Continuous culture experiment: effects of increasing
concentrations of glycerol in the feeding medium
Little research has looked at the synthesis of biolipids
from sugars or renewable feedstock by nitrogen-limited
continuous cultures [27–29]. Papanikolaou and Aggelis
conducted the only study to date to examine biolipid
synthesis by Y. lipolytica under continuous culture conditions
using glycerol as the sole substrate [15].
In this experiment, we used a stepwise continuous
fed-batch (SCFB) approach to test the effect of glycerol
concentration on lipid production. The cultures started
as batch cultures that were grown with molasses to produce
biomass; once the sugar supply was exhausted,
continuous culturing was initiated. Glycerol was used
as feed, and the dilution rate was 0.01 h−1
. The glycerol
concentration in the feeding medium was increased
from 100 to 450 g/L in steps that took place every 100 h
(Figure 4). It has been shown that the dilution rate and
had the highest lipid production, these conditions
were used in a second experiment, in which a higher
airflow rate of 3.5 L/min (the high-oxygen condition,
“Oxy-high”) was utilized. As a consequence, the lag phase
lengthened, sucrose hydrolysis began later—after 30 h
(Figure 3)—and lipid accumulation was limited. Citric
acid production exceeded 40 g/L, and the compound was
not reconsumed (Figure 3). The biomass yield was 59 g/L,
and final lipid content was 7.7 g/L, which corresponds to
a YL/gly of 0.077 g/g (Table 1). These results indicate that
increasing the oxygenation rate did not improve yeast
growth and lipid production.
The fed-batch experiments revealed that the highest
QL, YL/gly, and YL/X values were obtained using a lowdensity
inoculum and unregulated oxygenation. Consequently,
these conditions were used in the continuous
culture pilot experiment.