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Lignocellulosic biomass to biodiesel 145

behavior in presence of a carbon source excess. In fact, in a medium with
abundant carbon source and limiting amount of nitrogen, if all nitrogen
source has been consumed, oleaginous microorganisms utilize the remain-
ing carbon source for the synthesis of lipids, mainly triacylglycerols
(TAGs); on the contrary, non-oleaginous microorganisms convert carbon
sources into polysaccharides (glycogen, glucans, mannans, etc.) and are
not predisposed to accumulate lipids (maximum up to 10% 20%).
Although, the biosynthetic pathway to fatty acids substantially the same,
in oleaginous yeasts, the reason of this difference is related to the produc-
tion of acetyl-CoA in the cytosol as a precursor for FAS (fatty acids syn-
thetase) and to NADPH (nicotinamide adenine dinucleotide phosphate
hydrogen) production, which is used as a reducing agent in the synthesis
of fatty acids [105]. In oleaginous yeasts, the most abundantly produced
fatty acids are oleic acid (C18:1, n-9), linoleic acid (C18:2, n-6), palmitic
acid (C16:0), and palmitoleic acid (C16:1), as well as by C18:3 or alpha-
linolenic acid that, in general, represent less than 10% of the total [106].
In the recent years, it has been proven that yeasts such as Rhodotorula
glutinis [107], Rhorosporidium toruloides [104], Trichosporon fermentans [108],
and Lipomyces starkeyi [103,109] show a potential to microbial oil produc-
tion also because they can be cultivated in simple media containing low
cost substrates [110] (Table 4.3).
TAGs and steryl-esters (SE) are the lipids most abundantly accumu-
lated in the cells during stationary growth phase. TAGs have a distribution
of acyl groups very similar to the plant oils, in particular, the central posi-
tion is occupied almost exclusively by an unsaturated group. Since these
molecules are without positive or negative charge, they cannot be part of
cell membranes. However, they are sequestered in hydrophobic particles
called lipid particles or lipid bodies and are accumulated in the form of
micro droplets. Though the lipid fraction of the cells is composed almost
entirely of TAGs, the latter are extracted together with other lipid frac-
tions (phospholipids, sterols, SE, and others) associated to the cell mem-
brane and free fatty acids related to an uncontrolled lipolysis which can
occur during the extraction process.

4.6.2 Molds
Some oleaginous molds can store up to 80% of their biomass as lipids. As
in most other microorganisms, the increase in lipid yield is related to C/N
ratio, temperature, and pH. Table 4.4 shows the values in terms of lipid