Views: 24 Author: Site Editor Publish Time: 2021-11-16 Origin: Site
Freeze dryer principle
The freeze-drying machine is a kind of low-temperature drying equipment that uses the principle of sublimation to dehydrate materials, also known as a vacuum freeze dryer! The principle of the freeze dryer is to freeze the water-containing material into a solid, and use the sublimation performance of water under low temperature and low pressure conditions to dehydrate the material at a low temperature to achieve a new drying method. The product obtained by vacuum freeze drying is called a freeze-dried product.
To put it simply, the principle of the freeze dryer is to place the sample in a vacuum environment through the process from high temperature to low temperature, so that the free water in the sample is volatilized, and the bound water in the sample cannot be volatilized. The purpose is to increase Longer storage time for samples.
Water has three phases: solid, liquid, and gas. According to the phase equilibrium theory in thermodynamics, as the pressure decreases, the freezing point of water does not change much, but the boiling point gets lower and lower, approaching the freezing point. When the pressure drops to a certain degree of vacuum, the boiling point and freezing point of water coincide, and the ice can be directly vaporized into gas without being liquid. This process is called sublimation. The vacuum freeze-drying of food means that the frozen water in the food is sublimated and removed under the condition of low temperature and low pressure below the triple point of water.
Since freeze-drying technology is carried out in a low temperature and low oxygen environment, most biological reactions are stagnant, and there is no liquid water in the treatment process, and the water directly sublimates in a solid state, so that the original structure and shape of the material are protected to the greatest extent, and the appearance and appearance are finally obtained. High-quality dry products with both inherent quality. At present, freeze-drying technology has been widely used in many fields, especially the use of this technology in food processing to obtain high-quality dehydrated food.
The source and application of raw materials are different, and their pretreatment methods are also different. For example, the pretreatment of fruits is selection, washing and cutting, vegetables need to be blanched, and instant porridge needs to be steamed and plated.
After the raw materials are pre-treated, they are generally pre-frozen. This is because when there is a lot of water inside the material, if the vacuum treatment is directly carried out, the gas dissolved in the water will escape due to the reduction of external pressure, forming bubbles, resulting in cavities inside and on the surface of the material, affecting the sensory quality.
In order to solidify the moisture inside the material, quick freezing is usually required. The shorter the freezing time, the faster the material freezes, the smaller the internal crystallization, and the less mechanical damage to the material cells. Therefore, it is necessary to select the appropriate freezing speed according to the actual production.
Vacuum drying is usually divided into two stages: sublimation drying and analytical drying. Sublimation drying is mainly aimed at free water in materials; analytical drying is mainly to remove adsorbed water that is strongly combined with solids. The vacuum degree, temperature and plate thickness during the drying process directly affect the drying process and product quality.