In the early phase of yeast cultivation, the glycerol
concentration decreased rapidly and was almost zero after
72 h. A rapid decrease of the DO was also observed during
24 h (from over 70 % it dropped to the level less than 20 %),
and it gradually continuously decreased to nearly 0 % during
24–60 h. Since oxygen requirements depend on specific
growth rate, oxygen uptake rate would be high during
growth phase and low during lipid accumulation phase.
An increase of the DO after 72 h would be resulted from
the exhaustion of the glycerol. A cascade control of DO by
increasing the agitation speed and aeration rate was also
attempted (Fig. 4d). However, this strategy could not recover
the DO level. The maximum biomass of 11.3 g/L and
lipid production of 5.01 g/L with a lipid content of 44.3 %
were achieved at the optimal condition of controlled pH of
6.0 and an aeration rate at 1 vvm.