foam-mat freeze drying of egg white and ... - McGill University

To achieve drying we need to supply latent heat of sublimation (for ice at 0 °C
this is 2840 kJ/kg (Flink & Knudsen 1983; Liapis & Bruttini 1995)). During sublimation,
latent heat is absorbed which results in the temperature reduction in the frozen layer. If
there is no heat input, the water vapour pressure of the frozen product comes into
equilibrium with the drying chamber and there will be no further sublimation. The latent
heat supply can be achieved either by conduction, convection or radiation. During freeze-
drying, conduction and radiation heat transfers are more common than convection. The
amount of heat that could be supplied without affecting the product is one of the limiting
factors in freeze drying. One of the important limiting factors for heat input is the
maximum temperature the frozen layer could withstand and remain frozen. Melting of
frozen layer can have adverse effect on the product quality in many ways. Collapse can
occur, which results in structural deformation of the dried product. Some other factors
which affect the maximum heat input are colour sensitivity of the product, biosensitivity
of the product and structural deformation in the dried product (Mellor 1978; Flink &
Knudsen 1983; Liapis & Bruttini 1995).
Usually the sublimation of water starts at the surface and water vapour is directly
removed. After some time of drying, the product will have two distinct layers: the ‘Dry
layer’ and the ‘Frozen layer’. Once a dry layer is formed, the water vapour is transferred
through it. In ideal drying conditions both layers can be distinguished properly (Figure
3.6 (a)). But in practical drying conditions that may not be the case, usually there will be
a transition layer between frozen and dry layer (Figure 3.6 (b)). The end of the primary
drying stage is marked by a sharp decrease in drying rate. By the end of the primary
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