Freeze-drying

The advantages of freeze-drying

Unlike other drying processes, with freeze-drying the consistency, shape, colour and the intense flavour of the original product remain unaltered. As the process involves only a slight warming, the vitamins and minerals are preserved as far as is possible.

During further processing, the freeze-dried ingredients rehydrate within seconds and are once again as they were when freshly harvested.  Through contact with moisture they recover their original character, pure flavour, colour and aroma.

Fresh weight

With freeze-drying, the original structure of the products remains unaltered. Only the water is gently removed. The initial weight of the raw materials before freeze-drying is called fresh weight. By removing the water content, the products become much lighter and have a much lower weight than before. We obtain, for example, only 9g of freeze-dried chives from about 90g of fresh chives. When the freeze-dried products come into contact with moisture once more, they absorb it like a sponge and return to their approximate original weight. This is called the instant character. The same phenomenon occurs with freeze-dried instant coffee.

The 3 Steps of Freeze-Drying

1. Freezing

To start the freeze-drying process, it is first necessary to freeze the raw material. At this stage, it is very important that the structure of the raw material remains unchanged and therefore products are quick frozen directly after harvesting at a temperature of -18°C (-0.4°F)

2. Main Drying

The second step of the process is the extraction of the water from the raw material. The IQF product is placed in a vacuum chamber. Under very low pressure, the frozen water contained in its structure is removed in the form of steam. This is called sublimation. To create sublimation, energy in the form of heat is needed. This energy can either be gained from the temperature difference between chamber and product or be supplied by built in heating systems. The drying chamber and the condenser area are kept under vacuum in order to support the migration of water vapour to the condenser where it is deposited in the form of ice, and to make sure that the vacuum is below the threshold required for sublimation.  Freeze-drying starts to be possible at a temperature of 35°C (95°F).

3. Post Drying

After the free ice has been removed by sublimation, the product still contains bound water which could affect shelf life and quality. During post-drying the most strongly bound water inside the product is converted into steam. This is a slow process. Post drying is called the third step, but begins during the main drying.

The Physics

The freeze-drying process uses a particular physical property of water. Water is extracted from the frozen product under such low pressure that it changes directly from its solid phase to its gas phase (sublimation)

Complementary Information

Pressure

Freeze-drying takes place in a vacuum. Products are put in a vacuum chamber and pressure is generated by compressors. The pressure must be under 6 mbar for sublimation to start.

Production curves

Each product and each cut size has its own production profile, obtained after extensive testing in the pilot drier but above all thanks to years of experience. Successful freeze-drying is the result of the perfect tuning of vacuum pressure and temperature to product type and volume.

Volume

Since freeze-drying takes place under a vacuum, the quantities produced are dependent on the size of the tunnel. Production is therefore organized in batches.

Time

Drying time is dependent upon the product type and cut size. In some cases, the planned usage of the freeze-dried product can influence the drying time when customer specifications require very low drying temperatures.

Triple point

The triple point is the phase where a balance between temperature and pressure is reached and where the water is at the same time and in the same proportions in its three phases (solid, liquid and gas).

Change one of the parameters and the balance will be lost. The water will turn back into one of its various forms: solid (ice), liquid (liquid water) or gas (steam).

Energy

Freeze-drying carried out at low temperatures and under low pressure requires  energy. Once the drying is done, the energy needed for storage of the product is very low when compared to that needed for storage of fresh or frozen products.

Microbiology

The freeze-drying process has no effect on the microbiological values of the final products. Only very clean raw materials are selected for being freeze-dried in order to achieve the best results.

Moisture

Freeze-dried products generally have a lower moisture content than products dried by other methods. Residual moisture is usually in the region of 4%.