To identify a critical balance of different substrates and cultivation
modes that achieve a high-density cell culture for microbial lipid
fermentation, various parameters were optimized at flask level.
The effect of initial pH, ranging from 5.0 to 7.0, on yeast biomass
production was studied. A maximum OD of 16.86 (DCW,
7.2 g L1) was obtained when the fermentation was carried out at
pH 5.5 (data not shown). Studies on effect of inoculum age on yeast
biomass showed that a 24 h old inoculum gave maximum OD of
19.6 with DCW of 8.1 g L1 (data not shown). Among the carbon
sources tested, glycerol at 1% (v/v) was found to be the best carbon
source supporting yeast growth (Fig. 2) resulting in an OD of 25.35
and DCW of 8.3 g L1, though increasing the glycerol concentration
to 5% (v/v) did not significantly affect the biomass production (OD
of 23 and DCW of 7 g L1) (data not shown). Since the effect was
same irrespective of whether commercial or crude glycerol was
used, all further experiments were carried out with crude glycerol.
Carbon source plays a role in both biomass and lipid production
and glycerol in pure as well as crude form has been shown to be
utilized effectively by another well known oleaginous yeast C.
curvatus [18,27,28]. Among the nitrogen sources tested, CSL at 2%
(v/v) was found to be the best nitrogen source (Fig. 3) where the
OD was boosted to 31.92 with a DCW of 12 g L1. Incubation at
28 C gave maximum OD of 38.47 with DCW of 13.3 g L1 while
a inoculum size of 20% (v/v) gave a highest OD of 41.37 and
DCW of 15.5 g L1. The final set of optimized conditions at flask level
included growing the yeast in a medium having glycerol, 1% (v/
v); CSL, 2% (v/v) at a pH of 5.5, agitation of 200 rpm and a temperature
of 28 C. A 20% (v/v) of 24 h old inoculum was used to start
the fermentation.