The Role of Vacuum Technology in Freeze Drying

There are several ways to extend the shelf life of food. One of the oldest methods belong of smoking and salting is drying – which was used long before freezing, boiling or vacuum packaging. Freeze drying is a modern extention of traditional drying methodsto protect food from spoilage for longer. This process is onlymade possible by modern vacuum technology because freeze drying food takes place under vacuum.

Where is it used?

Freeze drying is suitable for fruit and berries as well as for cooked meat or vegetables. Herbs and spices are also preserved through freeze drying by removing all but a few percent of the water from these products. The best-known freeze-dried food is probably soluble (instant) coffee. Many ready meals, which are only dissolved in boiling water for preparation as granules or in powder form, have been freeze-dried. In principle, food can also be dried in a simpler way, for example through desiccation. However, this has the disadvantage of changing the structure and appearance of the food. In addition, aromas can react with atmospheric oxygen, which has an effect on the taste.

During freeze drying, the structure remains unchanged and the dried cells can easily absorb water again during preparation. By drying under vacuum, the aromas are largely preserved.

Freeze drying process

Freeze drying is a purely physical process that uses the principle of sublimation. Due to the vacuum in the drying chamber, the frozen water sublimates into water vapor. The ice skips the "liquid" state and becomes directly "gaseous" from "solid".

Before the actual drying process, the product, whether berries, pieces of fruit or coffee extract, must be deep-frozen in a cooling chamber. For this purpose, the product is spread over trays that are then placed in the cooling chamber either directly or on trolleys. The frozen product is then placed in the actual drying chamber.

In some freeze dryers, cooling takes place directly in the drying chamber. Also freeze dryers for continuous processes are used in food processing.

After cooling, the actual drying process takes place in the air-tight chamber by reducing the air pressure in the chamber via a vacuum pump until a vacuum of 1 to 0.5 mbar is reached. The evaporation process of the frozen water now starts at -50 to -40° Celsius. The water vapor is suctioned out of the drying chamber by the vacuum pump into a downstream condenser. In this "ice trap" cooled to at least -70° Celsius, the water vapor sublimates and condenses as ice on the cooling coil. Most of the moisture is removed from the product in this process step, called primary drying.

During sublimation, thermal energy is extracted from the drying chamber. This means that the temperature in the chamber would drop during this process. The chamber therefore needs to be heated. Care must be taken to ensure that only as much thermal energy is added as is taken from the water as sublimation energy in order to keep the temperature constant or allow a slight increase in temperature up to -20° Celsius.

For most foodstuffs, primary drying is immediately followed by secondary drying. This involves removing more strongly bound residual moisture from the product by lowering the vacuum level to 0.01 mbar or below and raising the temperature above the freezing point. The drying chamber is then ventilated to atmospheric pressure and the dried product with a water content of one to four percent is removed for further processing. Dry air or an inert gas is used for ventilation so that the dried product cannot absorb moisture from the ambient air.