Lyophilization

Freeze-drying, formerly known as cryodessication, is a low-temperature dehydration process which eliminates most of the water contained in a product by sublimation. It enables long-term preservation by lowering the product's water activity.

Because of its specific features, freeze-drying occupies a unique position among drying techniques. It produces high-quality end products. The shape and appearance of the products are well preserved, and their aromatic quality is far superior to that of dried products. The transition of the product from the frozen to the dehydrated state, in the absence of a high proportion of liquid water, reduces the potential for the development of spoilage reactions. Another major technological advantage of freeze-drying is the ability of the freeze-dried product to rehydrate instantly.

On the other hand, the development of freeze-drying in the food industry is limited by its cost, which remains much lower than that of drying. The low productivity of freeze-drying is due to the vacuum mode of operation, which is usually discontinuous, resulting in long processing times (from ten hours to several days). Investment and operating costs are also high (energy consumption of 1,500 to 2,500 kWh per tonne of water to be eliminated).

As a result, freeze-drying can only be applied to products with high added value. In the food industry, these include coffee, herbs and spices, ready-made meals, and ingredients (vegetables, fruit, seafood, etc.) for instant dehydrated soups, culinary preparations and breakfast cereals. The pharmaceutical (vaccines, serum, medicines) and bio-industry (sourdoughs) sectors are much more strongly concerned by the freeze-drying process, which alone enables them to obtain the technique's most characteristic property, i.e. the preservation of an active principle (biological and/or medicinal activity) in a product to be stored at near-ambient temperature.