Enzymes in Quality and Processing of Tropical and Subtropical Fruits - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 35

the formation of brown or black pigments (Mayer, 1987;
Vamos-Vigyazo, 1995).

acid and analogues, sulfites); (2) chelating agents (ethylene
diamine tetra acetate [EDTA], sodium diethyldithiocarba-
mate [DIECA], sodium azide); (3) complexing agents (cy-
clodextrins, chitosan); (4) acidulants (ascorbic acid, citric
acid, malic acid, phosphoric acid); (5) enzyme inhibitors
(substrate analogues, halides); and (6) enzyme treatments
(proteases, o -methyltransferase).
These compounds inhibit the rate of the browning reac-
tion by eliminating from the reaction one or more of the
active reaction elements (i.e., enzyme, substrate, oxygen,
copper or a reaction intermediate, o -quinones) (Macheix
et al., 1990; Ahvenainen, 1996). PPO from different sources
may react similarly with inhibitor compounds. However,
effectiveness of inhibitors against different PPOs could
significantly vary; therefore, specific control measures
for individual systems would also be needed (Ferrar and
Walker, 1996).

Latency and activation of PPO
One unique characteristic of PPO is its ability to exist
in an inactive or latent state (Moore and Flurkey, 1990).
In fact, it is found in either latent or active form as
well as in both forms at the same time in many sources
(Whitaker, 1995). PPO, located exclusively in the plas-
tids of healthy plant tissues, is activated once it crosses
the plastidic envelope and comes in contact with the phe-
nolic compounds in the vacuoles during cellular disruption
(Zawistowski et al., 1991). Consequently, endogenous phe-
nolics are enzymatically oxidized to o -quinones, which
polymerize

to polyphenols, responsible for enzymatic

browning (Mayer, 1987).

Substrates and inhibitors of PPO
The type and concentrations of naturally occurring phe-
nolic compounds vary widely for different plant sources.
Studies have shown that different substrates exhibit differ-
ent degrees of browning. It is believed that the reason for
the varying degree of PPO activity in the plant kingdom to-
ward dihydroxyphenylalanine as a substrate for instance is
the diverse substrate specificity of PPO in plants (Yoruk and
Marshall, 2003a). Nature of the side chain, number of hy-
droxyl groups, and their position in the benzene ring of the
substrate, also have a major effect on the catalytic activity
of the enzyme (Macheix et al., 1990). It also appears that
substrate specificity of PPO is dependent on species and
cultivars (Yoruk and Marshall, 2003a). In addition, PPO
isoforms in plant tissue may also exhibit varying substrate
specificities and relative activities toward monophenols and
o -diphenols (Oba et al., 1992).
PPO activity can be inhibited by various compounds
that are categorized based on their mode of action as fol-
lows (McEvily et al., 1992): (1) reducing agents (ascorbic

PPO in tropical and subtropical fruits
PPO is of significant importance within tropical and sub-
tropical fruits, as uncontrolled activity could result in severe
postharvest losses. Since PPO is involved in the browning
of injured, peeled or diseased fruit tissues, undesirable qual-
ity changes could occur during handling, processing, and
storage (Nicoli et al., 1991; Tomas-Barberan and Espın,
2001). Table 3.2 shows the optimal reaction conditions for
PPO activity in different tropical and subtropical fruits.
Avocado is reported to have one of the highest PPO ac-
tivities among the common tropical and subtropical fruits
(Weemaes et al., 1998), making the control of browning
difficult. The different rates of browning observed in avo-
cado varieties have been shown to directly correlate to the
PPO activity and the total phenol content of each fruit vari-
ety (Mayer, 2006). In banana, PPO was suggested to be the
cause of endogenous oxidation of dopamine (Yang et al.,
2000). PPO has also been shown to be responsible for the

Table 3.2. Polyphenol oxidase (PPO) in tropical and subtropical fruits.

Fruit Substrates Properties or Role in Processing Reference

Mango Catechol Optimum pH 6.0, temperature

25 ◦ C

Arogba et al. (1998)

Espın et al. (1997)

Avocado 4-Hydroxyanisole Optimum pH 5.0

Banana Dopamine Optimum pH 6.5-7, temperature

30 ◦ C

Yang et al. (2000)

Guava Catechol Optimum pH 7.2 Augustin (1985)

Olive 4-Methylcatechol Optimum pH 7.0 Segovia-Bravo (2007)

Pineapple Catechol Optimum pH 6.0-7.0 Das et al. (1997)