568 Produce Degradation: Reaction Pathways and their Prevention
Starch solutions are unstable at low temperatures. On standing in dilute solutions,
the linear amylose component crystallizes. Many branches of amylopectin may also
crystallize. Rapid cooling of concentrated starch dispersions creates stiff gels, which
crystallize more slowly. Amylose, and to a lesser degree the outer branches of
amylopectin, can assume helical conformations that have a hydrophobic core (Figure
19.6). Each turn of the helix comprises about six monomer units. Iodine, fatty acids,
lipids, alcohols, and other materials may enter the core of the helix to form stable
complexes with starch. Small amounts of crystalline amylose-lipid V-type complexes
are usually found in starches such as corn and wheat, which contain free fatty acids
and phospholipids [8–10].
Starch molecules readily depolymerize into glucose monomer units when heated
in acidic solutions or when treated with a variety of amylolytic enzymes. They are
generally stable under alkaline conditions at moderate temperatures. When heated
with amines under alkaline conditions, they undergo complex Maillard reactions to
form brown-colored products with caramel-like odors. If the produce is damaged
during its processing, storage, or transportation via mechanical injury or is subjected
to conditions that can alter its surface morphology and cause starch to be exposed
on the surface, the starch will degrade fairly quickly.
FIGURE 19.5Scanning electron micrograph showing cornstarch granules, bar = 10 μm.