Produce Degradation Pathways and Prevention

298 Produce Degradation: Reaction Pathways and their Prevention

10.1.2.1 Important Enzymes

A lipoxygenase (linoleic acid oxygen oxidoreductase, EC 1.13.11.12) enzyme occurs
in many plants and also in erythrocytes and leucocytes. It catalyzes the oxidation
of unsaturated fatty acids to their corresponding hydroperoxides. These hydroper-
oxides have the same structure as those obtained by autooxidation. In comparison
with lipid autooxidation the reaction is characterized by all features of enzyme
catalysis: substrate specificity, peroxidation selectivity, occurrence of a pH optimum,
susceptibility to heat treatment, and a high reaction rate in the range of 0 to 20°C.
Lipoxygenase oxidizes only the fatty acids with a 1-cis,4-cis pentadiene system;
therefore, linoleic and linolenic acids are the preferred substrates in fruit and vege-
table tissues. Hydroperoxides formed enzymatically are precursors of odorants, green-
grassy or cucumberlike smelling aldehydes hexanal, 3-cis-hexenal (leafy aldehyde), 3-
cis,6-cis-nonadienal, and others formed by subsequent enzymatic degradation catalyzed
by lyases. These products are typical in fresh produce such as peeled cucumber and
peeled and cut apples, but they are also undesirable in the case of frozen vegetables or
fruits after some length of storage (Grosch, 1982; Whitaker, 1991).
Polyphenoloxidase (PPO) catalyzes two reactions: hydroxylation of monophenol
to o-diphenol (E.C. 1.14.18.1, monophenol monooxygenase), which is followed by
an oxidation to o-quinone (E.C. 1.10.3.1, o-diphenol: oxygenoxidoreductase). Both
activities are also known as cresolase and catecholase activity. PPO is widely dis-
tributed in the plant and animal kingdom and is primarily responsible for enzymatic
browning. It affects numerous plant organs that are rich in oxidizable phenols
(Zawistovski et al., 1991).

TABLE 10.3

Important Endogenous Enzymes and the Effects

of Enzymatic Changes During the Processing

of Fruits and Vegetables

Endogenous Enzyme Effect on Food

Lipoxygenase

Lipase

Protease

Off-flavors, off-odors

Pectolytic enzymes

Cellulolytic enzymes

Texture changes (softening,

sediments in citrus juices, etc.)

Polyphenoloxidase

Chlorofylase

Peroxidase (to some extent)

Color changes (browning,

chlorophyll degradation)

Askorbase

Thiaminase

Polyfenoloxidase

Lowering of nutritive value

(vitamin degradation, lowering

of digestibility of protein, etc.)