is seen, the initial rate of hexanol formation decreasing rapidly and tending towards zero.
Nevertheless, while it was clear when using isolated enzymes that deactivation was due
to a thermal denaturation of the proteic material, it remains unclear in this system. The
decrease in initial rate at high water activity can be related to limitations of diffusion
caused by the presence of a distinct liquid phase, thus limiting mass transfers due to the
Figure 9.11 Effect of water activity on
initial reaction rate of the reduction of
hexanal catalysed by baker’s yeast ( ).
Isotherm sorption curve of the baker’s
yeast. The reaction was carried out
at 65°C with 200 mg of yeast. The
total flow passing into the bioreactor
was 680 μmoles/min. The hexanal
activity was fixed at 0.05 and the
ethanol activity at 0.2.
appearance of free water on the catalytic material as indicated on the sorption isotherm
curve of the preparation. Nevertheless, while high hydration levels resulted in dramatic
modifications at the macroscopic level to the dried cells, a reproducible measurement of
the remaining activity was impossible.
Concerning possible diffusional limitations in the system, it was also important to
check the effect of yeast quantity on the initial rate. The results presented in Figure 9.12
show
Solid/gas systems, theory and applications 279