1. Principle of vacuum freeze drying
(1) Basic principle of freeze drying
Water has three phases (solid, liquid and gas). According to the relevant theory of thermodynamics, reducing the pressure can make the boiling point and freezing point of water coincide. Then, by heating, ice can be directly sublimated into gas without passing through the liquid state.
The basic principle of freeze drying by freeze drying machine is to use the three-phase change of water under low temperature and low pressure. First, the material to be dried is cooled and frozen to make the liquid water in the material become solid ice. Then, the material is heated under vacuum conditions to make the ice directly sublimate into water vapor and remove it, thereby obtaining a low-temperature dehydrated dry product. A typical freeze drying process can be divided into three typical thermodynamic processes: constant pressure cooling, constant temperature and pressure reduction, and constant pressure and temperature increase.
(2) Basic structure of freeze drying machine
The freeze drying of vaccines is inseparable from the freeze drying machine. The freeze drying machine is mainly composed of a drying box, a refrigeration system, a heating system, a vacuum system, a control system, etc. The structure of a classic freeze drying machine is shown in the figure. On the premise of ensuring the quality of the product, choosing a suitable freeze dryer can achieve a win-win situation of economic benefits and practical effects.
2. Advantages of freeze drying
Compared with the usual sun drying, boiling drying, spray drying and vacuum drying, freeze drying has the following outstanding advantages.
① Freeze drying is dried at low temperature, which will not denature the protein, but can make microorganisms lose their biological activity. This is particularly suitable for the drying and preservation of bioactive products, biochemical products, genetic engineering products and blood products with poor thermal stability.
② Because it is low-temperature drying, the loss of volatile components and heat-denatured nutrients and aromatic components in the substance is very small, so it is a high-quality drying method for chemicals, medicines and foods.
③ During the low-temperature drying process, the growth of microorganisms and the action of enzymes are almost impossible, so the original properties of the substance can be best maintained.
④ After drying, the volume and shape remain basically unchanged, the substance is spongy, without shrinkage, and the contact surface with water is large when rehydrated, and it can quickly return to its original shape.
⑤ Because it is usually dried under vacuum, there is very little oxygen, so that easily oxidized substances are protected.
⑥ It can remove 95% to 99.5% of the water in the substance, and the product has a long shelf life.
3. Application of freeze-drying
Freeze-drying technology has become more and more popular due to its unparalleled advantages over other drying methods. It has been widely used in various fields such as medicine, biological products, food, and active substances. Its application scale is still expanding rapidly. Vacuum freeze drying will surely become an important application technology in the 21st century.
(1) Application of freeze-drying technology in biological products:
① Live bacterial vaccines. For example, BCG, meningococcal vaccine, tuberculosis vaccine, oral dysentery live vaccine, Salmonella, Streptococcus, etc.
② Fire virus vaccine. For example, measles vaccine, influenza vaccine, rabies vaccine, fowl plague vaccine, etc.
③ Other biological products and biochemical drugs. Hepatitis B surface antigen diagnostic snowball, human leukocyte interferon, coenzyme A (CoA), adenosine triphosphate (ATP), urokinase, etc.
(2) Application of freeze-drying technology in the production of western medicine: Most of them are injections, mainly antibiotics, circulatory organ drugs, central nervous system drugs, vitamins and tumor drugs. For example, ampicillin, erythromycin ethylsuccinate tablets, etc.
(3) Application of freeze-drying technology in the production of traditional Chinese medicine: In recent years, freeze-drying has been used to process Chinese patent medicines. The prepared Chinese patent medicines are made into powders, tablets or injections after being soaked, extracted, filtered, concentrated and freeze-dried.
(4) Application in medicine: Freeze-drying technology can be used to preserve various organs such as blood, arteries, bones, skin, cornea and nerve tissue for a long time. Cells are not destroyed during freeze-drying, and organisms can be revived after rehydration. For example, freeze-dried bones can be stored at room temperature or in a refrigerator for up to 2 years.
(5) Freeze-drying of food. Foods processed by freeze-drying include:
① Cooking ingredients: meat, eggs, fish, vegetables, etc.
② Raw materials for the food industry: milk powder, egg powder, plant protein powder, tea, dried fruit, meat powder, bean powder, etc.
③ Beverages: coffee, fruit extract, etc.
④ Supplements: pollen, honey bee powder, turtle powder, etc.
(6) Other applications of freeze-drying technology:
① Microorganisms and algae: such as long-term preservation of various bacteria, yeast, enzymes, protozoa, microalgae, etc.
② Biological specimens and biological tissues: such as making various animal and plant specimens, drying and preserving marginal tissues such as skin, bones, aorta, heart valves, etc. for animal xenotransplantation or homologous transplantation.
③ Making small tissue slices for optical microscopes, electron scanning and transmission microscopes.
④ Food drying: such as coffee, tea, meat, fish and eggs, seaweed, fruits, vegetables, seasonings, tofu, convenience foods, etc.
⑤ Advanced nutritional products and Chinese herbal medicines: such as royal jelly, honey, pollen, Chinese herbal medicine preparations, etc.
⑥ Preparation of ultrafine powders: such as the preparation of ultrafine powders such as Al2O3, ZrO2, TiO2, Ba2Cu3O7~8, Ba2Ti9O20, etc.
⑦ Other aspects: such as catalysts in chemical industry, which can increase catalytic efficiency by 5 to 20 times after freeze-drying; freeze-drying and preserving plant leaves and soil to study the effects of soil, fertilizer, climate on plant growth and the role of factors; freeze-drying damp wooden cultural relics, flooded book manuscripts, etc.