In boiling type drying, the product temperature reaches 100°C at atmospheric
pressure as long as activity of water in the product is a,=l, which is usually the case
at the beginning of the drying process; the heat flux is then very high. At the end of
drying, for a, < 1, the boiling temperature of the product goes over 100°C and the
temperature difference between product and metallic wall decreases, leading to a
low heat flux at the end of drying.
The desorption isobar (1 bar) was obtained using Bassal’s method (Bassal &
Vasseur, 1992). This method consists of drying a product in a pure steam
atmosphere until it reaches equilibrium. This curve determines the relation between
moisture content X of a product and its boiling temperature Tpe.
To describe the desorption isobar, we used the transformed (Bassal & Vasseur,
1992) Oswin model to take into account the effect of temperature (eqn (1)).