3.3. Cultivation of heterotrophic C. protothecoides in flasks
The cell cultivation kinetics of different mediums was presented
in Fig. 3. As shown, the dry cell yield reached the maximum value
of 3.7 g L1 after 120 h culture with the substrate of glucose, while
the maximum value was 3.3 and 5.1 g L1 with ASJ (acid hydrolysate
of sweet sorghum juice) and ESJ (enzymatic hydrolysate of
sweet sorghum juice), respectively. Compared with glucose, the
biomass production of ASJ was decreased by 10.8% and that of
ESJ was increased by 37.8%. The results indicated that, during acid
hydrolysis SJ might produce some inhibitory components to C.
protothecoides that inhibit cell growth, while ESJ might contain
some beneficial components to C. protothecoides, such as protein,
amino acids or other components. Therefore, the enzymatic hydrolysis
of SJ was preferable. Lipid content in algal cells was 53.3%
with glucose feeding, and 52.5% with ESJ feeding, which was not
significantly different. Thus, the lipid yield of C. protothecoides
was 394.4 mg L1 day1 on glucose and 535.5 mg L1 day1on
ESJ. The results indicate that not only can sweet sorghum be used
efficiently for C. protothecoides cultivation, but also the lipid yield
from sweet sorghum has exceeded that from glucose.
Yeast extract is one of the most suitable nitrogen sources to promote
growth of C. protothecoides [9]. In order to confirm the optimal
yeast extract amount in culture medium, a series of
experiments using ESJ supplemented with 1–3 g L1 yeast extract
were carried out. As shown in Fig. 4, when the initial yeast extract
was 1, 2 and 3 g L1, the biomass achieved 5.30, 5.85, 6.00 g L1,
respectively. However, the lipid content was 50.2%, 49.2% and
48.9%. With the increase of yeast extract from 2 to 3 g L1, the biomass
increased slightly and the lipid content seldom changed. The
highest lipid mass 586.8 mg L1 day1 (6.00 g L1 48.9%/5 day)
was achieved with the yeast extract concentration of 3 g L1,
8.5% higher than that before adding yeast extract.