1. Physical principles

In practice, the freeze-drying process is based on two chronological operations: freezing and dehydration. Freezing is considered to be a preliminary stage to the dehydration operation, and although it is crucial to the final result, it will not be detailed in this article. Dehydration involves two physical principles: the sublimation of ice (crystals formed by freezing) and the final desorption of the residual, unfrozen water. This latter water fraction can represent from 10 to 30% of the water initially present in the product.