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Lignocellulosic biomass to biodiesel 143

(SSF). Unfortunately, hydrolysis and fermentation require different oper-
ating conditions: the optimal temperature for the growth of yeasts is 30°
C 37°C, whereas the activity of hydrolytic enzymes is maximum at 40°
C 50°C [92 94].
SSF of lignocellulosic biomass is commonly studied for bioethanol
production [95,96]. SSF can potentially be developed to obtain oleaginous
microorganisms, able to produce intracellular lipids, though this field is
still substantially unexplored [97]. The reason is attributable to the lower
metabolic activity of oleaginous microorganisms and to the difficulty of
oxygen supply due to the increase of culture medium viscosity especially
in the case of high concentrations of processed biomass [33,97,98].

4.6 Oleaginous strains and their productivity
The oleaginous microorganisms have the ability to produce and accumu-
late a large amount of lipids if compared with their dry mass [99]. They
consist of different families, such as microalgae, bacillus, and fungi (molds
and yeasts) [100].

4.6.1 Yeasts

The increasing interest to the applications directly connected to the lipid
production by oleaginous fermentation, in the nutraceutical and pharma-
ceutical fields, as well as the possibility to use them also to produce bio-
diesel, has created scientific appeal aimed to obtain alternative ways to
those offered by oleaginous yeasts.
In comparison to oleaginous plants, oleaginous yeasts are not influ-
enced by climate variability and can grow in the presence of different car-
bon sources, for example, hexose and pentose sugars with high growth
rates [101]. Generally, they have the ability to accumulate microbial oils,
commonly called single cell oils (SCOs). These oils are usually more than
20% 25% of their total dry weight [98,102] reaching 65% of their dry
weight in specific growth conditions [103].
When cultured at low concentrations of nitrogen, the oleaginous yeasts
have the ability to trigger a cascade of reactions leading to intermediate
compounds formation such as acetyl-CoA [104], enabling oil accumulation
mechanisms related to the tricarboxylic acid cycle. In eukaryotic microor-
ganisms, these mechanisms take place in the mitochondria (Fig. 4.5) [32].
Though oleaginous and non-oleaginous yeasts share the same biosyn-
thetic pathways, there is a fundamental difference concerning their