Freeze dry

Freeze-drying (also known as lyophilization or cryodesiccation) is a low temperature dehydration process that involves freezing the product, lowering pressure, then removing the ice by sublimation. This is in contrast to dehydration by most conventional methods that evaporate water using heat.

Process

Freeze-drying works by freezing the material and then reducing the surrounding pressure to allow the frozen water in the material to sublimate directly from the solid phase to the gas phase. The process consists of three stages: pre-freezing, primary drying, and secondary drying.

Pre-freezing

The preparation stage of freeze-drying, pre-freezing, involves cooling the material below its triple point. This ensures that sublimation rather than melting will occur in the following steps.

Primary drying

During the primary drying phase, the pressure is lowered and enough heat is supplied to the material for the water to sublimate. The amount of heat necessary can be calculated using the sublimation heat (the heat of sublimation) and the rate of sublimation.

Secondary drying

The secondary drying phase aims to remove any unfrozen water molecules, since the ice was removed in the primary drying phase. This part of the freeze-drying process is governed by the principles of desorption.

Applications

Freeze-drying is used in a variety of sectors, from pharmaceutical companies to the food industry. In the pharmaceutical industry, it is used to dry vaccines, antibiotics, and other drugs. In the food industry, it is used to preserve food, increase its shelf life, and make it more convenient for transport.

Advantages and Disadvantages

Freeze-drying has many advantages over other drying methods. It maintains the physical structure of the material, and preserves the nutritional content and flavors of the food. However, it is a complex and expensive process, which can be a disadvantage for some applications.

See also

• Dehydration
• Sublimation (phase transition)
• Pharmaceutical industry
• Food industry