Freeze drying or lyophilisation
Freeze drying, or lyophilisation, is a dehydration process taking place at low temperatures. In contrast to alternative methods such as spray drying where the water is evaporated, in freeze-drying the water is sublimated. Sublimation is a transformation process of the solid phase (ice) to gas (vapor) at temperatures below -27°C and pressures below 0.5mbar.
The freeze drying process is separated into two major processes – the primary and the secondary drying. The primary drying cycle is the beginning of the freeze drying process. The frozen solvents e.g. ice (water) is removed from the product by sublimation. When all the ice is removed, the secondary drying is initiated. This process is where the bound water molecules (non ice) are removed through the process of desorption (see illustration beside for indication of typical drying cycle curves).
The freeze drying process results in a high-quality product because of the low temperature load of the product during processing. Product shape, texture and color is maintained, and the quality of the rehydrated product is often outstanding.
Freeze drying is often used for biological products such as bacteria (nutraceuticals) and yeasts. It is also used for biomedical and heat sensitive food products due to color, volatile taste components or chemical resistance.