Figure 9.10 Initial reaction rate of the
reduction of hexanal catalysed by
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, the ethanol
activity at 0.1 and the water activity at
0.57.
the water activity at 0.57 (93 μmoles/min). The initial rate of hexanol formation increases
up to a maximum rate equal to 8 μmoles/min.g of yeast, corresponding to 32% hexanal
conversion. The steady state is obtained after S hours. Since hexanal conversion is
dependent on both the amount of available NADH, H+ and its turnover when regenerated
using ethanol, the ethanol activity was fixed to 0 (0 μmoles/min) after 20 hours of
reaction. Consequently, the regeneration of NADH, H+ becomes impossible and the
initial rate of hexanol formation decreases rapidly and tends towards zero. If ethanol is
added, the conversion of hexanal is stimulated and the initial rate of hexanol formation
increases again. This stimulation is the result of the conversion of ethanol by ADH
leading to NADH, H+ production. Note also that the stability of ADH in this system is
important, since, after addition of ethanol in the inlet gas, the restored reaction rate is
almost equal to what was observed at the beginning under steady state conditions.
Water activity was also found to play a very important role. In a second experiment
the same conditions as before were applied, except that the catalytic activity was assayed
under different hydration conditions. As shown in Figure 9.11, the initial rate of hexanol
formation is strongly influenced by water activity. A critical water activity of 0.4 is
necessary for the yeast to become active. Then, the initial rate of bio-transformation
increases with water activity to reach maximum initial rate equal to 12 μmoles/min.g of
yeast, obtained for a water activity near to 0.7. For higher water activity a dramatic effect
Multiphase bioreactor design 278