Biomedical Engineering Reference
In-Depth Information
(190 bu/acre) by 2030 from the current 940 kg/ha (150 bu/acre), assuming that current 1%
annual genetic gain in corn grain yield can be sustained over time (Duvick and Cassman,
1999). In an average year corn is grown on 30-32 million hectares in the United States. Yet,
in 2007, 36.5 million hectares (90 million acres) was dedicated to corn because of the high
demand for ethanol production. Presuming that production area is maintained at the 2007
level, United States' corn production could reach 430 million tonnes (approximately 17
billion bu/year) accounting for the future yield increases (Dhugga, 2007). Even under these
conditions United States would be far short of producing enough corn needed to produce
enough ethanol to replace gasoline and meet the increasing demand of grain corn for feed
and food (Dhugga, 2007 ).
1.2.2.2
Chemical Composition
Corn is categorized as dent, flint, flour, sweet, pop or pod based on its kernel characteristics.
The majority of corn grown in the United States is yellow dent, yellow referring to the
color of endosperm. Chemical composition of corn kernel varies significantly depending on
type, variety, environmental conditions and agronomic practices used during crop production
(Reynolds
et al
., 2005). Starch is the most economically important component of corn.
Typical starch content of dent type corn is about 60% (NCGA, 2010). Mature kernels of
some corn hybrids may contain as high as 75% starch (w/w, dry basis). Morphological,
rheological, functional and thermal properties of starches from different corn types vary
significantly (Li
et al
., 1994). Amylopectin and amylose are two structural components of
starch. Regular corn starch consists of 75% branched amylopectin and 25% linear amylose
while waxy corn hybrids may contain 100% amylopectin. A clear understanding of starch
properties in different corn types and lines is vital for selecting corn for breeding purposes
and producing starch with specific properties that are required by manufacturing industry.
Sweet corn is primarily grown for fresh consumption. Young sweet corn kernels are
succulent because of a mutant recessive sugary-1 gene that retards the conversion of sucrose
into starch during endosperm development (Dickerson, 2003). Immature sweet corn contains
about 10% sucrose, which is rapidly converted to starch after harvest. Kernels can lose up to
50% of their sucrose at ambient temperature within 24 hours after harvest.
The protein content of corn kernel is about 10%. Zein is the major storage protein and
comprises 45-50% of the total protein in corn. Zein cannot be digested efficiently by humans
and other non-ruminants. Since corn is an important staple in many countries, its protein
quality is important. Corn kernels tend to be low in lysine and tryptophan, two of the eight
essential amino acids, requiring corn-based diets to be supplemented with other proteins such
as bean proteins. Corn kernels containing the Opaque-2 gene are shown to have lower
amounts of zein and higher amounts of lysine and tryptophan in their endosperm than
standard dent corn (Paulis
et al
., 1991). High lysine corn containing increased levels of lysine
and tryptophan has been developed. Although high lysine corn has demonstrated nutritional
advantage over field corn it still has several disadvantages. It has soft texture, dull appearance
and very little hard endosperm, which make high lysine corn difficult to harvest and prone to
attack by pests. The value of high lysine corn in poultry rations is limited because of its
insufficient methionine content. Furthermore, high lysine corn has lower crop yield and must
be segregated from other corn varieties to preserve protein quality (Dickerson, 2003).
Field corn contains 4-6% oil. Corn oil is a healthy energy source for both humans and
livestock because of its high polyunsaturated fatty acid, specifically linoleic acid content.
High oil corn, which contains 7-8% oil, is preferred by cattle feeders for its high calorie
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