Before point A, the water is highly structured. Water molecules are located in ionic
groups and polar groups. They constitute the first hydration “layer”. Point O corresponds
to the tightly bound water (buried water).
Between A and B, the water content changes in a linear way while increasing the aw. It
corresponds to the formation of new hydration layers. The protein dissolves molecules of
water, thus forming a solid solution.
Point B corresponds to the appearance of free water. Once the water activity is higher
than the abscissa value of point B, an aqueous phase exists, with many water molecules
having no interaction with the protein. Point C corresponds to the total quantity of non
solvent water, i.e. structured by the solid sample.
Realisation of the isotherms of the different components of the system allows a more
accurate prediction concerning the behavior of an enzyme placed in solid/gas systems.
Figure 9.5 Schematic diagram of an
isotherm sorption curve. (From
Drapron R. (1985), see details in text)
Nevertheless, one must be aware that because water activity is an equilibrium parameter,
the exchanges of water between different phases have to be in a steady state.
As an example of the effect of water activity on the catalytic rate of a F. solani pisi
cutinase, the following reported experiments (Lamare and Legoy, 1995, 1997) highlight
the importance of controlling all the thermodynamic activities. In the first experiment, 1
mg of enzyme adsorbed on 15 mg of Chromosorb P was placed at 60°C or 70°C in the
reactor described in Figure 9.1. The total flow passing through the bioreactor was set to
500 or 470 μmoles/min giving a volumetric flow of 12.5 ml/min and a residence time
close to 0.4 s under both temperature conditions. The effect of water activity was studied
on transesterification between n-propanol and propionic acid methyl ester, because water
does not participate in the reaction scheme. apropionic acid methyl ester (aester) and an-propanol
(aalcohol) were set to 0.200 giving partial pressures equal to 0.100 and 0.051 at 60°C and
Multiphase bioreactor design 272