BIOCHEMICAL CONVERSION: BIOFUELS BY INDUSTRIAL FERMENTATION - Biomass as a Sustainable Energy Source for the Future - page 422

substrate and is the rate-limiting step. In the secondary hydrolysis, the cellobiose is
further hydrolyzed by -glucosidases to glucose. An important drawback of cellulase

is that it suffers from end-product (cellobiose and glucose) inhibition. This results in
reduction of the hydrolysis rate in the course of the reaction unless the product is
removed as soon as it is being formed. As source for these enzymes, the same fungi
used for biological pretreatment (e.g., Trichoderma reesei ) are being used.
Hemicellulose can also be enzymatically hydrolyzed. Being a diverse group of
heterogeneous polymers with various side groups, the complete hydrolysis of
hemicellulose requires a more complex system of enzymes including xylanase,
β

β

-xylosidase, and several other complementary enzymes, such as acetylxylan
esterase,

-galactosidase, and ferulic and/or p-coumaric acid
esterase. While the number of enzymes required for hemicellulose hydrolysis is much
greater than for cellulose hydrolysis, accessibility to the substrate is easier as xylan,
the main constituent of hemicellulose, does not form tight crystalline structures
(unlike cellulose).
The enzymatic hydrolysis of both cellulose and hemicellulose is confronted with a
number of obstacles:

α -arabinofuranosidase, α

￿ Price : Although enzyme prices have decreased due to intensive research by, e.g.,

Novozymes, Genencor (acquired by DuPont), and DSM, enzyme loading should
be minimized in order to reduce production costs. This, however, increases the
time needed to complete hydrolysis. An alternative is to recycle the enzymes,
since much of them remain active. Many methods, with or without enzyme
immobilization, are currently being researched.

￿ Product inhibition : As mentioned previously, this results in lower performance

and limits the use of high substrate concentrations. Alternative process
configurations, in which the product is continuously removed, are being
currently evaluated (see also Section 13.3.5).

￿ Lignin interference : Lignin shields the cellulose chains and adsorbs the enzymes,

resulting in decreased efficiency and hampering enzyme recovery.

An alternative to the use of commercial enzymes is their production on the target
lignocellulosic material. It has been reported that these enzyme preparations perform
better than standard commercial enzyme preparations produced on substrates such as
purified cellulose. This approach could beneficially be employed in biorefineries to
produce enzymes on-site. Moreover, on-site enzyme production could reduce enzyme
costs due to less need for purification and stabilization of enzyme preparations.

13.3.4 Fermentation of C 5 and C 6 Sugars into Ethanol
The hydrolysis of lignocellulosic feedstocks results in a mixture of hexose (C 6 ) and
pentose (C 5 ) carbohydrates. Table 13.9 shows typical compositions for a few feed-
stocks. Glucose and xylose are the most abundant sugars. There are many naturally
occurring microorganisms that can ferment glucose into ethanol, S. cerevisiae being