productivity was observed. This difference might be due to difference
in the oxygen uptake rate which is closely related to the
oxygen deficiency in the mycelial mat, because the production
rate of citric acid by Aspergillus niger (ca 3 g/(L d)) is much higher
than that of cordycepin by C. militaris. The efficient supply of the
substrates from the liquid medium might be potentially more influential
on the cordycepin production than that of oxygen from the
upper surface of the mycelia in the liquid surface culture of G81-3.
Mao et al. also reported that the DO shift to a lower level during
the latter stage of the submerged culture was effective in obtaining
a higher cordycepin production [43]. Therefore, the best way
of scale-up to have the same efficiency as the small scale might be
fixing not the medium depth but other factors like the ratio of the
perimeter to depth. This is the subject for a further study.