566 Produce Degradation: Reaction Pathways and their Prevention
The addition of more monosaccharides in this manner yields very large polysaccha-
ride molecules containing thousands of monosaccharide units (residues).
Starch polymer is composed of two major components, amylose and amylopec-
tin. The amylose is mostly composed of linear α-D-(14)-glucan (Figure 19.3),
whereas amylopectin is a highly branched α-D-(14)-glucan with α-D-(16) linkages
at the branch points (Figure 19.4). The linear amylose molecules comprise about
30% of common cornstarch and have molecular weights of 200,000 to 700,000,
while the branched amylopectin molecules have molecular weights as high as 100
to 200 million.
Factors that influence the properties and functions of starch are briefly described
here. Starch is stored in plants as granules or solid particles composed of molecules
of both amylose and amylopectin. The granules vary in size from a few micrometers
to more than 50 μm, depending on their botanical source. A scanning electron
micrograph of starch granules recovered from corn is shown in Figure 19.5.
FIGURE 19.1Chemical structure of glucose, a monosaccharide sugar.
FIGURE 19.2Chemical structure of lactose, a disaccharide sugar.
FIGURE 19.3Linear amylose molecule.
OHOHO OHOHHOO
O
HOOHOHOOH OHOHHO
OHOHO
HO
OH OHO OHO
OH OHO OHO
OH OHHOnG Gn G