Comparison with Liquid-Phase Drying
Lyophilization gives the opportunity to avoid denaturation caused by heating the product,
by maintaining it frozen throughout drying.
This is the most obvious advantage over liquid-phase drying.
Equally important is that in liquid-phase drying there is an undesirable shrinkage and
concentration of active constituents that causes damage as well as a movement of these
constituents to the surface of evaporation, where they form a dense, impermeable skin
that inhibits drying, and later, rehydration. Such effects can be avoided by spray drying,
but this requires brief exposure to temperatures around 100 C.
Further advantages of lyophilization for parenteral products are that the wet material
can be accurately dispensed and can be sterile filtered just before filling into final
containers so that particulate and bacterial contamination is reduced.
Thus, the principle advantages of lyophilization as a drying process are:
- Minimum damage and loss of activity in delicate heat-liable materials
- Speed and completeness of rehydration
- Possibility of accurate, clean dosing into final product containers
- Porous, friable structure
The principle disadvantages of lyophilization are:
Lyophilization should be used when the product meets one or more of the following criteria:
unstable; heat liable; minimum particulates required; accurate dosing needed;
quick; complete rehydration needed; high value.
- High capital cost of equipment (about three times more than other methods)
- High energy costs (2-3 times more than other methods)
- Long process time (typically 24 hour drying cycle)
Some other less common applications of lyophilization are recovery of water-damaged
books and manuscripts and preservation of archaeological specimens, tissue for
spare-parts surgery, museum specimens for display such as plants and animals,
and vegetable matter for research programs.
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