What is freeze-drying

Overview

Vacuum Freeze Drying is an advanced High-Tech dehydration technology. It makes hydrous material frozen in the low temperature status, then under the vacuum condition, it adopts thermal radiation method for heating, to sublimate ice to gas directly. After the moisture comes out, it uses ice-condenser (cold trap) and vacuum device to dehydrate the water content. It is a combined application technology based on multidisciplinary development, including refrigeration, heating, vacuum, biological, electrical and etc.

Vacuum freeze drying technology has widely application, such as for chemical products, biological, health product, herbs, agricultural products (meat,poultry, egg, sea food, vegetables and fruits and etc.).

Freeze-drying Processes

In principle, the vacuum freeze drying process includes three main stages:

1. First stage, the quick freezing. By freezing, the water content in the products turns into solid status from liquid status. In this step, the final frozen temperature should be below its own eutectic point temperature (got by test), which guarantee the material is totally frozen. The material freezing speed depends on different material features. In this stage, it uses quick blast freezing room for pre-freezing.

2. Second stage is the primary dehydration stage, which is also called sublimation dehydration stage. The frozen material with below eutectic point temperature will be dehydrated in vacuum condition, by sublimation method to remove its moisture. During sublimation, the heating plate temperature and vacuum condition should be controlled strictly, to prevent the material from melting or the temperature higher than eutectic point. Also it should prevent the dried parts temperature no higher its disintegration temperature which change the shape or even collapse. In this stage, the heating plates heat the material by thermal radiation, to provide the energy for sublimation. The vacuum tankmust under vacuum condition. The ice-condenser (cold trap) will catch the moisture which comes from material, and condense to ice on the cold trap coils surface.

3. Third stage is the secondary dehydration stage. It is also called desorption drying. The purpose of this step is to remove bound moisture. Because of the adsorption energy of bound moisture is very large, it need to provide large heat energy in this stage, which means the heating plates temperature will be a little higher, to approach the highest temperature for material to bear. When the material moisture within specified data, the final dehydration is done. To decide whether the vacuum freeze drying is finished, it can depend on experience for material temperature curve, sample status, shape and etc. Also we can judge it by terminal point testing (air pressure increase).

Advantages of freeze-drying:

Comparing with normal sun drying, hot air drying, spray drying and vacuum drying, there are several outstanding advantages of vacuum freeze-drying:

a. Vacuum freeze drying is dehydration process in low temperature, which will not damage protein. While it will make the microorganism losing vitality.

b. For the same reason, it makes little loss of volatility content, nutrition, aromatic and flavor in the material.

c. During the low temperature dehydration, microorganism's growing and enzyme almost cannot work, which keeps material original features.

d. After dehydration, the material volume, shape will not change. The final product is in cavernous status, no shrinkage. When rehydration, because of the efficient contactingarea is large, it will regain the original shape quickly.

e. In vacuum condition for dehydration, there is very little oxygen content, which will protect the oxidized material.

f. Vacuum freeze drying can remove 95%- -99.5% of the moisture from the material, which brings the final product with long shelf life.