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

3.6.1.1 Freezing
Freezing is one of the important steps if freeze drying. The freezing of the
product can be achieved either within the freeze drying apparatus or in a separate
freezing chamber and the choice mainly depends on the cost and operating methods used.
Freezing has some important functions like, partial dehydration and increasing the
rigidity of the structure. Freezing temperature should be much lesser than that of freezing
temperature of pure water as water is mostly present in the combined form. The most
common freezing temperature is – 30 °C. During freezing most of the free water is
completely frozen and only bound water remains unfrozen. (Flink & Knudsen 1983;
Liapis & Bruttini 1995).
During freezing the rigidity of the material increases due to ice formation and this
may not be uniform throughout the frozen material. This is due to the presence of bound
water, which remains unfrozen. These unfrozen regions are supported by the surrounding
ice, which prevents deformation of the product to some extent. The method of freezing
(quick freezing or slow freezing) can play a major role in determining the structural
integrity of the product during freezing (Mellor 1978; Flink & Knudsen 1983; Liapis &
Bruttini 1995).
3.6.1.2 Primary drying Stage
During primary drying, the frozen water is removed by sublimation. To achieve
drying during primary stage the pressure inside the drying chamber must be less than or
in equilibrium with the vapour pressure of the frozen water. For pure water the absolute
pressure should be 4.58 mm Hg or less at 0 °C (Figure 3.4). For practical drying
applications the temperature and pressure combination must be lower than that of pure
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