The freezing step is the most important and critical step 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 chill the material to below its freezing point.
Primary Drying Stage
The primary drying removes physical water in material. As known, >90% of the water in products are physical water, so this stage takes quite 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 removes bound water in material. In this stage, more energy and higher vacuum is needed.
Typically, in this stage, shelves & materials temperature will exceed 0C(some product can be 30C), and the pressure will pull down to <30Pa to encourage the desorption.