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The four main stages of the vacuum freeze drying process

Categories：	Textile machine / Dryer
Usage：	vacuum freeze drying

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Description

The vacuum freeze-drying process can generally be divided into four main stages: pre-treatment, pre-freezing, drying and post-treatment.
First, vacuum freeze drying pretreatment
The pretreatment method is different for different materials to be dried. Some processes are more complicated, and some processes are relatively simple. However, in order to obtain safe and hygienic products, it is an indispensable process to sterilize freeze-drying equipment. Commonly used disinfection methods are:
(1) Gas Sterilization Method This disinfection method utilizes gaseous or vaporized chemicals to treat equipment or materials. Commonly used disinfectants are: ethylene oxide, formaldehyde, propylene oxide, methyl bromide and ? propanol. fat.
(2) Heat sterilization: dry heating: killing bacteria by oxidation, in fact, most of them are incinerated; direct steam heating: using a special low-pressure steam sterilization steamer, the steam temperature is required to be maintained at about 120 ° C, duration of at least 30 minute. In order to prevent steam from heating up to a dead angle, the entire system should be evacuated to 100 Pa before steaming. Spray with 70% alcohol in the box, dry with a sterilizing cloth after 24 hours; heat the sterilizing agent with negative pressure steam: vacuum the lyophilization box and the whole system to below 10Pa, and pass steam at 70-90 °C. Marlin solvent, keeping the container in a hot state for 2 hours.
(3) Radiation sterilization UV radiation: low-pressure mercury discharge lamp is used as the radiation source to generate ultraviolet radiation with a wavelength of 2.537×10-3m. These ultraviolet rays are harmful to the skin and eyes of the human body. Pay special attention when using it. Protection; X-ray: X-rays generated under high pressure have strong penetrating power and can be used to disinfect foods and medicines. It has a killing effect on various microorganisms, but it takes a long time to kill all the bacteria; Cathode ray: Cathode ray has a rapid bactericidal effect.
Second, vacuum freeze-drying pre-freezing
Pre-freezing is to solidify the free water in the material to ensure that the dried product has the same shape as before drying, preventing irreversible changes such as foaming, shrinkage, concentration and solute movement during vacuum drying, and reducing the temperature after vacuum drying. The decrease causes a decrease in the solubility of the substance and a change in the life characteristics of the substance. Pre-freezing plays a crucial role in the quality of the lyophilized product, so two basic requirements must be met: an appropriate cooling rate. When the cooling rate is too fast, when the extracellular solution starts to freeze, the water inside the cell can't penetrate outside the cell through the cell membrane, so that the internal solution of the cell is too cold to form intracellular ice; the cooling rate is too slow, the cell is excessively dehydrated, and the wrinkles are severely wrinkled. Shrinking, cells experience too long in high concentrations of solution at too low temperatures, resulting in cell damage and even death. The rate of cooling depends on the temperature of the cold source, the number of samples, the volume and surface area of ??the vessel, and the heat transfer between the sample vessel and the cold environment. After years of exploration, people have found that there are different kinds of cold for different cells and different protective agents.
Third, dry
The drying stage of the material is generally divided into two stages, a sublimation drying stage and an analytical drying stage.
(1) Sublimation drying stage
The sublimation drying stage is also referred to as *stage drying, and the frozen product is heated in a closed vacuum vessel to sublimate the ice crystals into water vapor, thereby dehydrating and drying the product. The drying is gradually moved inward from the outer surface, and the residual space after the ice crystals sublimate becomes the escape passage of the sublimated water vapor. The interface between the dried layer and the frozen portion is called the sublimation interface. When all the ice crystals are removed, the * stage drying is completed, at which time about 90% of the total moisture is removed. However, during the drying process, the undried portion of the material should always be kept frozen. If the temperature is too high, the ice crystals will melt, so the temperature of the frozen layer of the material should be lower than the temperature of its eutectic point. In addition, it cannot be heated too fast to make the temperature of the dried layer exceed its disintegration temperature. The so-called disintegration temperature is the phenomenon that some dried products lose their rigidity when the temperature reaches a certain value, which is similar to collapse, and loses the porous nature. The dry product is sticky, the specific gravity is increased, and the color is deepened. After the disintegration of the dried product, the passage of water vapor from the lyophilized layer is affected, and the production portion is melted. In order to increase the thermal conductivity of the lyophilized layer, the higher the pressure in the drying chamber, the better, but the excessive pressure in the drying chamber makes it difficult for water vapor to escape from the sublimation interface, resulting in excessive sublimation interface temperature, melting of the frozen layer and drying of the layer. solution. Considering comprehensively, the pressure in the drying chamber is generally controlled between 13 and 150 Pa.
(2) Desorption drying stage
The desorption drying stage is also referred to as the second stage drying. After the end of the * stage drying, a portion of the water is also adsorbed on the capillary walls and polar groups of the dried material, which is not frozen. When they reach a certain level, they provide conditions for the growth and reproduction of microorganisms and certain chemical reactions. Experiments have shown that even a low water content below the molecular layer can be a solution of certain compounds, resulting in the same mobility and reactivity as the aqueous solution. Therefore, in order to improve the storage stability of the product and extend its shelf life, it is necessary to remove the moisture.
Fourth, post processing

The product after lyophilization not only has low water content, but also is porous and porous, so that it has strong hygroscopicity, and the surface area of ??the material is relatively increased, which is susceptible to oxidation. For ease of preservation, post-treatment cannot be ignored. Post-processing capacity is packaged and stored. Different products and different packaging methods. For biological products, after lyophilization, the box is filled with sterile dry air, and then the container is sealed in a sterile room or vacuum-packed or packaged in a freeze-drying box after lyophilization.

vacuum freeze-drying :

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