Produce Degradation Pathways and Prevention

Structure and Function of Complex Carbohydrates in Produce 569

The presence of many hydroxyl groups on starch permits easy alteration of its
properties through chemical derivatization. Acetate esters and carboxymethyl and
hydroxypropyl ethers exemplify starch derivatives. Starch polymer aged at constant
temperature and moisture level results in starch embrittlement. Differential scanning
calorimetric (DSC) studies [11] have shown that this phenomenon is due to structural
relaxation of starch chains, leading to decreases in enthalpy and free volume with
time. This type of aging is typical of most amorphous polymers [12,13]. The rates
of aging seem to vary with polymer structure but the reasons for such differences
are not fully understood at present. Gelatinized starch also tends to swell in water,
leading to its hydrolytic degradation.
Considerable literature is available indicating that under appropriate conditions
the starch polymer is susceptible to biodegradation, photodegradation, or chemical
degradation processes. Particularly, biodegradation of starch and starch-based mate-
rials has been studied in detail [14–16]. Most efficient and rapid starch degradation
usually requires a combination of degradation mechanisms and occurs within living
organisms and involves their hydrolytic enzymes. Native starch is unquestionably
biodegradable, because microorganisms such as bacteria, fungi, algae, and yeasts
readily metabolize it. Additionally, biosynthetic enzymes inherently present and
stored within the cells and tissues of fruits and vegetables also participate in the
starch degradation process.
Mechanisms for starch biodegradation are numerous and dependent on the type
of hydrolytic enzymes present [6]. Enzymes are extremely efficient and specific
biocatalysts produced by plants, animals, and microbes and are known for their
catalytic efficiency and regio- and stereoselectivity. These biocatalysts are involved
in the: (1) biosynthesis of polysaccharides to assemble complex polymeric structures;
(2) polymer modifications to render useful functionalities; and (3) biodegradation
process (recycling) of natural polymers. A comprehensive monograph on the roles
of enzymes in food science has been published [7].

FIGURE 19.6Helical conformation in amylose and the outer branches of amylopectin. Each
turn of the helix comprises about six monomer units.

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