Everything you need to know about freeze dryers

A freeze dryer lowers the temperature of food to minus 30 to minus 50 degrees Fahrenheit. It then creates a vacuum in the food chamber, allowing easy preservation.

What is freeze drying?
Freeze drying, otherwise known as Lyophilisation is the process of removing ice or frozen solvents from perishable items through desorption. Freeze drying involves three main stages:

Freezing stage
Freezing is done on a shelf in a freeze dryer where the material is cooled below its triple point. This ensures sublimation rather than melting to help preserve its physical form.

Primary drying (sublimation) stage
In this stage, the material is exposed to low pressure and heat in order for the water to sublimate. The vacuum created in the freeze dryer speeds up the process of sublimation. Primary drying can be a slow process, as excessive heat can alter the structure of the material. However, it removes 95% of water from the material.

Secondary drying (absorption) stage
This stage involves removing the ionically-bound water molecules by raising the temperature higher than that in the Primary Drying stage. The material is then sealed and retains only about 1-5% moisture.

What makes up a freeze dryer?

Refrigeration system
This system works towards cooling the condenser that is in the freeze dryer. It is also used for cooling the product shelves inside the chamber for the product.

Vacuum system
This system has a vacuum pump that is separate and is connected to a condenser which is airtight. It is attached to a product chamber and helps to reduce the water molecules.

Control system
Control systems help in maintaining temperature and allow monitoring the freeze drying process.

Product chamber or manifold
These chambers are one of two types – a manifold with flasks attached to it and the second – a chamber with shelves where the product is placed.

Condenser
Attracting the vapors that are sublimed from the product is the purpose of the condenser. The collapse temperature and freezing point and of the product prescribe the condenser temperature.

What are the different types of freeze dryers?
Freeze dryers are divided into the following based on the type of product chamber.

Manifold dryers
These dryers are used when the product or material is pre-frozen and is in flasks.

Shelf dryers
Shelf dryers are used when the material or product is directly placed on the shelf or tray of the freeze dryer.

Combination units
Combination units have both options – manifold as well as shelf drying.