Fruit and Vegetable Freeze-drying Process

Fruit And Vegetable Freeze-drying Process

Fruit and vegetable freeze-drying process and food freeze-drying machine production process: raw material selection → finishing → pre-freezing → sublimation drying → post-processing → packaging and storage.

Operation points：

⑴ preprocessing

General pretreatment refers to all treatments before sublimation drying, so freezing before drying also belongs to pretreatment. The pretreatment of raw materials is the same as that of conventional fruit and vegetable drying and fruit and vegetable quick-frozen products, such as selection, cleaning, peeling, cutting, blanching, cooling, etc. When freeze-drying fruit or vegetable juices, it is pre-concentrated using less expensive processing methods, and the product is then granulated during pre-freezing.

Pre-freezing is freezing the pre-treated raw materials, and it is an important process of freeze-drying. Since a series of complex biochemical and physicochemical changes will occur in the freezing process of fruits and vegetables, the quality of pre-freezing will directly affect the quality of freeze-dried fruits and vegetables. The key consideration in the freezing process is the effect of the freezing rate of the frozen material on its quality and drying time. The difference between quick freezing and slow freezing is as follows: the ice crystals produced by quick freezing are smaller, and those produced by slow freezing are larger; large ice crystals are conducive to sublimation, and small ice crystals are not conducive to sublimation; small ice crystals have less impact on cells, and the smaller the ice crystals, the more it can reflect the original organizational structure and performance of the product after drying. However, the freezing rate is high and the energy consumption required is also high. It should be considered comprehensively to select an appropriate freezing rate to minimize the required freezing energy consumption while ensuring the quality of freeze-dried food.

(2) Sublimation drying is the core process in the production process of freeze-dried food. To control the process conditions:

①Loading capacity When drying, the wet weight loading capacity of the freeze dryer, that is, the quality of the drying plate per unit area, is an important factor in determining the drying time. The thickness of the food being dried is also a factor that affects drying time.

During freeze-drying, the drying of the material is pushed from the outer layer to the inner layer. Therefore, when the material to be dried is thicker, a longer drying time is required. During actual drying, the dried material is cut into a uniform thickness of 15-30mm. The amount of material that should be loaded on the drying plate per unit area should be determined according to the heating method and the type of dried food. When using an industrial large-scale device for drying, if the drying cycle is 6-8h, the material loading of the drying board is 5-15kg/m2.

②Drying temperature

In freeze-drying, in order to shorten the drying time, it is necessary to efficiently supply the heat required for the sublimation of ice crystals, so various practical heating methods have been designed. The drying temperature must be controlled within a range that does not cause ice crystals in the material to be dried to melt, and the dried part will not cause thermal denaturation due to overheating. Therefore, in a single heating method, the temperature of the drying plate should be controlled at 70-80°C in the early drying stage when sublimation is vigorous, 60°C in the middle drying stage, and 40-50°C in the later drying stage.

③ Judgment of drying endpoint

The drying endpoint can be judged by the following indications: the temperature of the material and the heating plate are basically consistent and maintained for a period of time; the vacuum gauge of the pump group (or cold trap) and the vacuum gauge of the drying chamber tend to be consistent and maintained for a period of time; the vacuum of the drying chamber The temperature of the cold trap basically returns to the index when the equipment is unloaded and remains for a period of time; for the freeze dryer with a large butterfly valve, the large butterfly valve can be closed, and the vacuum machine basically does not drop or drops very little. The above four judgment criteria can be used alone, or in combination, or in combination.

(3) Post-processing Post-processing includes unloading, sorting of semi-finished products, packaging, and other processes.

After freeze-drying, inject nitrogen or dry air into the drying chamber to break the vacuum, and then immediately remove the material in a closed environment with relative humidity below 50%, a temperature of 22-25 °C, and less dust. Selection and packaging of semi-finished products. Because the freeze-dried material has a huge surface area and is very hygroscopic, it is necessary to complete these operations in a relatively dry environment.

⑷ Packaging and storage

Freeze-dried food in airtight packaging.

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