Freezing Technology

3.6.3. Principle of the equipment

Effectiveness of a freezes drier depends on rate of heat transfer to the sublimation front and rate of mass transfer.There are three methods of heat transfer.

Heat transfer through the frozen layer. The rate of heat transfer depends on the thickness and thermal conductivity of frozen layer. As the drying proceeds, the thickness of ice is reduced and the rate of heat transfer increases.

Heat transfer through dried layer. The rate of heat transfer to sublimation front depends on the thickness of the dried layer and the surface area of food, thermal conductivity of dry layer and the temperature difference between the surface of the food and ice front. The dried layer has very low thermal conductivity and therefore offers high resistance to heat flow. As drying proceeds, this layer becomes thicker and the resistance increases.

Heating by microwaves : Heat is generated at the ice front and the rate of heat transfer is not influenced by thermal conductivity of ice or dry matter or the thickness of dry layer. However it is difficulty to control microwave heating.

Rate of mass transfer: It depends on the water vapour pressure gradient inside the drying chamber which is controlled by

1. The pressure in drying chamber
2. Temperature of the vapour condenser – both should be as low as possible.
3. Temperature of ice at the sublimation front and it should be on high as possible (at O^oC) without melting.

Equipment : freeze drier consists of a vacuum chamber containing trays to hold food during drying, source of heat to supply latent heat of sublimation, refrigeration coils to condense vapour with automatic devices to keep maximum area of the coil free of ice for vapour condensations and vacuum pump to remove non expensible vapour.

Types of equipments :

1. Accelerated freeze driers : Food is held between expanded metal mesh for heat transfer.
2. Radiation freeze driers : In paired radiations from radiant are used. Heating is more uniform. Close contact between food and the heat is not necessary; float trays can be used and cheaper. Easier to clean.
3. Microwave and dielectric freeze driers