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.