0.143 and 0.064 atm at 70°C. The effect of water activity was monitored by scanning
from 0 to 0.8 using plateaus of 2 h at a given aw value. The same enzymatic preparation
was used for the complete run due to its excellent stability. The partial pressures of water
ranged from 0 to 0.160 atm at 60°C and form 0 to 0.250 at 70°C.
As depicted in Figure 9.6, it is clear that the correct parameter to use with a gas/solid
process is the thermodynamic activity of water and not its volumetric concentration or its
partial pressure. If the results are plotted against the partial pressure of water, the two
curves obtained at 60 and 70°C exhibit different optima and some misunderstanding can
arise from using this parameter.
When the two sets of data are plotted against water activity, the two curves obtained at
60 and 70°C are well superimposed, and present the same optimum at aw=0.6.
Furthermore, the catalytic activities measured at 60 and 70°C are very close, indicating
also the necessity to use the thermodynamic activity of the other compounds present in
the system (i.e. substrates).
This was confirmed using thermodynamic activities when studying the kinetics of two
reactions, transesterification and hydrolysis, and the same results were observed (Lamare
and Legoy, 1997).
Moreover, kinetic studies showed that an interaction exists between all the
components of the gaseous phase, since organic compounds were found to solvate,
thereby enhancing catalytic activity when using restricted water conditions.
Since it was possible to carry out reactions involving lipolytic enzymes at the solid/gas
interface, it was also important to check the stability of enzymatic preparations in systems
requiring relatively high temperatures for non- thermostable enzymes.
For these experiments the reactors used previously for 24 hours at the different water
activities were reused at a fixed water activity corresponding to the maximal hydration
state before appearance of free water (see isotherm on Figure 9.7).
The experimental conditions were as follows: an-propanol=amethyl propionate=0.2. Fixed aw,
total molar flow set at 500 μmoles/min, and a temperature of 70°C.
The activity was measured for 4 hrs. Figure 9.7 shows that 80% of the maximal
activity observed during the first run at aw=0.6 could be restored in all cases, once the
reactor was used between aw=0 and aw=0.6. Figure 9.8 compares the thermal stability of
the solid/gas and liquid system of a C. rugosa lipase for a transesterification reaction.
Whilst the thermodynamic activities are controlled, biocatalyst stability in solid/gas
systems is excellent.
Hydrolysis (Lamare and Legoy, 1997) and esterification reactions were also studied,
and the competitiveness of solid/gas catalysis led to the development of an industrial
platform for the production of natural esters using a commercial enzymatic preparation.
Results of this are presented in the conclusion.
Solid/gas systems, theory and applications 273