The basic principle of freeze-drying is based on the change of three states of water. Water has solid state, liquid state and gas state, and the three states can both transform and coexist. To obtain high-quality products, we should have a comprehensive understanding of the theory and technology of freeze-drying. The freeze-drying process includes three stages: pre freezing, sublimation and re freeze-drying. Reasonable and effective shortening of freeze-drying cycle has obvious economic value in industrial production.

There are various drying methods, such as sun drying, boiling drying, drying, spray drying and vacuum drying, but common drying methods are usually carried out at temperatures above 0 ℃ or higher. The dried products generally have the problems of shrinking in size and hardening in texture. Most volatile components will be lost, some heat sensitive substances will be denatured and deactivated, and some substances will even be oxidized. Therefore, there is a great difference in properties between the dried products and those before drying.