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

partial differential equations have very complex solutions but they can better explain the
drying process (Liapis & Bruttini 1995; Barbosa-Canovas & Vega-Mercado 1996; Bala
1997; Jafar & Farid 2003; Karel & Lund 2003).
Earlier drying models were based on pure mass transfer and the effect of heat
transfer was completely neglected. Mass transfer mechanism was assumed to be either by
capillary or diffusion. But lot of research has been done in this area, with more
parameters and conditions. Many mathematical models have been proposed to describe
drying (Karim & Hawlader 2005). Freeze drying involves simultaneous heat and mass
transfer and this has to be taken into account when models are developed. However, the
conditions governing heat and mass transfer are often simplified to model freeze drying
to determine drying rate and diffusion coefficient. Complex models are necessary to
better describe drying kinetics during freeze drying. But it would be a better approach to
start with simple models and improve the model to better describe the process (Karel
1975; Flink & Knudsen 1983; P. Sheehan 1998).
3.8.1 Heat Transfer
In case of solid foods, heat transfer is usually modelled using Fourier’s
conduction equation. Most of the models can be solved analytically but they are often
very complex, hence numerical solutions are widely used for solving heat transfer models
(Wang & Sun 2003). Some researchers analyzed radiation heat transfer method for freeze
drying. In this case, heat passes through the dry layer and is absorbed at sublimation
interface. If radiation is a major source of heat supply, the freeze drying is heat transfer
controlled and sublimation temperature becomes constant. Internal heat transfer
resistance is much higher than mass transfer resistance. Quasi-steady state model is
29