Technology Of Freeze-Dried Fruits And Vegetables

The production technology of freeze-dried fruits and vegetables

Material selection → finishing → pre-freezing → sublimation drying → post-processing → packaging and storage.

Operating points

Pretreatment: general pretreatment refers to all treatment before sublimation drying, so freezing before drying also belongs to pretreatment. Raw material pretreatment and conventional fruit and vegetable drying and fruit and vegetable quick-frozen products are the same, such as the need for selection, cleaning, peeling, cutting, hot blanching, cooling, and other treatment, in the juice or vegetable juice freeze-drying, it is first with a cheaper processing method in advance of its concentration, after the pre-freezing product into granular.

Sublimation drying is the core process of freeze-drying food production. To control the process conditions:

(1) load: when drying, the wet heavy load of the lyophilizer that is, the quality of drying on the unit area drying plate, is an important factor to determine the drying time. The thickness of the dried food is also a factor affecting the drying time.

During freeze-drying, the drying of the material is pushed from the outer layer to the inner layer. Therefore, when the material being dried is thicker, it needs a longer drying time. In actual drying, the dried materials are cut into a uniform thickness of 15 ~ 30mm. The amount of material to be loaded on the drying plate per unit area shall be determined according to the heating mode and the types of dried food. If the drying period is 6 ~ 8h, the material loading capacity of the drying plate is 5 ~ 15kg/m2.

(2) drying temperature: freeze-drying, in order to shorten the drying time, must effectively supply the heat required by the sublimation of ice crystals, so a variety of practical heating methods are designed. The drying temperature must be controlled within the range so as not to cause the melting of ice crystals in the dried material and not to cause thermal denaturation of the dried part due to overheating. Therefore, in a single heating mode, the temperature of the drying plate should be controlled at 70 ~ 80℃ in the initial stage of sublimation vigorous drying, 60℃ in the middle stage of drying, and 40 ~ 50℃ in the later stage of drying.

(3) the endpoint of drying judgment: drying endpoint can be used to determine the following indicators: material temperature and heating plate temperature is basically consistent and maintain a period of time; Pump set (or cold trap) vacuum gauge and drying chamber vacuum gauge tend to be consistent, and maintain for a period of time; The cold trap temperature of the vacuum gauge in the drying chamber is basically returned to the index when the equipment is empty and kept for a period of time. For a freeze-drying machine with a big butterfly valve, it can close the big butterfly valve, the vacuum machine basically does not drop or drop very little. Above 4 decision basis, can use alone, it can also be used in combination or union.

Post-processing: post-processing includes unloading, semi-finished product selection, 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 a relative humidity of 50%, the temperature of 22 ~ 25℃, less dust discharge, and in the same environment for the selection and packaging of semi-finished products. Because of the large surface area and high hygroscopicity of the lyophilized materials, it is necessary to complete these operations in a relatively dry environment.

Packaging and storage: food after freeze-drying has a large surface area, some components in the food directly exposed to the air, easy to contact with oxygen in the air and adsorption of water, which leads to the gradual deterioration of freeze-dried food. Secondly, most of the freeze-dried food has a natural color, these natural pigments are easy to light down. Temperature is also an important factor in the process of oxidation and pigment degradation. Therefore, the packaging of freeze-dried food mainly considers how to prevent or mitigate the above four factors.