Page 188 - Lignocellulosic Biomass to Liquid Biofuels

152 Lignocellulosic Biomass to Liquid Biofuels

culture, cellular viability increases and free fatty acids are oxidized to
unsaturated fatty acids since yeast cells need unsaturated fatty acids to con-
tinue their growth.
When lignocellulosic materials are used as feedstock, the cultivation
strategies commonly adopted are based on the separate execution of
hydrolysis and fermentation (separate hydrolysis and fermentation,
SHF). The major advantage of SHF method is to carry out hydrolysis
(40°C 50°C) and fermentation (30°C 37°C) in separate stages at their
own optimum conditions. The main drawback of SHF is the inhibition of
hydrolytic enzymes activity by released sugars. A new perspective is
offered by combining enzymatic hydrolysis of pretreated lignocelluloses
and fermentation in a single step or in a single reactor, also called SSF. A
main advantage of this method is that the sugars produced by hydrolysis
are immediately consumed by fermenting microorganisms, limiting sub-
strate inhibition of the enzymes during the hydrolysis [94]. This process
has been studied mainly for bioethanol production. Possible limitations to
the applicability of the SSF processes in single reactors are the different
requirements of hydrolytic enzymes and oleaginous yeasts in terms of
operating temperatures and pH. Therefore for application at industrial
scale, new improvements in enzyme technology (e.g., thermostable cellulases
and higher inhibitor tolerance) are required [158].
Another interesting perspective, though not adequately deepened by
the scientific community, is represented by mixed cultures of microor-
ganisms, very common in natural ecological system. When a mixed
culture is used, two or more microorganisms are synchronously culti-
vated within the same medium, so that these microorganisms can mutu-
ally exploit complementary metabolic activities to survive, grow, and
reproduce [159].
Different systems have been developed for microalgae cultivation, such
as open ponds or closed-up photobioreactors or bioreactors [160].
Currently, commercial cultivations of microalgae are mostly carried
out in open ponds, where algae are directly exposed to sun irradiation,
because of their economic feasibility and simplicity of maintenance.
However, in open systems, the light provided to microalgae growth is a
limiting factor, since it is correlated to the seasonal changes. In these sys-
tems the biomass density is not higher than 0.5 g/L [160], and the simul-
taneous presence of bacteria can be considered a competitive factor, since
they are able to consume the organic source supplied to microalgae, in
particular when lignocellulosic hydrolysates are used as feedstock. This