ℃.

(2) Vacuum

After the food is heated, it is sent to a vacuum box and the sublimation pressure of the ice crystals is increased by a vacuum pump, which increases the thermal conductivity of the dried layer of the food. At this time, if the sublimation pressure is too small or too large, it will affect the sublimation speed of the ice crystals. Different foods have different sublimation pressures. In the process of sublimation drying, only by accurately grasping the pressure of the vacuum box can unnecessary energy loss be reduced, and the quality and freeze-drying rate of freeze-dried foods be guaranteed as much as possible.

(3) Refrigeration

After the vacuum pump processing in the vacuum box is completed, the ice crystal sublimation work is completed, and the next step is to enter the drying process of water vapor. In this stage, the main use of the vacuum pump is to eliminate non condensable gases in the drying box.

5、 Packaging process

After the processing of freeze-dried food is completed, it is necessary to package it for easy transportation and storage.

① Considering the low moisture content of freeze-dried food, the packaging materials used must be waterproof in order to avoid moisture damage to freeze-dried food.

② Freeze dried food is prone to chemical reactions such as oxidation and hydrolysis, which can lead to spoilage. Therefore, the selected packaging material must be able to isolate oxygen.

③ The irradiation of light can raise the temperature of food, further promoting the decomposition of nutrients in food and causing food decay. Therefore, the selected packaging material must be able to block light.

Based on the characteristics of freeze-dried foods and the transportation requirements of different foods, the selected packaging of freeze-dried foods must be waterproof, oxygen resistant, and light shielded. In general, packaging materials are selected as vacuum aluminum plated thin films or PP composite materials, and food is packaged in the form of vacuum packaging or vacuum inflatable packaging through a vacuum pump.

In addition, considering that the residual microorganisms in food may affect food quality and storage time, various effective sterilization and disinfection methods such as radiation sterilization and ultraviolet sterilization must be adopted to sterilize the food and its packaging materials. At the same time, considering the potential risks during transportation, it is necessary to humidify the freeze-dried food before packaging to facilitate storage and transportation.

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Conclusion

Vacuum freeze-drying food processing is an important means to extend the shelf life of food and improve the convenience of food transportation. In the process of freeze-drying food processing, it is necessary to choose the correct processing equipment and process according to the food variety, and strictly ensure that each process is executed according to standards. At the same time, by improving processing technology, adopting drying combinations and other methods to improve food processing quality and reduce production costs, it has to some extent promoted the expansion of the production and operation scope of food enterprises and the rapid development of the logistics industry.

Source: Shi Shuoli Analysis of vacuum freeze-drying food processing technology Jiangsu Siyang Vocational School, reprints must indicate this source.