Page 120 - Lignocellulosic Biomass to Liquid Biofuels

94 Lignocellulosic Biomass to Liquid Biofuels

hydrolysis [222]. In particular, many strains of K. marxianus appear to be
very promising; indeed, these strains are able to (1) grow well at tempera-
tures of 45°C 52°C; (2) efficiently produce ethanol at temperatures of
38°C 45°C; and (3) use several sugar substrates, such as arabinose, galac-
tose, mannose, xylose [53].
The difficulty in implementing continuous fermentation, by recirculat-
ing and reusing the yeast, is a further obstacle of the SSF process. Indeed,
owing to the presence of the lignin residues from the hydrolysis, it’s prov-
ing hard to separate yeast from these solid residues after fermentation
[220]. Consequently, a yield loss in an SSF process represents necessarily
weakness of this method [221]. High-solid loadings are generally needed
to achieve high ethanol percentages in the fermentation broths: solid load-
ings of pretreated biomass, greater than 30% (w/w), may be required to
obtain an ethanol concentration of 4 5 wt.% that is considered a thresh-
old level for a sustainable distillation.
The SSF process is usually carried out in a batch mode; however, the
high content of solids in a bioreactor could lead to a deteriorating enzy-
matic activity and a viscosity increase, hampering the homogeneous and
effective distribution of the enzymes in the bioreactor [220]. It was
reported that the difficulty of attaining a good ethanol yields with a high
loading of pretreated material contained a WIS (water insoluble solid)
fraction around of 10% [211]. This problem could be overcome by a fed-
batch SSF, in which a gradual or stepwise addition of hydrolyzate allows a
continuous conversion of inhibitors and a gradual hydrolysis of added
fibers [223].
The aim of a fermentation process is to achieve a complete assimilation
by fermenting microorganisms of all sugars formerly released from ligno-
cellulose biomass during pretreatment and hydrolysis steps. The cofermen-
tation of hexoses and pentoses could be carried out by using of mixtures
cultures of yeasts capable of assimilating both C6 and C5 sugars.
However, there are significant differences with respect to temperature tol-
erance between the hexose-utilizing microorganisms and pentose utilizing
microorganisms: usually, the first one grows faster than the latter.
Consequently, the conversion efficiency of hexoses to ethanol is higher
than that of pentoses [224].
The SSF process required two bioreactors and two biomass produc-
tions setup to convert C6 and C5 sugars into ethanol: in SSF bioreactor,
only the hexoses fermentation is performed; pentose sugars can be fer-
mented in another bioreactor by using different microorganisms. The