Freezing may be the most under-rated step in the freeze
drying process. If the product is not frozen properly, freeze
dry can be exceptionally long.
In general, the larger the ice crystals the easier the
product is to freeze dry. The larger crystal makes it easier
for the vapor to escape from the structure.
The type of freezing will vary depending on whether the
product forms a crystalline structure or is amorphous
when frozen. A crystalline material is typically easier to
freeze dry. Amorphous materials may have lower freezing
points and may require an annealing step to form a
structure that can be more easily freeze dried.
Fast freezing produces small crystals (harder to dry), but
provides even distribution. Fast freezing inhibits
precipitation of solids.
Slow freezing produces large crystals (better for drying),
but with a possibility of skin formation.
Annealing is the process of freezing, then warming the
material to allow the crystals to grow.