Freeze-drying, also known as lyophilization or sublimation drying, is a method of careful drying of high-quality products. In freeze-drying, the ice crystals sublimate without passing into a liquid state. The end product of freeze-drying is known as lyophilization which is completed in a batch process.

Why Lyophilization Is Used?

A lyophilizer consists of

• A vacuum chamber containing product shelves which are capable of cooling and heating containers and their contents.
• A vacuum pump, a refrigeration unit, which is associated controls are connected to the vacuum chamber.

Samples are generally placed in containers such as glass vials that are placed on the shelves within the vacuum chamber.

Cooling elements within the shelves freeze the product. Once the product is frozen, the vacuum pump evacuates the chamber and the product is heated. Heat is transferred by thermal conduction from the shelf, through the vial, and ultimately into the product.

Lyophilization involves the removal of water or other solvents from a given product by a process called sublimation. This occurs when the ice of a frozen product converts directly to the gaseous state without passing through the liquid phase. This enables the preparation of a stable product that is easy to use and store at ambient temperatures.

A low pressure environment is pre-requisite to allow this process to take place. In order to start the removal of water, the pressure inside the freeze dryer must be below the “triple point value” for the product, whilst also maintaining the temperature of the sample below its freeze point in the lyophilization process.

Pre-freezing – first stage of the lyophilization process

The sample is frozen, which means the water in the product is converted to ice, thereby the phase has changed from liquid to solid.
Slow pre-freezing will produce lager ice crystals, which are easier to lyophilize, whilst fast pre-freezing results in smaller crystals.

Primary drying – Second stage of the lyophilization process

In the second stage of lyophilization the sublimation process starts. The ice formed in the pre-freeze step is removed from the sample by the direct transition of the “solid” ice to a vapour without passing through a liquid phase. The resultant vapour is collected by the condenser, which has a lower temperature and pressure than the product. The vapour is thus converted back to ice on the condenser surface.
The “energy” required for this process to occur is provided by a gentle heating of the sample, which will start the sublimation process and eventually the sample will dry.

If too much energy (heat) is applied to the sample during this stage the condenser of the lyophilizer may not be able to condense the volume of vapours fast enough, the ice condenser temperature will subsequently rise along with its vapor pressure, thus increasing the risk of the sample melting.

Secondary drying – Third stage of the lyophilization process

Finally, any residual water present, which is strongly bound to the molecules of the sample, is converted to vapour and removed from the sample.

This water has invariably a vapour pressure lower than that of water in its “free” form.
Removal of the water in this final stage of lyophilization will be performed at higher product temperatures, consequently, any biological activity of the sample will not be impaired or affected. This usually involves increasing the temperature and lowering the pressure to provide enough energy to break down the molecular bonding. A process called desorption.

Lyophilization has many advantages over the other drying and preserving techniques.

1. It maintains food/ biochemical and chemical reagent quality because they remains at a temperature that is below the freezing-point during the process of sublimation.The use of lyophilization is particularly important when processing lactic bacteria, because these products are easily affected by heat.
2. Food/biochemicals and chemical reagents which are lyophilized can usually be stored without refrigeration, which results in a significant reduction of storage and transportation costs.
3. Lyophilization greatly reduces weight, and this makes the products easier to transport. For example, many foods contain as much as 90% water. These foods are 10 times lighter after lyophilization.
4. Because they are porous, most freeze-dried products can be easily rehydrated. Lyophilization does not significantly reduce volume, therefore water quickly regains its place in the molecular structure of the food/ biochemicals and chemical reagents.