Biomedical Engineering Reference
In-Depth Information
2010). The United States, Nigeria, India and Sudan are the largest sorghum growers. Annual
sorghum production in the United States was about 12 million tonnes in 2008.
In the 1960s, there were concerns about the low level of genetic diversity in the existing
grain sorghum cultivars due to breeding practices used at the time. The Sorghum Conversion
Program, which was created as a US Department of Agriculture (USDA) and Texas A&M
University partnership, introduced genetic variation from tropical species into cultivated
sorghum lines grown in the United States to alleviate the problem (Vandenbrink et al .,
2010). As a result, 701 sorghum varieties were released by the Sorghum Conservation
Program in 2004. The S. bicolor genome was sequenced in 2009 (Paterson et al ., 2009 ).
This achievement could lead to further improvements in sorghum chemical and physical
properties, including its nutritional composition, starch and cellulose hydrolysis yields and
fermentation efficiency.
In terms of their use sorghum is classified into three groups: grain, grass/forage and
sweet sorghum (Monk et al ., 1984). Grain sorghums are grown for their seeds which are rich
in starch. Grassy sorghums grow fast and can reach ten feet (about 3.1 m) in height, generating
significant amount of biomass. Sweet sorghums have sweet juicy stems and produce the
highest amount of biomass among the sorghum varieties. Over three decades ago it was
proposed that development of high energy sorghum (HES) hybrids that would combine
characteristics of both sweet and grain sorghums could increase sorghum production
potential (Monk et al ., 1984). Eventually in 2008 two sorghum varieties, BTx623, a grain
sorghum, and RIO, a sweet sorghum, were crossed to create recombinant inbred lines
(RILS) (Murray et al ., 2008a, 2008b). RILS have been used to genetically map quantitative
traits, such as wet and dry yields, and the amounts of structural carbohydrates, such as
cellulose, lignin and hemicelluloses (Vandenbrink et al ., 2010). It is expected that new
sorghum lines will lead to the discovery and introduction of specific genes for traits such as
lower lignin and higher starch content, resulting in higher biomass hydrolysis yields during
pretreatment of sorghum for bioproduct manufacturing.
1.2.4.2
Chemical composition
The structure of sorghum grain is similar to that of other cereal grains. The endosperm, germ
and pericarp account about 85%, 9.55% and 6.5% of the whole grain, respectively. Like corn,
sorghum has a large germ relative to its endosperm size. Oil content in sorghum (3.4%) is
higher than that in wheat kernel (2.2%) (Kent and Evers, 1994). The endosperm is rich in
starch (higher than 80%) and relatively poor in protein (about 10%) and lipid (less than 1%
lipid) (Serna-Saldivar and Rooney, 1995). The prolamins, storage proteins in sorghum, also
referred to as kafirins, account for about 80% of the total grain proteins (Taylor and Dewar,
2001). Kafirins are low in essential amino acids, specifically in lysine which comprise only
0.2% of the total amino acids in kafirin. The World Health Organization (WHO) recommendation
for lysine content in sorghum protein is 5.5 mg lysine/g protein (FAO, 1973).
Cell wall components of sorghum, lignin, cellulose, protein and hemicellulose, vary
significantly by variety (Corredor et al ., 2009). Endosperm cell wall and non-starch
polysaccharides are rich in water-insoluble glucuronoarabinoxylans that affect the end use
of this crop (Verbruggen et al ., 1995). Sweet sorghum contains about equal quantities of
soluble (glucose and sucrose) and insoluble carbohydrates (cellulose and hemicellulose)
(Monk et al ., 1984). Sweet sorghum produces both grain and large amounts of sugar-rich
stems. The sugar in stems is mainly comprised of sucrose (up to 55%, dry basis), fructose
and glucose (Billa et al ., 1997 ).
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