When crude glycerol was used as substrate, only Y. lipolytica
TISTR 5151 could grow well on crude glycerol and accumulated a
high lipid content of up to 64% of its dry cell mass (Fig. 2b). Other
strains also accumulated lipid higher than 40% of their dry cell
mass but they gave a relatively low amount of lipid because they
could not grow well on crude glycerol. It has been reported that
crude glycerol contains not only glycerol but also impurities such
as potassium and sodium salts, methanol and non-glycerol organic
matter (Chatzifragkou et al., 2011). These impurities could repress
the cell growth of those yeasts. On contrast, the serum latex was
more suitable for cell growth but not for lipid accumulation. The
yeasts accumulated lipid only 8–13% of their dry cell mass
(Fig. 2c). This was probably due to the low C/N ratio of the serum
latex (Table 1). It should be noted that the yeasts grew well on the
wastes with low C/N ratio (serum latex) but only accumulated high
lipid content on the wastes with high C/N ratio (palm oil mill effluent
and crude glycerol). It has been reported that at a low C/N ratio,
the carbon flux is distributed to allow cellular proliferation and this
resulted in a high number of cells obtained but with a low lipid
content (Li et al., 2006). Therefore, the addition of external carbon
source to the serum latex was attempted to increase the C/N ratio and consequently lipid production. Molasses was added as external
carbon source to the diluted serum latex at a concentration of 4%.
This increased the C/N ratio up to 327 (Fig. 2d). The yeasts grew
better and gave higher lipid production with higher lipid content
up to 14–20% of the dry cell mass.