There are various methods that can be used to freeze the product. Freezing can be done in a cold bath with liquid nitrogen, in a shockfroster, a regular freezer, or on a shelf in the freeze dryer. Cooling the material below its triple point ensures that the product is not thawed but sublimated. In this way, the shape of the product is optimally preserved.
Large ice crystals created by slow freezing are the easiest for the freeze dryer to dry. However, if the crystals are too large, they can destroy the cell walls of the product, resulting in a lower quality end product. To prevent this, the freeze process is carried out as quickly as possible.
Freeze-Drying Process Through Drying Cycles
The second phase of freeze-drying is the so-called primary drying (sublimation), in which the pressure is lowered (a vacuum is created) and heat is added to the product so that the water can sublimate. The vacuum promotes sublimation. The condenser provides a surface for the water vapor. In the case of WAVE freeze dryers, the vacuum chamber wall is the condenser to which the water vapor adheres and solidifies in the form of ice. The condenser also protects the vacuum pump from the water vapor. Too much heat at once can affect the structure and shape of the material. We recommend adding heat in small steps in order not to overheat the material as the ice will melt and the structure will be lost.
This is followed by further phases of freeze-drying, secondary drying (adsorption), in which water molecules that are ionically bound are removed. By setting the temperature higher in small steps than in the primary drying phase, the bonds between the material and the water molecules are broken. These drying stages can take place at 5°C to 10°C intervals, for example, and can be flexibly adjusted by the Siemens control of the WAVE freeze dryers. Freeze-dried materials retain a porous structure. After the freeze-drying process is complete, the vacuum can be broken before the material is sealed airtight. Most materials can be dried to 1-5% residual moisture thanks to this process.