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The products obtained from drying generally have the problem of volume becoming small, material hardening, most of the volatile ingredients will be damaged, some heat-sensitive substances produce transmutation, inactivity, and some substances and even the occurrence of air oxidation.

　　Freeze-drying is a drying method aimed at better preservation of objects. Items often spoil, rot, or deteriorate due to two factors: external factors and internal factors. External factors include the effects of gases, water, temperature, and biological agents; internal factors primarily involve the metabolic activity of the biological material itself. If the effects of external and internal factors can be reduced to a low level, the goal of maintaining the stability of the object over a period of time can be achieved. Freeze-dryers are now widely used in chemistry, pharmaceutical manufacturing, food processing, and scientific fields, particularly in the field of biopharmaceuticals containing bioactive substances.

　　What are the differences between freeze-drying and conventional drying?

　　Drying is a very old operation, but heating drying of powders and biological products often leads to irreversible agglomeration of particles, especially in the field of ultrafine powder systems. Nanoparticles in high-performance liquid chromatography agglomerate into inseparable clumps after drying. The root cause is that during conventional particle drying, water is released from the inter-particle pores, and the surface tension creates very high additional pressure, compressing the particles together into clumps. Using a freeze-dryer can prevent this problem. Freeze-drying first freezes the gel or blood to be dried into a solid state, and then, under appropriate conditions, removes the water content through sublimation. Because the sublimation process of ice does not affect the agglomeration of solid particles, the original particles can be perfectly preserved without being compressed and agglomerated. In addition, freeze-drying is particularly suitable for processing biological products such as blood, because it does not involve high-temperature operations, avoiding the deterioration of biological products.

　　Dried products generally exhibit reduced volume and hardening of the material. Most volatile components are lost, some heat-sensitive substances undergo denaturation and loss of activity, and some substances even undergo oxidation. Therefore, the properties of the dried product differ significantly from those before drying. Freeze-drying generally takes place below 0℃, i.e., while the product is frozen. The product is only raised to a temperature above 0℃ in the later stages to reduce the residual moisture content, but generally not exceeding 40℃. Under vacuum conditions, when water vapor is directly removed, the drug remains in the ice matrix at the frozen state, forming a honeycomb-like loose and porous structure. Therefore, the volume of the product after freeze-drying hardly changes. Before reuse, simply adding injection water will immediately dissolve it.

　　Freeze-drying methods vary, such as air drying, boiling drying, baking, spray drying, and vacuum drying, but conventional drying methods are usually carried out at temperatures of 0℃ or higher. Dried products generally exhibit reduced volume and hardening of the material. Most volatile components are lost, some heat-sensitive substances undergo denaturation and loss of activity, and some substances even undergo oxidation. Therefore, the properties of the dried product differ significantly from those before drying. Freeze-drying generally takes place below 0℃, i.e., while the product is frozen. The product is only raised to a temperature above 0℃ in the later stages to reduce the residual moisture content, but generally not exceeding 40℃. Under vacuum conditions, when water vapor is directly removed, the drug remains in the ice matrix at the frozen state, forming a honeycomb-like loose and porous structure. Therefore, the volume hardly changes after drying. Before reuse, simply adding injection water will immediately dissolve it.

　　Compared to conventional methods, freeze-drying offers the following advantages:

　　1. Many heat-sensitive substances do not deform or lose activity.

　　2. When drying at lower temperatures, the loss of volatile components in the substance is minimal.

　　3. During freeze-drying, microbial growth and enzyme activity are inhibited, thus maintaining the original properties.

　　4. Because drying is carried out in a frozen state, the volume hardly changes, maintaining the original structure and preventing extraction.

　　5. Because the water in the raw material exists in the form of ice crystals after pre-freezing, the dissolved carbonate substances in the original dissolved water are evenly distributed in the raw material.