Produce Degradation Pathways and Prevention

Structure and Function of Complex Carbohydrates in Produce 577

microbes such as bacteria and fungi, as well as the biosynthetic enzymes stored
within the plant cells, bring about the biodegradation of the plant carbohydrates.
The enzyme-mediated biodegradation in turn alters the appearance, color, texture,
flavor, and taste of the infected fruits and vegetables [107,108].

19.3.1 STARCH DEPOLYMERASES

The enzyme specificity and the availability of multiple enzyme reactive sites on the
starch polymer are two characteristics that offer potential for produce biodegradation.
The amylases act on starch polymers to hydrolyze α-1,4 and α-1,6 glucosidic bonds.
These bonds are cleaved via an exo or endo mode of hydrolysis. In complex branched
polymers such as cellulose and starch, branching creates a large number of nonre-
ducing terminal residues, which require a combination of both exo- and endo-
splitting enzymes to achieve a complete polymer hydrolysis. For example, β-amylase
(1,4-α-D-glucan maltohydrolase, EC 3.2.1.2), an exo-splitting enzyme, acts only on
residues at the nonreducing terminus until it reaches α-1→6 linkage, whereas
α–amylase (1,4-α-D-glucan glycanohydrolase, EC 3.2.1.1) is an endo-splitting
enzyme that can randomly cleave α-1→4 glycosidic linkages throughout the linear
chain. Glucoamylase (1,4-α-D-glucan glucohydrolase, EC 3.2.1.3 or 1,4-α-glucosi-
dase) can act on α-1→4 bonds but can also hydrolyze α-1→6 linkages, albeit at a
much slower rate. The enzymatic breakdown of highly branched amylopectin poly-
mer by α- and β-amylases and glucoamylases is depicted in Figure 19.12. Loss of
viscosity, which is critical for texture and taste of the juicy fruits, is related to the
type of amylase used. In starchy fruits and vegetables, the loss of starch polymer
viscosity is much more rapid with α–amylase; however, both –amylase and glu-
coamylase are slow-acting and would have a lesser impact on the polymer viscosity.
It has been observed that the oxidation and etherification of starch significantly alter
the extent of starch degradation by commercial amylases without affecting the rate

FIGURE 19.12Possible mode of actions for enzymatic breakdown of an amylopectin poly-
mer mediated via α- and β-amylases and glucoamylase.

GGGGGGGGGG GG

G

G

G

α-amylase

β-amylase

Glucoamylase

GGG

G

G

G

GGGGG

G

G

G GGGGG

GGG

G

G

G G G

G

GGG

G

G GGG

G

G

G

G

GG

G

GG

G

G

G