Environmental Engineering Reference
In-Depth Information
the demand and petroleum was costly. Sugar-based bioethanol allowed Brazil to achieve energy
self-sufficiency (Gressel 2008).
Sucrose produced by sugarbeet is a disaccharide molecule made up of glucose and fructose.
Monosaccharides like glucose and fructose serve as base material for ethanol production. Therefore,
the sucrose molecule first needs to be broken down into its component sugars. This is done by
enzymatic hydrolysis of sucrose catalyzed by the enzyme invertase, which converts sucrose into
glucose and fructose:
C
12
H
22
O
11
→ C
6
H
12
O
6
+ C
6
H
12
O
6
(29.1)
(Sucrose)
(Glucose) (Fructose)
Enzymatic hydrolysis is followed by the fermentation process for the production of bioethanol.
Commercial yeast such as
Saccharomyces cerevisiae
converts glucose into ethanol under anaerobic
conditions. The recently introduced strain
S. cerevisiae
ATCC 36859 has proven to be an efficient
microbe in enhancing ethanol production from sugarbeet molasses (Atiyeh and Duvnjak 2003a).
C
6
H
12
O
6
→ 2C
2
H
5
OH + 2CO
2
(29.2)
(Glucose)
(Ethanol)
Many other microbes are also capable of ethanol formation. Many bacteria such as
Enterobacteriaceas, Spirochaeta,
and
Bacteroides
spp. and yeasts including
S. cerevisiae,
S. uvarum, Schizosaccharomyces pombe,
and
Kluyueromyces
sp. metabolize glucose under
anaerobic conditions by the Embden-Meyerhof pathway. In this pathway, one molecule of glucose
yields two molecules of pyruvate, which are then decarboxylated to acetaldehyde and then reduced
to ethanol. The ethanol thus produced is low in concentration (8-12%). A distillation process is used
to increase the concentration of ethanol to the required levels. Modified techniques are used in this
process to recover a large portion of ethanol and to prevent ethanol emission into the atmosphere or
ethanol losses with water (Krylova et al. 2008).
All of the intermediates produced during sugarbeet processing can be used as raw material for
ethanol production. All of these have advantages and limitations. Direct use of beet or pulp as raw
material is not very efficient (Turquois et al. 1999) because there is a slow release of sugars from the
pulp into fermented solution and the long-term storage of beet leads to a loss of sugar due to enzyme
action (Berghall 1997). Solids from raw juice contain 85-90% sugars and 10-15% nonsugar;
therefore, raw juice can be used as raw material for fermentation after slight pH adjustments. Easy
decomposition by microbes and low storability limits the use of raw juice for direct fermentation.
However, thin juice is very suitable for ethanol production (Figure 29.1). Molasses, produced during
the crystallization process, is a traditional raw material for distilleries, and more than 90% of
ethanol is produced from this raw material.
29.5 suGarBeet ImProvement
Sugarbeet continues to be a major crop meeting our daily needs for sugar consumption; thus, major
genetic studies have been initiated in recent years to improve the quality and productivity. Other
objectives include selection for resistance against insects, pests, and diseases. Crop modeling is
another area that should be explored to increase productivity. A specific areawise crop model should
be developed that will help us to decide on agronomic practices and harvesting schedules. Sugarbeet
can be grown as an intercrop between the main crops.
Thus, increasing the productivity of sugarbeet is of paramount importance and for it to become
a successful biofuel crop the need of the hour is to identify and exploit genetic potential of the
crop through better selection of traits (Doney and Theurer 1984; Kuhn 1998) that contribute to the
