Aeration
Industrial producers of citric acid have long known that
variations in the rate of aeration can have a detrimental effect
on performance. If the aeration rate is too high, the partial
pressure of dissolved CO2 in the broth may be too low. Carbon
dioxide is important as a substrate for pyruvate carboxylase
that replenishes the supply of oxaloacetate for citrate synthase.
Sufficient CO2 is produced by the reaction catalyzed by
pyruvate decarboxylase, but excessive aeration leads to some
losses. On the contrary, high levels of CO2 in the gas are
detrimental for the final concentrations of citrate and biomass
(65). The effect of dissolved O2 has been studied in some
detail. Even short periods of reduced O2 tension cause
irreversible changes in the citric acid productivity (44).