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Wednesday, November 23, 2011

General Terms in Freeze-Drying (Lyophilization)




A Freeze - Dryer (Lyophilizer)


Accelerated Storage: Exposure of freeze dried products to elevated temperatures to accelerate the degradation process that occurs during storage.

Batch Freeze Drying: Freeze drying multiple samples of the same product in similar sized vessels at the same time in a shelf tray dryer.

Bulk Freeze Drying: Freeze drying a large sample of a single product in one vessel such as the bulk drying pans designed for
shelf tray dryers.

Collapse: A phenomenon causing collapse of the structural integrity of a freeze dried product due to too high a temperature at the drying front.

Collapse Temperature: The temperature above which collapse occurs.

Collector: A cold trap designed to condense the water vapor flowing from a product undergoing freeze drying.

Internal Collector: A collector located in the same area as the product. All water vapor has a free path to the collector.

External Collector
: A collector located outside the product area connected by a small port through which all water vapor must pass. Allows isolation of the product from the collector for drying end point determinations and easier defrosting.

Ethylene Oxide: A colorless, odorless gas used for gas sterilization of tray dryer systems.

Eutectic Temperature: The temperature at which all areas of concentrated solute are frozen.

Evaporative Cooling
: Cooling of a liquid at reduced pressures caused by loss of the latent heat of evaporation.

Freeze Drying: The process of drying a frozen product by creating conditions for sublimation of ice directly to water vapor.

Glass Transition Temperature: The temperature at which certain products go from a liquid to a vitreous solid without ice crystal formation.

Isothermal Desorption: The process of desorbing water from a freeze dried product by applying heat under vacuum.

Lyophilization: The freeze drying process.

Manifold Freeze Drying: A freeze drying process where each vessel is individually attached to a manifold port resulting in a direct path to the collector for each vessel.

Prefreezing: The process of cooling a product to below its eutectic temperature prior to freeze drying.

Pressure Gauge (Vacuum Gauge): An instrument used to measure very low pressures in a freeze drying system.

Thermocouple Gauge: A pressure gauge that measures only the condensable gases in the system. This gauge can be used as an indicator of drying end points.

McLeod Gauge: A mercury gauge used to measure total pressure in the system (i.e. condensable and non-condensable gases.)

Primary Drying: The process of removing all unbound water that has formed ice crystals in a product undergoing freeze drying.

Residual Moisture: The small amount of bound water that remains in a freeze dried product after primary drying. Residual moisture is expressed as the weight percentage of water remaining compared to the total weight of the dried product.The amount of residual moisture in a freeze dried product can be reduced during secondary drying.

Secondary Drying: The process of reducing the amount of bound water in a freeze dried product after primary drying is complete. During secondary drying, heat is applied to the product under very low pressures.

Shell Freezing: Freezing a product in a thin layer that coats the inside of the product container. Shell freezing is accomplished
by swirling or rotating the product container in a low temperature bath.

Sublimation: The conversion of water from the solid state (ice) directly to the gaseous state (water vapor) without going through the liquid state.

Vapor Pressure: The pressure of the vapor in equilibrium with the sample.

Reference:
A Guide to Freeze Drying for the Laboratory, LABCONCO, An Industry Service Publication.