3. Dehydration kinetics

Like drying processes, freeze-drying is characterized by the coupling of heat and matter transfer, with a change of state. Sublimation in food products is commonly represented by a (sublimation) front moving from the free surface of the product (subject to vacuum) towards the core of the sample (frozen zone). The dry matter, distributed after freezing in a more or less dispersed interstitial network depending on the size of the ice crystals, forms a porous surface layer containing all or part of the uncrystallized water.

Freeze-drying kinetics will be illustrated in the case of a monodirectional transfer within a flat plate of surface area A and thickness e. The position of the sublimation front is marked by the distance x from the top surface of the product (figure 5...