The effect of aeration on the HA fermentation is shown in
Fig. 1.With an aeration rate of 1 vvm, the dissolved oxygen (DO)
level fell off continuously to 25% air saturation at the end of the
exponential growth phase. Cell concentration achieved 2.8 g/L
with a maximum specific growth rate (μmax) of 0.56 h−1 for both
aerobic and anaerobic cultures; aeration seems to have no effect
on the cell growth. Nevertheless, aeration could increase the
HA yield significantly, from 1.5 to 2.3 g/L. Fig. 1 reveals three
clues to the fermentation. First, the present strain is an aerotolerant
anaerobe, that is, it always gains energy by an anaerobic
metabolism and grows as well under aerobic environment. Second,
HA is a growth-associated metabolite, with a 45 min-lag
correlated to cell growth; and the presence of oxygen stimulates
HAsynthesis. Third, the cells consume dissolved oxygen during
the exponential growth phase.