Dryness is one of the means of keeping materials from spoilage.
There are many methods of drying, such as sun drying, boiling drying, drying, spray drying and vacuum drying.
However, these drying methods are carried out at temperatures above 0℃ or higher.
The product that dry gets, it is volume is reduced commonly, quality of a material becomes hard, some material produced oxidation, a few volatile composition can lose most, the material with some heat sensitive sex, if protein, vitamin can produce denaturation.
Microorganisms will lose biological vitality, dry material is not easy to dissolve in water and so on.
Therefore, the properties of dried products are very different from those before drying.
The freeze-drying method is different from the above drying methods. The product is basically dried at a temperature below 0℃, that is, under the freezing state of the product. Until the later stage, in order to further reduce the residual moisture content of the product, the product is not allowed to rise to the temperature above 0℃, but generally does not exceed 40℃.