Freeze drying refers to the process of removing water or other solvents from frozen biological products through sublimation. Sublimation refers to the process in which a solvent, such as water, like dry ice, changes from a solid state to a gaseous state without going through a liquid state. The product obtained from freeze-drying is called Lyophilizer, and this process is called Lyophilization.
Traditional drying can cause material shrinkage and damage cells. The structure of the sample will not be damaged during the freeze-drying process, as the solid components are supported by the solid ice in their positions. When ice sublimates, it leaves pores in the remaining dry material. This preserves the integrity of the biological and chemical structure and activity of the product.
In the laboratory, freeze-drying has many different uses, and it is widely used in many biochemical and pharmaceutical applications. It is used to obtain biomaterials that can be preserved for a long period of time, such as microbial culture, enzymes, blood, and drugs. In addition to the stability of long-term preservation, it also retains its inherent biological activity and structure. For this purpose, freeze-drying is used to prepare tissue samples for structural studies (such as electron microscopy). Freeze drying is also used in chemical analysis, which can obtain dried samples or concentrate samples to increase analysis sensitivity. Freeze drying stabilizes the composition of the sample without changing its chemical composition, making it an ideal analytical aid.
Freeze drying can occur naturally. Under natural circumstances, this process is slow and unpredictable. Through freeze-drying systems, people have improved and subdivided many steps, accelerating this process.
A basic freeze-drying system includes:
·A drying chamber or multiple manifolds
·A vacuum pumping system overcomes obstacles and accelerates gas flow
·A heat source provides energy
·A low-temperature condenser is used to maximize the vapor pressure difference and capture the vapor to freeze it, avoiding water vapor pollution of the vacuum pump.
The freeze-drying process consists of three steps:
·Pre freeze to prepare samples for the subsequent sublimation process.
·Primary drying, during which ice sublimates without melting.
·Secondary drying is a process in which residual moisture from bonds and solid substances is removed, leaving behind a dried sample. This step is crucial for maintaining the stability of the sample. In shell pre freezing, the sample in the freeze-drying bottle is immersed in a low-temperature heat conduction liquid and rotated. The liquid sample freezes along the inner wall of the freeze-drying bottle to achieve a larger surface area. This thin frozen layer makes it easier for water molecules to pass through. Once the sample freezes, it can be connected to the freeze-drying system. Primary and secondary drying occur when the sample bottle is connected to the freeze-drying system, immediately exposing the sample to a vacuum condition to overcome airflow resistance. At the same time, heat is provided as energy. The heat source for providing heat to freeze-drying bottles and other glass containers connected to a drying oven or multiple manifolds is a room temperature air bath. On the automatic capping box, it is supplied by the heating layer. The conditions of vacuum and heat can help the water vapor sublimated from ice to more easily flow out of the sample and the freeze-dried material on the surface.
The sublimation efficiency of frozen samples depends on several factors. The most important one is the pressure difference between the frozen product and the collector. The most effective freeze-drying occurs when the sample is at the highest temperature it can withstand while still maintaining a frozen state, while the collector temperature and system vacuum are kept at the lowest possible values. The variation of drying time depends on the eutectic temperature of the material being freeze-dried. For the vast majority of biomaterials, this temperature is below 0 ℃, and some even as low as -40 ℃. High air pressure and temperature differences will result in effective drying. After the primary freeze-drying is completed, all the ice is sublimated. However, the bound water still exists in the product, and during secondary drying, the water that is firmly bonded to the solid sample after drying is called absorbed water and converted into vapor. This process is called desorption. Desorption is a slow process because absorbed water has a lower air pressure than liquid water at the same temperature. Freeze drying is completed when the vapor pressure of the sample and collector is equal. If the sample leaves the system too early before drying, it may quickly degrade and lose its structure and biological properties.