3.3.3. Verification of optimized culture conditions
The optimal conditions determined from the central
composite design were further verified by comparing the
experimental data obtained at these conditions with that
predicted from central composite design (Eq. (1) and Table 9).
Two verification experiments were performed: one was at the
optimal culture condition for cell dry weight; the other was that
for DHAyield (Table 11). It was found that the cell dry weight
prediction had around 11–14% deviation, while the deviation of
DHA yield between the experimental and predicted data were
less than 5% (Table 11), suggesting the experimental designs
used in this work are effective for optimizing algal culture
conditions for producing DHA from biodiesel-derived crude
glycerol. Based on the experimental data of DW and DHA
yield, the biomass and DHA productivity at the two optimal
conditions were calculated (Table 11).
In summary, the above results demonstrate that it is feasible
to use biodiesel-derived crude glycerol for algal DHA
production. A similar growth rate and cell density was obtained
when using glucose and glycerol. For a typical heterotrophic
algal culture, however, glucose is usually expected as an easier
consumed carbon source versus other substrates, and oxygen is
the main factor determining the yield of culture. To test if the
similar results from glucose-based and glycerol-based cultures
were caused by oxygen-limitation; a further experiment was
performed in flask cultures by loading the flasks with different
volumes of medium, and comparing the algal growth. The
hypothesis is that if oxygen in the flask is the limiting factor, the