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

3.6 Freeze drying
Freeze drying or lyophilisation was first introduced in the forties for large scale
production of dry plasma and blood products (Vega-Mercado et al. 2001). Freeze drying
is one of the drying methods, which can produce high quality dried product. Freeze
drying has two important steps; one is freezing of the material and then drying under
vacuum. This method takes advantage of triple point of water. Triple point is a state in
which substances coexists in different states such as solid, liquid and vapour and remain
in thermodynamic equilibrium. Water has triple point at 0.01 °C and 611.73 Pa (Figure
3.4) (Geankoplis 1993). Below triple point (temperature and pressure combination) ice
can be directly converted to vapour, a process known as sublimation.
Some of the advantages of freeze drying, which makes it an attractive process are
(Liapis & Bruttini 1995; Liapis & Sadikoglu 1997; Vega-Mercado et al. 2001; Jafar &
Farid 2003),
i) The absence of air during processing prevents oxidative and chemical
deterioration of the product
ii) Lower drying temperature, which preserves volatile and flavour components
iii) Absence of moisture movement within the material during drying and
iv) Dried product has excellent re-hydration properties
Freeze dried food products have low density and higher nutritional value, which
makes a popular choice for astronauts and mountaineers. The drying time is usually
longer, which results in higher cost of drying. During freeze drying the temperature of
the condenser and vacuum level should be maintained to the lowest possible value and
this also contributes towards increased drying costs. This is one of the major reasons its
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