Production process of freeze-dried fruits and vegetables

1. Process flow

Raw material selection → finishing → pre-freezing → sublimation drying → post-processing → packaging → storage.

2. Operation points

(1) Pretreatment. General pretreatment refers to all treatments before sublimation and 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 quick-frozen fruit and vegetable products. If processing such as selection, washing, peeling, cutting, blanching, and cooling is required, it will not be described in detail here. When fruit juice or vegetable juice is freeze-dried, it is first concentrated by a cheaper processing method, and then the product becomes granular during pre-freezing.

Pre-freezing is to freeze the pre-processed raw materials, and it is an important process of freeze-drying. Since fruits and vegetables undergo a series of complex biochemical and physical and chemical changes during the freezing process, 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.

Quick freezing and slow freezing have the following differences: the ice crystals produced by quick freezing are smaller, and the ice crystals produced by slow freezing are larger; large ice crystals are good for sublimation, and small ice crystals are not good for sublimation; small ice crystals have less influence on cells, and the smaller the ice crystals , The more it can reflect the original structure and performance of the product after drying. However, the freezing rate is high, and the required energy consumption is also high. Comprehensive consideration should be given to selecting an optimal freezing rate to ensure the quality of freeze-dried food while minimizing the required freezing energy consumption.

(2) Sublimation drying is the core process in the production 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 dried food is also a factor that affects the drying time.

During freeze-drying, the drying of the material is advanced 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 materials to be dried are cut into a uniform thickness of 15-30mm. The amount of materials that should be loaded per unit area of the drying board should be determined according to the heating method and the types of dried food. When the industrialized large-scale device is used for drying, if the drying cycle is 6-8h, the drying board material loading capacity is 5-15kg/m2.

②Drying temperature

In freeze-drying, in order to shorten the drying time, the heat required for the sublimation of ice crystals must be efficiently supplied. Therefore, various practical heating methods have been designed. The drying temperature must be controlled so as not to cause the melting of ice crystals in the material to be dried, 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℃ in the initial stage of sublimation and vigorous drying, 60℃ in the middle of drying, and 40~50℃ in the latter stage of drying.

③Judgment of drying end point

The drying end point can be judged by the following indications: the material temperature and the heating plate temperature basically tend to be the same and keep it for a period of time; the pump set (or cold trap) vacuum gauge and the drying chamber vacuum gauge tend to be the same and keep it for a period of time; the drying chamber vacuum The temperature of the cold trap is basically restored to the index when the equipment is empty and maintained for a period of time; for freeze dryers 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 4 criteria can be used alone, in combination or in combination.

3) Post-processing

Post-processing includes unloading, semi-finished products sorting, packaging and other processes. After the freeze-drying is fast, inject nitrogen or dry air into the drying chamber to break the vacuum, and then immediately remove the material in a closed environment with a relative humidity of 50% or less, a temperature of 22 to 25 ℃, and less dust, and in the same environment Carry out the 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 procedures in a relatively dry environment.

(4) Packaging and storage

Freeze-dried food has a huge surface area. Some ingredients in the food are directly exposed to the air, and are easily exposed to oxygen in the air and adsorb moisture, which leads to the gradual deterioration of freeze-dried food. Secondly, most freeze-dried foods have natural colors, and these natural pigments are easily degraded under light. In the process of oxidation and pigment degradation, temperature is also an important factor affecting such chemical reactions. Therefore, the packaging of freeze-dried food mainly considers how to prevent or reduce the impact of the above four factors.

The porous sponge-like structure of freeze-dried food has its advantages and disadvantages: First, once the product is exposed to the air, it is easy to absorb moisture and oxidize and degrade. Therefore, freeze-dried food should be vacuum-packed. Good nitrogen-filled packaging; second, freeze-dried food occupies a relatively large volume, which is not conducive to packaging, transportation, and sales. Therefore, freeze-dried food is often compressed and packaged. In addition, the porous and loose structure makes freeze-dried foods extremely easy to break during transportation and sales. Therefore, for those freeze-dried foods that are inconvenient to compress and package, packaging materials or packaging forms that have a certain protective effect should be used.

The commonly used packaging materials for freeze-dried food are PE bags and composite aluminum platinum bags. PE bags are often used for large packaging and composite aluminum platinum bags are often used for small packaging. The outer packaging is usually made of kraft corrugated cardboard boxes, the size of which is suitable for container transportation. If necessary, when using PE bags as inner packaging, in order to strengthen its function of isolating oxygen, water and vapor, double layers are often used. If necessary, iron cans can also be used for packaging, but the price is higher and the application is not very common. No matter what kind of packaging materials are used, vacuum nitrogen filling is required, and oxygen scavenger and desiccant are added.

Freeze-dried food should be stored in a cool, dry place. If possible, it is best to place it in a low-temperature and low-humidity environment. The shelf life is usually 1 to 2 years. It can be appropriately extended when it is packaged in iron cans.