Freeze drying is actively used in canning along with other methods of processing food.
A high-performance equipment is used for dehydration. The machine is featured with big overall dimensions and high speed of operations.
Applications of freeze drying
Freeze drying is used not only in food production, but also is well known in the pharmaceutical industry. At the same time the use in the food industry and productivity is higher. For comparison, moisture removal can be from thirty to one hundred kilograms in the pharmaceutical industry, and from one hundred to five hundred kilograms in the food industry.
A certain unit of product is placed in the freeze drying chamber, therefore the drying process takes place in several cycles, the duration and stop of which must be monitored so that the machine does not stop, the production process remains uninterrupted.
Drying of pharmaceutical drugs is carried out in ampoules or vials. The containers are stored in cassettes, their capacity is designed for up to one hundred ampoules. For drying food raw materials baking trays with an area of 0.2 to 0.6 m2 are used.
Causes of uneven drying
The product to be dried is unevenly affected by the drying agent. It depends on many factors, for example, on the heterogeneous structure of the product, its moisture indicators, density; unequal conditions of contact between the material and the heat-conducting medium; the degree of radiation intensity, if the product is placed in a drying chamber with a radiation power line; unequal pressure supplied to the chamber. To dry a liquid or pasty product, it is frozen. In this case, the layer of the product will have a non-uniform structure on the baking sheet and deviations in the thickness of the layer.
In order to avoid such situations, it is necessary to clearly know the technology of the drying process. It is required to outline the zones of intensive and less intense action of the drying agent on the product before starting, and also to think about choosing the optimal drying speed.
A feature of the technology of the sublimation drying process is the choice of the optimal energy supply, which would not simultaneously cause burning and at the same time be effective in removing moisture. Heat can be supplied by radiation. That means that energy is supplied through the product layer, which is already dried and has a low thermal conductivity. Thus, it is necessary to clearly adjust the radiation supply, because the duration of dehydration depends on this factor.
The energy supply can be carried out through a dried or frozen layer. The drying time of the product in the latter case is noticeably reduced. The frozen product has a high degree of thermal conductivity.
The induction of the baking sheet walls with low heat-conducting materials like a foam is used to dry a pasty or liquid product. It is important to care that the foam does not interact with the product being dried.
The conductive energy supply contributes to an uneven distribution of heat among the food to be dried on a baking sheet. While materials close to the heat are already losing moisture, the side layers of the product are just beginning to lose the moisture. This significantly increases the drying time and slows down the production process.
Optimum conditions for freeze drying
The main points to create optimal conditions for freeze drying are:
- The stove and baking sheet must be in «complete understanding» with each other, ie. the stove should have as many points of contact with the product as possible. However, with such perfect contact with the baking sheet, it will deform and quickly become unusable.
- The use of a radiation energy supply when the baking sheet is at a distance of two to three millimeters from the stove. It is possible to use a higher heating temperature of the stove with this location of the baking sheet.
- Use a baking sheet with a thick bottom as the main one. The thick bottom compensates the energy costs in case of uneven contact between the plate and the baking sheet.