Chemistry Reference
In-Depth Information
13.3 reSIStaNt StarCh
Starch is the major source of carbohydrates in the human diet (Ratnayake and Jackson 2008). It
occurs in many plant tissues as granules, usually between 1 and 100 μm in diameter, depending on
the botanical source. Chemically, starches are polysaccharides composed of α-d-glucopyranosyl
units linked together with α-d-(1-4) and/or α-d-(1-6) linkages. Native starch granules have a crys-
tallinity varying from 15% to 45% (Aprianita et al. 2010); thus, most native starch granules exhibit
a Maltese cross when observed under polarized light. It is clear from the level of starch crystallinity
that most starch polymers in the granule are in an amorphous state (Aprianita et al. 2010). The two
major macromolecular components of starch are amylose and amylopectin (Figure 13.1). Amylose
is the predominantly linear α-(1-4)-linked α-glucan with a DP as high as 600. Amylopectin is the
major component of the granule (30%-99%) and is α-(1-4)-linked α-glucan with α-(1-6) branch
points. Amylopectin contains about 5% of branch points, which impart profound differences in its
physical and biological properties compared to amylose. Amylopectin possesses several popula-
tions of polymer chains that can be classiied into short chains (12 < DP < 20), long chains (30 <
DP < 45), and very long chains having an average DP > 60. The chains are further classiied into
A-, B-, and C-chains, where A-chains do not carry any other chains, B-chains carry one or more
chains, and the C-chain is the original chain carrying the sole reducing end (Sharma and Yadav
2008; Haralampu 2000).
Two crystalline structures of starch have been identiied (an “A” and a “B” type), which con-
tain differing proportions of amylopectin. A-type starches are present in cereals, whereas B-type
starches are found in tubers and amylose-rich starches. The third type, called “C-type,” is a mix-
ture of both A and B forms and is mainly present in legumes. In general, digestible starches are
hydrolyzed by an enzymatic cascade in the small intestine to yield free glucose, which is subse-
quently absorbed (Nugent 2005). However, not all starch in the diet is digested and absorbed in the
small intestine (Ratnayake and Jackson 2008). This recognition that some starch is neither com-
pletely digested nor absorbed in the small intestine has brought scientiic and commercial interest
in nondigestible starch fractions and their potential to perform functions similar to DF in the large
intestine (Fuentes-Zaragoza et al. 2010). Resistant starch is not digested by pancreatic amylases
H
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b)
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Figure 13.1
Schematic diagram of (a) amylose and (b) amylopectin. The Merck index, Eleventh edition 1989;
S. Budavari, Merck co. Rahway, NJ.
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