Structure and Function of Complex Carbohydrates in Produce 565
The role of enzymes in the syntheses, degradation, and modification of polysaccha-
rides has been the subject of several excellent reviews [1–7]. In addition to enzymes,
plants also synthesize and/or utilize any required cofactors and, eventually, inhibitors
to terminate reactions once the polymer synthesis is complete or not needed anymore.
Because nature assembles these structures, it has both the inherent ability and the
capacity to degrade and depolymerize these complex structures into simple mole-
cules or polymer building blocks (precursors) via specialized enzymes. Once the
polymer degrades back to simple molecules, naturally occurring microbes and/or
their enzymes can utilize them as a carbon source, recycling these materials back
into the biosphere.
After the produce is harvested, it proceeds through a variety of phases that
involve ripening, processing, packaging, transportation, and storage before it is
finally delivered to the customers. During these phases, produce is exposed to a
variety of stresses such as heat, irradiation, altered humidity conditions, mechanical
injury, suffocation, infestation, etc., that can lead to postharvest deterioration of
complex carbohydrates in fruits and vegetables, which alters their color, texture,
flavor, and appearance (Table 19.1). This chapter focuses on the chemistry of the
major structural carbohydrates generally found in produce, including their structure-
function properties, interactions, and biochemical mechanisms that lead to their
biodegradation and physical deterioration.
19.2 CHEMISTRY, FUNCTIONAL PROPERTIES, AND REACTION
MECHANISMS OF PLANT CARBOHYDRATES19.2.1 STARCH
Starch, a major storage product found in fruits and vegetables, is essentially a
condensation polymer of glucose molecules connected by acetal linkages. Glucose
sugars (Figure 19.1) in starch are the principal food molecules of the cell. All sugars
contain hydroxyl groups and either an aldehyde (H > C = O) or a ketone (> C = O)
group. The hydroxyl group of one sugar can combine with the aldehyde or ketone group
of a second sugar with the elimination of water to form a disaccharide (Figure 19.2).
TABLE 19.1
Possible Factors That Can Induce Changes in Postharvest Produce
Factors That Can Induce or Initiate Changes Leading
to Produce Deterioration and Degradation Associated Changes in Produce
Light (photo)
Heat (thermal)
Water (hydrolysis)
Mechanical injury (physical)
Chemicals (chemical)
Microbes and enzymes (bio)
Molecular chain scission
Depolymerization of polysaccharides
Viscosity
Appearance
Texture
Flavor
Color