Freeze Drying Steps
There are mainly 3 steps in a freeze drying process:
Freezing Stage, Primary Drying Stage and Secondary Drying Stage.
The freezing step is the most important and critical in the whole freeze drying process. Because incomplete frozen product can be boiled and spoiled under vacuum environment. Keep in mind that materials completely frozen is the insurance of a success lyophilization, also it guarantees the final products' form.
Products freeze can be done inside freeze dryer or in external freezer. The key point is to cool the material below its freezing point, to ensure the solid water will sublimate not melt in the following steps.
Primary Drying Stage
The primary drying step is to remove physical water in material. As known, >90% of the water in products are physical water, so this stage takes longer time.
During this stage, pressure is lowered to a few mbars, gradually apply heat to frozen materials is helpful to speed up the process. But too much energy offered in a short time may melt the materials. So, you need program the right freeze drying time chart.
Secondary Drying Stage
The secondary drying step is to remove bound water molecules. Since the bound water is hard to remove, so in this phase, more energy and higher vacuum will is needed.
Typically, this stage, the shelves & materials temperature may exceed 0C(some product can be 30C), and the pressure will pull down to <30Pa to encourage the desorption.
Freeze Drying Time Chart Example
As above mentioned, the freeze drying recipes actually is a temperature raise process, step by step. This rule fit all products, all industries, the main difference is the steps temperature set and lasting hours, should based on the temperature sensitivity of raw material.
Example of Freeze Drying Time Chart:
Step1 is raw material freezing stage, No vacuum is applied.
Step 2 to 6 is primary drying stage, temperature gradually raise (Energy applied). Vacuum degree 60~80Pa is an ideal range for rapid sublimation.
Step 7 is secondary drying stage, temperature raise to 40C, the highest raw material can bear, and pressure pull down to 20~30Pa.
Big pressure difference between drying chamber and raw material internal, force bound water morlecules come out.
Step 8 the temperature reduce to 30C again, this is to prepare dried material harvest, 30C is a good condition.