196 Introduction to Renewable Biomaterials
6.4.4 Fermentation
The monosaccharides produced by hydrolysis are used in the fermentation process
to produce ethanol. A new method known as simultaneous saccharification and
fermentation (SSF) is done at one stage, and great effort has been made to perform
saccharification and fermentation simultaneously at one stage (Ballesteroset al., 2004).
A part of saccharification is done to hydrolyze cellulose to monosaccharides. A new
method of SSF was studied. In this method, first, 6-carbon monosaccharides like glucose
are fermented and then 5-carbon monosaccharides such as pentose and arabinose are
fermented in the same way by means of enzymes. This procedure needs to be considered
closely because the pentose sugars fermentation is done hardly.
6.4.5 Formation of Fermentation Inhibitors
Pretreatment of lignocellulosic materials leads to the formation of different compounds
with inhibitory effect on the fermentation process. These inhibitors have toxic effects
on the fermenting microorganisms and therefore lead to decreased ethanol yield and
production. The toxicity level of these materials depends on fermentation variables such
as physiological conditions of microorganisms, dissolved oxygen concentration, and the
pH of the medium. Toxic compounds can affect fermenting organisms to a point beyond
which the efficient utilization of sugars is reduced thereby leading to decreased product
formation.
Furthermore, the fermenting organisms may be resistant to inhibitors slightly or may
become gradually adapted to their presence. Conversely, the best approach is to prevent
the formation of inhibitors as much as possible through (adaptation of) the pretreatment
process conditions or other measures.
The main types of inhibitors are summarized in Table 6.4 and discussed here. The
effect of inhibitors is higher when they are present together due to their synergistic effect
(Mussatto and Roberto, 2004).
6.4.6 Sugars Degradation Products
During hydrolysis, pentose sugars can degrade to a toxic compound, namely furfural,
with its toxicity depending on its concentration in the fermentation media.
Table 6.4Main types of fermentation inhibitors and their chemical structures.
Phenols
OH
Furfural
O
O
H
5-Hydroxymethylfurfural (HMF)
O
HO O
Acetic acid
O
O - H
Levulinic acid
O
OH
H 3 C
Formic acid
H O H
C
O
The relative toxicity of the different inhibitors for ethanol fermentation can be
summarized as phenolic compounds>furfural>HMF>acetic acid>extractives.
