The principle and procedure of freeze drying
Drying is one of the methods to keep materials from spoilage. There are many drying methods, such as sun drying, boiling drying, drying, spray drying and vacuum drying. However, these drying methods are carried out at temperatures above 0 ° C or higher. The product obtained after drying is generally reduced in volume and hardened in texture. Some substances have been oxidized, most of the volatile components will be lost, and some heat-sensitive substances such as protein and vitamins will be denatured. Microorganisms will lose their biological vitality, and the dried substances are not easy to dissolve in water. Therefore, the product after drying is very different from that before drying. The freeze drying method is different from the above drying method. The drying of the product is basically carried out at a temperature below 0 ℃, that is, the product is frozen, until the later period, in order to further reduce the residual moisture content of the product, let the product rise To a temperature above 0 ° C, but generally does not exceed 40 ° C.
Freeze-drying is to freeze a substance containing a large amount of water in advance to a solid, and then directly sublimate water vapor under vacuum, and the substance itself remains in the ice shelf when it is frozen, so its volume remains unchanged after drying. Loose and porous to absorb heat when sublimating. It causes the temperature of the product itself to decrease and slow down the sublimation speed. In order to increase the sublimation speed and shorten the drying time, the product must be properly heated. The entire drying is carried out at a lower temperature.
Freeze drying has the following advantages:
1. Freeze-drying is carried out at low temperature, so it is especially suitable for many heat-sensitive substances. Such as proteins and microorganisms will not be denatured or lose biological vitality. Therefore, it is widely used in medicine.
2. When drying at low temperature, the loss of some volatile components in the material is very small, which is suitable for drying some chemical products, medicines and foods.
3. During the freeze-drying process, the growth of microorganisms and the action of enzymes cannot be carried out, so the original sex clothes can be maintained.
4. Due to drying in a frozen state, the volume is almost unchanged, the original structure is maintained, and the concentration phenomenon does not occur.
Fifth, the dried material is loose and porous, showing a sponge-like shape. After adding water, it dissolves quickly and completely, and the original properties are restored almost immediately.
Sixth, because the drying is carried out under vacuum, there is very little oxygen, so some easily oxidized substances are protected.
7. Drying can remove more than 95-99% of water, so that the dried product can be stored for a long time without deterioration.
Therefore, freeze-drying is currently widely used in the pharmaceutical industry, food industry, scientific research and other departments. The freeze-drying of products needs to be performed in a certain device. This device is called a vacuum freeze-dryer, or freeze-dryer for short. The freeze dryer is divided into four main parts: refrigeration system, vacuum system, heating system, and control system. According to the structure, it is composed of freeze-drying box or drying box, condenser or water vapor condensator, freezer, vacuum pump and valve, electrical control element, etc. The freeze-drying box is a high and low temperature box that can be cooled to about -40 ° C and can be heated to about + 50 ° C. It is also a closed container that can be evacuated. It is the main part of the lyophilizer. The products that need to be lyophilized are placed on the layered metal plates in the box. The products are frozen and heated under vacuum to sublimate and dry the moisture in the products. The condenser is also a vacuum-tight container with a large surface area metal adsorption surface inside. The temperature of the adsorption surface can drop below -40 ° C and can maintain this low temperature constantly. The function of the condenser is to freeze and adsorb the water vapor sublimated from the product in the freeze-drying box on its metal surface. The freeze-drying box, condenser, vacuum pipes and valves, together with the vacuum pump, form the vacuum system of the freeze-drying machine. The vacuum system requires no air leakage, and the vacuum pump is an important part of the vacuum system to establish a vacuum. The vacuum system is essential for the rapid sublimation and drying of products. The refrigeration system is composed of a freezer, a lyophilizer, and pipes inside the condenser. The freezer can be two independent sets, or one set can be used together. The function of the freezer is to cool the freeze-drying box and the condenser to generate and maintain the low temperature required for their operation. There are two ways of direct cooling and indirect cooling.
The procedure of freeze-drying is as follows: before freeze-drying, the products that need to be freeze-dried are packed in suitable containers, generally glass bottles or ampoules, the amount should be uniform, the evaporation surface should be as large as possible and the thickness should be as thin as possible; Then put it in a metal tray that fits the size of the freeze-drying box. Before packing, first freeze the lyophilizer to cool it, and then put the product into the lyophilizer for pre-freezing. Before vacuuming, the condenser should be operated in advance according to the cooling rate of the condenser freezer. Should reach a temperature of about -40 ℃, after the vacuum reaches a certain value (usually should reach a vacuum of more than 100uHg), you can heat the product in the box. Generally, heating is carried out in two steps. The first step of heating does not cause the temperature of the product to exceed the temperature of the eutectic point; after the moisture in the product is basically dry, the second step of heating is performed. At this time, the product can be quickly raised. The highest temperature. After maintaining the highest temperature for several hours, the freeze-drying can be ended.
The entire sublimation and drying time is about 12-24 hours, which is related to the amount of product in each bottle, the total amount, the shape and specifications of the glass container, the type of product, the freeze-drying curve and the performance of the machine.
After lyophilization is completed, dry and sterile air should be put into the drying oven, and then plugged and sealed as soon as possible to prevent re-absorption of moisture in the air.
During the freeze-drying process, the temperature of the product and the board layer, the temperature of the condenser and the vacuum degree are plotted into a curve, called the freeze-drying curve. Generally, the temperature is the ordinate, and the time is the abscissa. Freeze-drying different products use different freeze-drying curves. When different freeze-drying curves are used for the same product, the quality of the products is also different. The freeze-drying curve is also related to the performance of the freeze dryer. Therefore, different products and different freeze dryers use different freeze drying curves.
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