Environmental Engineering Reference
In-Depth Information
Reaction conditions for cellulose hydrolysis are frequently in the ranges of 40-50°C, acetate or
citrate buffer pH 4.8-5, 15-60 FPU/g-glucan plus excess β glucosidase, 1-20 wt% solids, 24- to
72-h reaction time.
8.4.3.2 hemicellulases
Hemicellulose degrading enzymes act specifically on the hemicellulose. Because hemicellulose
is a branched heteropolymer, many different hemicellulases are required for efficient enzymatic
hydrolysis. Hemicellulases include the xylanases (E.C. 3.2.1.8), β-mannanases (EC 3.2.1.78), α-l-
arabinofuranosidases (EC 3.2.1.55) and α-l-arabinanases (EC 3.2.1.99), α-d-glucuronidases (E.C.
3.2.1.139), β-xylosidases (EC 3.2.1.37), xylan esterases (EC 3.1.1.72), and eruloyl esterases (EC
3.1.1.73) (Shallom and Shoham 2003).
8.4.4
f
ErmEntation
of
B
iomaSS
-d
ErivEd
S
ugarS
to
B
iofuElS
8.4.4.1 overview of Fermentation-Based conversions
Fermentation refers to the anaerobic conversion by microorganisms of substrates into products such
as acetone, 2-propanol, butanol, hydrogen, methanol, and hydrocarbons chains. Amongst the potential
products that could be obtained from fermentations, this chapter focuses mainly on ethanol obtained
from five- and six-carbon sugars. Traditional sources of sugars for ethanol production such as corn,
sugarcane, and sugarbeets are mostly composed of disaccharides or polysaccharides of six-carbon
sugars, but the most abundant feedstocks for ethanol production, lingocellulosic materials, contain six-
carbon sugars as well as five-carbon sugars in considerable amounts. The nature of having five- and
six-carbon sugars in these feedstocks raises the need of acquiring or developing one or more organisms,
or a mixture thereof, to release and ferment these sugars sequentially or simultaneously. The main
source of six-carbon sugars, specifically glucose, is cellulose (Zaldivar et al. 2001). Hemicellulose is
the source of five-carbon sugars such as arabinose and xylose as well some six-carbon sugars, such as
glucose, mannose, and galactose (Badal 2003). Glucose, the main component of cellulose, and xylose,
the main component of hemicellulose, are the most abundant carbohydrates on Earth (Zaldivar et al.
2001; Badawl 2003). As stated previously, these carbohydrates can be fermented into several products.
The biochemical reactions that summarize their biosynthesis are shown in Table 8.8. Biochemical
fermentation of five- and six-carbon sugars is presented in greater detail in the next sections.
8.4.4.2 Fermentation of six-carbon sugars
The preferred biochemical substrate for obtaining energy in the cell is glucose. When glucose
is used as a source of energy under anaerobic conditions, it undergoes a process called glycolysis.
Glycolysis consists of a series of reactions that mainly produce adenosine triphosphate (ATP), which
is the carrier and energy source of all biochemical processes inside of cells. Glucose fermentation or
glycolysis consists in a series of ten reactions that break down this carbohydrate into two molecules
of pyruvate, which could undergo subsequent reactions to finally obtain acetone, 2-propanol, ethanol
(Hahn-Hägerdal et al. 1994; Ostergaard et al. 2000; Dien et al. 2003), and others compounds. Once
that pyruvate is produced, it could undergo a series of transformations to acetone, 2-propanol, and
ethanol. To produce ethanol, pyruvate is first decarboxylated by the enzyme pyruvate decarboxylase
in the presence of magnesium(II) and thiamine pyrophosphate (TPP); as a product of this process,
acetaldehyde (2C) is produced. Acetaldehyde is further reduced to ethanol by means of the enzyme
alcohol dehydrogenase in the presence of NADH and H
+
. The overall process of ethanol production
is given by the following reaction with a theoretical yield of 0.51 g of ethanol per gram of glucose
(Nelson and Cox 2000):
Glucose → 2[Ethanol] + 2CO
2
+ 2H
2
O (8.4)
During the treatment of biomass for biofuel production, the six-carbon sugars mannose and
glucose are also produced. Also the feedstock for biofuels production could contain fructose. All of
