The experiments were carried out in two conditions. One was the
substrate level 1.0 with the water-supplement and the other was
the substrate level 0.75 without the water-supplement (Fig. 8). In
both cases, the final cordycepin productions were hardly affected
by the size of the culture bottle, that is, the expansion of the liquid
surface area resulted in an improvement of the net production
corresponding to the scale-up factor (×2.5). On the other hand,
the production rates of both cases were reduced by the expansion
of the liquid surface area. Fig. 9 shows the relation between
the cell surface density, which is defined as the dry cell weight at
the end of the culture divided by the surface area [g-DCW/cm2],
and the cordycepin productivity [(g-cordycepin)/(g-DCW d)] calculated
from the linear part of each time course in Fig. 4a (the cell
growth almost reached a maximum at the end of the linear part).
Generally, the increase in the cell surface density leads to an oxygen
deficiency inside the mycelial mat. In the previous paper about the
citric acid production by the surface culture, the productivity drastically
decreased with an increase in the cell surface density [42].
As shown in Fig. 9, the quite gradual decrease in the cordycepin