Article written by
Amy Miyashiro, Production Chemist, Analytical Products Group
At APG, our microbiological products consist of a varying number of freeze-dried samples in aseptically
sealed vials. Freeze drying or lyophilization is the process of drying a frozen product by creating conditions
for sublimation of ice directly to water vapor.
The Three Stages of Lyophilization
The process of lyophilization consists of three stages: prefreezing, primary drying and secondary drying.
In the prefreezing stage, the aqueous suspension is cooled until the water is separated from the solute (product).
This creates more concentrated areas of solute. In most cases, the solute freezes at a lower temperature than
the water so the suspension must be cooled until the eutectic temperature is reached. That is, the temperature
at which all areas of concentrated solute are frozen.
Once the product has undergone prefreezing, an environment in which the ice can be removed from the product
by means of sublimation must be established. Sublimation is the conversion of water from the solid state (ice)
directly to the gaseous state (water vapor) without going through the liquid state. Sublimation results in a dry,
structurally intact product. This is known as the primary drying stage.
After primary drying, secondary drying must occur. This is the process of reducing the amount of residual
moisture in the freeze dried product. In order to reduce the residual moisture of the product, continued drying
at a warmer temperature under very low pressures is necessary. However, this step is carefully controlled. If
this process occurs too quickly or too slow the structural stability of the product may become compromised.
Freeze drying is used in a variety of applications, most frequently in the food and pharmaceutical industries.
Sublimation of the ice preserves the product’s chemical and structural integrity allowing for long-term storage
of biological substances such as microbial cultures, blood, enzymes and pharmaceuticals. In fact, the freeze drying
concept has been around for hundreds of years. The ancient Incans used the Andes mountain peaks as a way to preserve
their food. The very cold temperatures and extremely low pressure at those high altitudes prohibited food from
spoiling in the same basic manner as a modern day freeze-dryer.
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