Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
that are used as commodity products for food and industrial applications. 5 Major
markets of starch applications are in corn syrups, high-fructose corn syrup, paper,
textile, and food industries. In recent years, there is increasing demand in alcohol
fuel produced from starch fermentation to partially replace gasoline and to increase
the octane number of the fuel. Alcohol containing gasoline has been reported to
reduce air pollution. Readers are encouraged to refer to other chapters for information
on starch structures 1,6 and properties. 7
7.2 CHEMICAL STRUCTURES AND STARCH
MOLECULES
Starch consists of two major types of molecules, primarily linear amylose and highly
branched amylopectin. Normal starch consists of about 75% amylopectin and 25%
amylose, waxy starches consist of mainly amylopectin and 0–8% amylose, and highamylose
starches consist of 40–70% amylose. A newly developed genetically modified
starch has more than 90% amylose content. 8,9 In addition to amylose and
amylopectin, most cereal normal starches also contain lipids and phospholipids, 10–13
which have profound impacts on the pasting property of the starch. 14 Most tuber
and root starches and some cereal starches consist of phosphate monoester derivatives
that are found exclusively on amylopectin molecules. 15,16 Sugary-1 starch consists
of phytoglycogen that is a water-soluble glucan with a highly branched structure
and substantially shorter branch chains. The presence of phytoglycogen is a result
of the lack of starch-debranching enzymes. 17,18 Many starches, such as high-amylose
maize starches 19-22 and sugary-2 starches, 23 also contain intermediate components
that are branched molecules with smaller molecular weights and longer branch-chain
lengths than does amylopectin. 24 Structures and properties of these components are
discussed in the following sections.
7.2.1 AMYLOSE
Amylose is a primarily linear polymeric molecule, consisting of α 1-4 linked Dglucopyranose
with a few branches. 25,26 Molecular weight of amylose varies from
ca. 500 anhydroglucose units (AGU) of high-amylose maize starch 19 to more than
6000 AGU of potato starch. 27 It is known that amylose molecules cannot be totally
hydrolyzed by β-amylase to produce maltose. 27 Hizukuri and coworkers 25 demonstrated
that some amylose molecules consist of a few branches, and the number of
branches seems proportional to the molecular size of the molecule. The same authors
propose that amylose contains some clusters of branches that resemble clusters of
amylopectin molecules. 28 The structure may be a result of transferring a cluster from
an amylopectin molecule to the amylose molecule catalyzed by the branching
enzyme.
The α 1-4 glycosidic bonds of amylose molecules, differing from the β 1-4
glycosidic bonds of cellulose, give random coil conformations to the molecule. The
hydrocarbon and the hydroxyl moieties of the anhydroglucose unit prompt the
formation of double helices by folding two linear chains of starch and having the
hydrocarbon moiety of the chains folded inside of the helix, away from a polar,