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Freeze Drying Steps

Freeze Drying Steps

There are mainly 3 steps in a freeze drying process:

Freezing Stage, Primary Drying Stage and Secondary Drying Stage.

Freeze Drying 3 Main Stages

Freezing 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.

Freeze Drying Recipes and Steps Setting

Example of Freeze Drying Time Chart:

Temp. Hrs Vac
-30C 2.5 No
-10C 2 60~80Pa
0C 1 60~80Pa
10C 2 60~80Pa
20C 2 60~80Pa
30C 10 60~80Pa
40C 2 20~30Pa
30C 0.5 20~30Pa
Explain:

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.