Environmental Engineering Reference
In-Depth Information
Sugar degradation products: After hydrolysis and at high temperature and
pressure, pentose and hexose sugars may dehydrate to furfural and
5-hydroxymethylfurfural (HMF), respectively. HMF, in turn, can be further
degraded to levulinic acid.
Acetic acid: This acid is derived from the acetyl groups in hemicellulose and is
inherently formed during hemicellulose hydrolysis.
Lignin products: A variety of compounds (e.g., aromatic, polyaromatic,
phenolic, and aldehydic) may be released from the lignin fraction due to long
exposure to high temperatures. At temperatures below 180
C, lignin degradation
is negligible, provided that no strong acid or alkaline conditions prevail.
In general, the relative toxicity of these inhibitors is phenolics > furfural > HMF >
acetic acid. However, these compounds also have synergistic effects. The yeast
S. cerevisiae
, e.g., grows in the presence of either furfural or HMF, but not in a mixture
of both.
In order to deal with these inhibitors, the following approaches are being followed:
Removal of the inhibitors before fermentation, also known as detoxification step.
The most widely used methods include overliming (i.e., treatment with lime,
typically at a pH of 9
60
C), evaporation, steam
−
11 and a temperature of 50
−
stripping, and absorption.
Optimization of the pretreatment method and process conditions for minimizing
the generation of inhibitors while maximizing the production of fermentable
sugars. Since such an optimum also depends on the feedstock, a comparison
is usually based on the pretreatment severity (temperature, residence time, and
pH) and the quality of the resulting hydrolysate (ratio of fermentable sugars
to inhibitors).
Strain engineering for higher tolerance to inhibitory compounds. The focus is
currently mainly on acetic acid, as this compound is inherently formed during
hemicellulose hydrolysis.
13.3.3 Enzymatic Hydrolysis
As discussed previously, many pretreatment methods partially hydrolyze the
hemicellulose, but only a few result in cellulose hydrolysis. Hence, an additional step
is required for obtaining the fermentable sugars. Enzymatic hydrolysis is preferred,
since enzymes have high substrate specificity and require only mild operating
conditions compared to chemical methods such as strong acid hydrolysis (e.g.,
a pH of 4.8 and a temperature of 45
50
C for cellulase).
The hydrolysis of cellulose requires cellulases, a group of enzymes that act
synergistically in two steps: primary hydrolysis and secondary hydrolysis. In the
primary hydrolysis, endoglucanases cleave the cellulose to form oligosaccharides,
and subsequently, cellobiohydrolases (also known as exoglucanases) release the
water-soluble dimer cellobiose. Primary hydrolysis occurs on the surface of the solid
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