Enzymes in Quality and Processing of Tropical and Subtropical Fruits - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 33

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Enzymes in Quality and Processing
of Tropical and Subtropical Fruits

Allan Liavoga and Norm Joseph Matella

INTRODUCTION
Enzymes are biochemical catalysts present in all living tis-
sues, including fruits. Enzyme-catalyzed reactions affect
the quality of processed fruits and foods in general by
producing chemical changes in fruits before, during, and
sometimes after processing. The major quality changes for
which enzymes are responsible are listed in Table 3.1 along
with respective enzyme(s). Many of the postharvest enzy-
matic reactions are undesirable; with few exceptions, they
result in loss of nutritive value and in poorer flavor, texture,
and color of the canned, frozen, or dehydrated product. To
preserve the quality of fruits and products thereof, these en-
zymes must be inactivated by heat (typically by blanching)
and/or enzyme-specific chemical treatment. This chapter
covers information on some of the most important enzymes
implicated in raw or processed fruit quality, with special
reference to tropical and subtropical fruits.

imidazoles, and thiols or as constituents of proteins to form
complex brown polymers (Taylor and Clydesdale, 1987).
Schematic representation of reactions involved in the pro-
cess of browning catalyzed by PPO and followed by nonen-
zymatic condensation is shown in Fig. 3.1 (this illustration
shows the sequence of biochemical reactions leading to
formation of melanin from the oxidation of phenolic amino
acid tyrosine).
On the basis of the substrate specificity, the Nomencla-
ture Committee of the International Union of Biochemistry
(NCIUM; 1992) has designated monophenol monooxyge-
nase, also referred to as monophenolase, cresolase or ty-
rosinase as EC 1.14.18.1. Ortho -diphenolase also referred
to as catechol oxidase or diphenol oxygen oxidoreductase
is designated EC 1.10.3.2 and laccase or para -diphenol
oxygen oxidoreductase as EC 1.10.3.1.

Occurrence and role of PPO in plants
PPO is a widely distributed enzyme in living organisms
and is found in many plant tissues and its distribution in
edible plant products has been reviewed by several authors
(Vamos-Vigyazo, 1981; Mayer, 1987). The role of PPO in
plants continues to be investigated; it is suggested that PPO
may be involved in immunity reactions, in biosynthesis of
plant components, and in scavenging of free radicals in
photosynthesizing tissues (Heimdal et al., 1994). PPO is
also believed to be involved in the defense mechanism of
plants during injury, herbivore, and/or pathogen attack as
well as during exposure to external stresses (Mayer, 2006).
The role of PPO in browning phenomena is well doc-
umented. The browning process catalyzed by PPO occurs
when the highly reactive quinones polymerize, leading to

POLYPHENOL OXIDASE (PPO)
Nomenclature and reactions catalyzed
Polyphenol oxidase (PPO) also referred to as tyrosinase
is a generic term for the group of enzymes that catalyze
the oxidation of phenolic compounds to their respective
o- quinones using Cu ++ as a prosthetic group (Vamos-
Vigyazo, 1981; Mayer, 2006). It uses molecular oxygen
to catalyze the ortho -hydroxylation of the monophenols
to o -diphenols (monophenolase activity) and subsequently
oxidizing the ortho -diphenols to colored and highly reac-
tive o -quinones (diphenolase activity) (Mayer, 2006). The
o -quinones undergo nonenzymatic self-polymerization or
condensation with a number of compounds like amines,