Sweetpotato - Bioenergy Crop Plants - page 739

2006). Because corn is a staple food not only in the United States but worldwide, this has led to
controversy and concerns related to the large amounts of arable land required for the crops and
the impact on grain supply. Studies have shown that sweetpotato storage root, carbohydrate, and
ethanol yields are approximately 3 times that of corn (Rodgers et al. 2007; ARS 2008). To improve
the potential of sweetpotato as a feedstock for bioethanol production, varieties with high yields and
dry matter content termed industrial sweetpotatoes (ISPs) have been developed at North Carolina
State University (NCSU), which have been shown to have ethanol yields as high as 67.8 g/L of
ethanol for flour-based fermentation and 34.9 g/L for fresh sweetpotato sugar fermentation (Santa-
Maria et al. 2006; Duvernay 2008). A limitation of using sweetpotato for bioethanol production is its
economical conversion to simple sugars before fermentation. Researchers are evaluating the potential
of transforming sweetpotato biotechnologically with a hyperthermophilic amylolitic enzyme under
control of a root-specific promoter. The genetically engineered sweetpotatoes so produced when
heated are expected to bioprocess their own starch into simple sugars (Santa-Maria et al. 2006).
Sweetpotato is a nutritious and generous food source for humans and animals as well as a raw
material for manufacturing starch, sugar, and alcohol (Kozai et al. 1997; Saiful Islam et al. 2002;
Zhang et al. 2002). Only orange flesh sweetpotato varieties are recommended for marketing in
the United States and they include Beauregard, Garnet, Hernandez, and Jewel, with Beauregard
being the no. 1 industry standard (USDA 2005). Orange flesh sweetpotatoes are especially preferred
for food because they contain significant amounts of dietary fiber, minerals, vitamins (especially
vitamins C, B 6 , and folate), and antioxidants, such as phenolic acids, anthocyanins, tocopherol, and
β-carotene (Woolfe 1992). Teow et al. (2007) reported significant variations in respect to β-carotene
content among sweetpotato genotypes, with orange flesh varieties having higher β-carotene content
than white flesh. That white flesh sweetpotatoes lack these nutritional qualities makes them more
suitable for ethanol production because they reduce the food fuel competition particularly in the
United States (Eggen 2006; Service 2007). Moreover, orange flesh clones generally have lower dry
matter and hence extractable starch content (Barbet et al. 1999).
31.2.1 S tarch
Starch offers a high yielding ethanol resource (Nigam and Singh 1995). Starch is the main storage
carbohydrate of plants. It is deposited as semicrystalline granules in storage tissues (grains, tubers, and
roots) and it also occurs to a lesser extent in most vegetative tissues of plants (Copeland et al. 2009). It
is a polysaccharide of α-d-glucose and consists of the polymers amylose and amylopectin. Amylose is
essentially linear, and the glucose units are bound together with α-(1,4)-linkages and very few α-(1,6)-
bonds. Amylopectin is larger than amylose, is highly branched, and has α-(1,6)-bonds in the branching
points in addition to the α-(1,4)-linkages in the linear chains (Stevneb et al. 2006). Amylose has a
molecular weight range of approximately 10 5 -10 6 , corresponding to a degree of polymerization (DP)
of 1000-10,000 glucose units. The molecular weight of amylopectin is approximately 10 8 and it has
a DP that may exceed 1 million. Most starches contain 60-90% amylopectin, although high-amylose
starches with as little as 30% amylopectin and waxy starches with 100% amylopectin are known.
Approximately 5% of amylopectin glucoses are in α-(1,6) linkages, giving it a highly branched, tree-
like structure and a complex molecular structure architecture that can vary substantially between
different starches with regard to placement and length of branches (Copeland et al. 2009). The moisture
content of native starch granules is usually approximately 10%. Amylose and amylopectin make up
98-99% of the dry weight of native granules, with the remainder including small amounts of lipids,
minerals, and phosphorus in the form of phosphates esterified to glucose hydroxyls. The proportion of
amylose and amylopectin depends on the source of the starch (Allen et al. 1997).
Starch granules range in size (from 1 to 100 μm diameter) and shape (polygonal, spherical,
and lenticular) and can vary greatly with regard to content, structure, and organization of the
amylose and amylopectin molecules; the branching architecture of amylopectin; and the degree of
crystallinity (Lindeboom et al. 2004) (Figure 31.1).