I .Vacuum freeze-drying principle
Vacuum freeze-drying is the method of freezing a water-containing substance to a solid state, and then subliming the water therein from a solid state to a gaseous state to thereby remove water and preserve the substance.

1. The advantages of freeze-drying:
Compared with the usual drying, boiling drying, spray drying and vacuum drying, freeze-drying has the following outstanding advantages.
a. Lyophilization is dry at low temperatures, does not denature protein, but can make microorganisms lose their biological activity. This is especially useful for the dry storage of bioactive, biochemical, genetically engineered, and blood products that have poor thermal stability.
b. During low-temperature drying, the growth of microorganisms and the enzyme’s action are almost impossible to process, thereby maintain the original properties of the material.
c. After drying, the volume and shape are basically the same, the material is sponge-like, no shrinkage, and the contact surface with water after rehydration is large, and can quickly return to the original shape.
d. Due to the fact that it is usually dried under vacuum, there is very little oxygen and the easily oxidized substances are protected.
e.Can remove 95% to 99.5% of water in the substance ,the finished product has long shelf life .

2. Freeze-drying applications
Freeze-drying technology is more and more popular because of the unparalleled advantages compared with other drying methods. It has been widely used in various fields such as medicine, biological products, foodstuffs and active substances. Its application scale is still rapidly expanding. Vacuum freeze drying is bound to become an important application technology in twenty-first Century.
(1) Application of freeze-drying technology in biological products
a. live bacteria vaccine. For example, BCG, meningococcal vaccine, tuberculosis vaccine, oral live dysentery vaccine, Salmonella, Streptococcus and so on.
b. fire poison vaccine. Such as measles vaccine, flu vaccine, rabies vaccine, chicken plague vaccine.
c. other biological products, biochemical drugs. Hepatitis B surface antigen diagnosis snowball, human leukocyte interferon, coenzyme A (CoA), adenosine triphosphate (ATP), urokinase and so on.
(2) Application of freeze-drying technology in western medicine production
Mostly are for injection, with antibiotic drugs, circulatory organ drugs, central nervous drugs, vitamins and oncology drugs for more. For example, ampicillin, bacilli (erythromycin ethylsuccinate tablets) and so on.
(3) Application of freeze-drying technology in the production of traditional Chinese medicine
In recent years, with the lyophilized method of processing proprietary Chinese medicines, prepared good proprietary Chinese medicines, after dipping, extraction, filtration, concentration, freeze-dried and then made in to powder , tablets, or injection.
(4) Medical applications
Using freeze-drying technology can be long-term preservation of blood, arteries, bones, skin, cornea and nerve tissue and other organs. When the cells are lyophilized, the cells are not destroyed, and the organisms can revive after rehydration. For example, freeze-dried bones can be stored at room temperature or in a refrigerator for up to 2 years.
(5) Food freeze-dried, foods using freeze-dried process are:
a. cooking ingredients: meat, eggs, fish, vegetables and so on.
b. Raw materials for the food industry: milk powder, egg powder, vegetable protein powder, tea, dried fruit, meat meal, soybean meal and the like.
c. Beverages: coffee, fruit drink and so on.
d. Tonic categories: pollen, honey powder, turtle powder and so on.
(6) Other applications of freeze-drying technology :
a. Microbial and algae aspects: Long-term preservation of various bacteria, yeasts, enzymes, protozoa, microalgae, etc.
b. Biological specimens, biological tissue: such as making a variety of animal and plant specimens, dry preservation for animal xenotransplantation of the skin, bones, aorta, heart valves and other marginal tissues.
c. Make small pieces of tissue for use in light microscopy, electron scanning and transmission microscopy.
d. Drying of food: such as coffee, tea, meat and fish eggs, seaweed, fruits, vegetables, spices, tofu, instant food and so on.
e. Advanced nutrition and Chinese herbal medicine: such as royal jelly, honey, pollen, herbal preparations.
f. Preparation of ultrafine powder: Preparation such as Al2O3, ZrO2, TiO2, Ba2Cu3O7 ~ 8, Ba2Ti9O20 and other ultrafine powder.
g. In other aspects, such as the catalyst in chemical industry, the catalytic efficiency can be increased by 5 to 20 times after lyophilization; the leaves and the soil are lyophilized and preserved for studying the influence of soil, fertilizer and climate on plant growth and the role of factors; Damp wooden artifacts, submerged book articles, etc. are dried by freeze drying method.

3. The basic process for freeze drying is as follows:
a. Preparation of products (pre-treatment): such as drug cultivation, sterilization, packaging, bottle washing, semi-stopper, food raw materials selection, cleaning, segmentation, enzyme, packaging and so on.
b. Product Freeze (Pre-freeze): The product is frozen to a solid state.
c. The first stage of drying (sublimation drying): The ice crystals in the product are removed by sublimation.
d. The second stage of drying (desorption drying): Part of the residual moisture in the product at a higher temperature can be evaporated,make content of the residual moisture to meet the predetermined requirements.
e. Sealed packaging: Processed products should generally be sealed under vacuum or inert gas packing conditions, in order to facilitate storage.

4. The physical basis of product freezing
Matter has three states: solid, liquid and gas. The state of matter is related to its temperature and pressure. (Figure 1) shows the state balance of water (H2O). The curves of OL, OK and OS in the figure respectively represent the relationship between pressure and temperature of ice and water, water and water vapor, ice and water vapor coexisting, which are respectively referred to as melting line, boiling line and sublimation line. The three curves divide the surface into solid phase region, liquid phase region and gas phase region.Arrows represent the process of melting ice into water, water vaporizing into water vapor and sublimation of ice into water vapor, respectively. Curve OK at the top of the point K, the temperature is at 374 ℃, known as the critical point. When the water vapor temperature is higher than its critical temperature of 374 ℃, no matter how to increase the pressure, the water vapor can not become water. Intersection point of the three curves O is a state in which three phases of solid, liquid and gas coexist, which are called three-phase points. The temperature is 0.01 ° C. and the pressure is below 610.5 Pa. If the ice is heated, the ice will directly change without going to the  liquid phase Into the gas phase, this process is called sublimation.

5. Eutectic melting point and eutectic point of freeze dried product solution
In general, solutions of lyophilized pharmaceuticals are colloidal suspensions in which the main functional components (such as bacterial and other medicinal ingredients), various added components (antifreeze, antioxidants, fillers, etc.) and distilled water are mixed. It is not exactly the same as a generally miscible solution and has a series of low eutectic temperatures. For freeze-drying, it is necessary to determine a higher operating temperature above which the product will have dissolved liquid, and below which the product will freeze completely, the temperature is the eutectic temperature the being the lyophilized product . The eutectic temperature of some products is listed in Table 1. Similarly, the product solution during the cooling process, there are a series of eutectic temperature, in which the lowest eutectic point temperature is called the eutectic point of the solution temperature. The eutectic point temperatures of some materials are listed in Table 2.

Solution name

Molar solubility

Observe the solution temperature/℃

Calculated temperature

Methylphenidate phosphate




Phenylamine phosphate










Sodium chloride




Potassium chloride




Potassium bromide




Glycerin water


Dimethylsulfoxide water


(Table 1) Eutectic temperature of some saturated solutions

Sublimation Dry
Sublimation drying means putting the frozen product in a sealed vacuum vessel to heat, the ice crystals will sublimate into water vapor to escape the product dehydration and drying. Drying is from the outer surface, and gradually move inward, the remnants of ice crystals after sublimation.

Substance name

Eutectic point

Substance name

Eutectic point

Pure water


2% gelatin 10% glucose solution


0.85% sodium chloride solution


2% gelatin 10% sucrose solution


10% sucrose solution


Horse serum


10% glucose solution


General biological products


Skim milk






Glycerin water


(Table 2) eutectic point temperature of some substances

Gaps become sublimed water vapor escape channels. The interface between the dried layer and the frozen section (actually a thin layer) is called the sublimation interface. In biologic drying, the sublimation interface is pushed inward at a rate of about 1 mm / h. When all the ice crystals are removed, the sublimation drying is complete, at which point about 90% of the total moisture can be removed.

Desorb Dry
Desorption drying is also called the second stage of drying. After the first stage of drying, a portion of the water is adsorbed on the capillary walls and polar genes of the dried material, which are not frozen. When they reach a certain level, they provide the conditions for the growth and reproduction of microorganisms and certain chemical reactions. Experiments have shown that even a low water content of the monolayer adsorption may be a solution of certain compounds, resulting in the same mobility and reactivity as an aqueous solution. Therefore, in order to improve the storage stability of the product and prolong its shelf life, it is necessary to remove most of the moisture and leave only the monolayer of moisture. This is the purpose of desiccation drying.

The first stage of drying removes water as ice crystals. Therefore, the temperature of the freeze-dried layer and the pressure at the sublimation interface must be controlled below the eutectic point (or disintegration temperature) of the product so that ice crystals do not melt. However, for adsorbed water, which have high adsorption energies, they can not be desorbed from the adsorption without providing them with enough energy. Therefore, the temperature of the product at this stage should be sufficiently high, as long as it does not exceed the maximum allowable temperature, does not burn the product, and does not cause the product denatured by overheating. In the meantime, in order for the desorbed water vapor to have sufficient impetus to escape the product, large vapor pressure differences must be created inside and outside the product, so the vacuum inside the tank must be high during this stage.
After the second stage of drying, the content of residual moisture in the product depends on the product type and requirements. Generally is between 0.5% ~ 4%.

II .Freeze Drying System
1. Control System
The control system is composed of programmable program controller (SIEMENS -PLC), touch screen, peripheral relay, sensor and so on.

2. Refrigeration System
The refrigeration system is made up of a refrigeration compressor and auxiliary facilities, which provides a cold source for the drying box and the cold trap. (in the refrigeration system, the pressure controller is also installed except for the high and low pressure gauges that are displayed. High pressure relay action, compressor stop working when high pressure pressure is too high.

3. Circulation System
Freeze-drying and heating are the technology of material transfer by temperature difference. The refrigeration and heating of the shelf are carried out through the heat transfer of the heat conducting oil. In order to make the heat conduction oil circulate continuously in the whole system, we add a shielding circulating pump in the pipeline.

4.  Vacuum System
The vacuum system is composed of a drying box, a cold trap, a vacuum valve, a vacuum pump, a vacuum pipe and a vacuum gauge tube. The vacuum pump group has strong suction ability, and on the one hand, it promotes the moisture of the drying box to sublimate in vacuum. On the other hand, the system forms a vacuum pressure difference between the cold trap and the drying box, which makes water in the drying box after the  sublimation is capture by cold trap .

5. Hydraulic System
It is mainly used to press the bottle stopper into the bottle at the end of the drying.

III.The Use of Feeze-Drying Machine
1. Before each start-up, first check if the vacuum pump oil is enough.
2. Unscrew the drain valve to let the remaining water flow out. Tighten the valve after draining.
3. Check if the contact area between the freeze-dried chamber and the cold trap is intact, if necessary, clean and readjust.
4. Close all valves (valves connected directly to the freeze dried bottle, with the white knob turned upwards is closed). Close the connected valve between the trap and the vacuum pump (behind the instrument, connect with the vacuum tube).
5. Open the cold trap power switch, turn on the vacuum pump power switch.
6. After 15 minutes, open the valve through which the cold trap is connected with the vacuum pump.
7. After 1 minute, the sample that has been pre-frozen (already in the freeze-dried bottle) can be hung on the bracket, and the corresponding valve can be opened to do freeze-dry.
8. After lyophilization is complete, close the corresponding valve (white knob turns up) and take off the lyophilized bottle.
9. After all lyophilized bottles have been removed, turn off the vacuum pump and cold trap power in turns.
10. After the frost in the cold trap has melted, open the drain valve to drain and then close.

IV.Matters needing attention

  1. The sample must be pre-frozen before lyophilization. The sample must be lyophilized in the solid state (frozen state). The longer the pre-freeze time, the better the lyophilization. The volume of the sample should not exceed 1/3 of the volume of the sample bottle.
  2. In the freeze-dried if there is sample melting phenomenon, and the vacuum pump and cold trap work in good condition, the reason is lack of vacuum , where there is a leak. Check all valves ,if they are closed, to check if the contact area between the freeze-dried chamber and the surface of the cold trap is sealed, if necessary, clean the cold trap surface and freeze-dried chamber seal.
  3. Regularly clean the cold trap chamber and freeze-dried chamber.
  4. 15 minutes after the boot can be lyophilized, the purpose is to let the cold trap pre-cooling, let the vacuum pump preheat. This will enable the instrument to work in the best working condition.
  5. After the end of the freeze-drying,when the valve is closed, freeze-dried bottle is connected to the outside world, until the pressure is balanced,the bottles  can be removed .
  6. Regularly clean the dust inside the compressed bottle, the vacuum pump should be refilled when the oil is in less.