High-density inocula were also used under both regulated
and unregulated oxygenation conditions. As shown
in Figure 1, the lag phase became shorter and culture
duration decreased significantly; the latter was 70 and
66 h under regulated and unregulated conditions, respectively.
When oxygenation was unregulated, the biomass
yield was 58 g/L; it reached 70 g/L when oxygenation was
regulated (Figure 1a). Citric acid production was similar
across the two conditions (19 and 23 g/L, respectively);
however, it was only reconsumed when DO was regulated
(Figure 1b). In both cases, compared to the unregulated/low-density
condition, QL was low, as were YL/gly
and YL/X (Table 1). Furthermore, in both conditions,
JMY4086 formed short true mycelia and pseudomycelia
(Figure 2c, d).
Because the fed-batch culture initiated with a lowdensity
inoculum and subject to unregulated oxygenation
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.