Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

274 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
Table 12.3
Different phenolic substrates of PPO in fruits
Fruit
Apple
Apricot
Avocado
Banana
Grape
Mango
Peach
Pear
Plum
Polyphenolic substrates for PPO
Chlorogenic acid, catechol, catechin, caffeic acid, 3,4-dihydroxyphenylalanine (DOPA),
3,4-dihydroxy benzoic acid, p-cresol, 4-methyl catechol, leucocyanidin, p-coumaric acid,
flavonol glycosides
Isochlorogenic acid, caffeic acid, 4-methyl catechol, chlorogenic acid, catechin, epicatechin,
pyrogallol, catechol, flavonols, p-coumaric acid derivatives
4-Methyl catechol, dopamine, pyrogallol, catechol, chlorogenic acid, caffeic acid, DOPA
3,4-Dihydroxyphenylethylamine (dopamine), leucodelphinidin, leucocyanidin
Catechin, chlorogenic acid, catechol, caffeic acid, DOPA, tannins, flavonols, protocatechuic
acid, resorcinol, hydroquinone, phenol
Dopamine-HCl, 4-methyl catechol, caffeic acid, catechol, catechin, chlorogenic acid, tyrosine,
DOPA, p-cresol
Chlorogenic acid, pyrogallol, 4-methyl catechol, catechol, caffeic acid, gallic acid, catechin,
dopamine
Chlorogenic acid, catechol, catechin, caffeic acid, DOPA, 3,4-dihydroxy benzoic acid, p-cresol
Chlorogenic acid, catechin, caffeic acid, catechol, DOPA
Adapted from Marshall et al. (2000).
and “Cortland.” This shows that catechin and epicatechin might play more decisive role than
chlorogenic acid in causing enzymatic browning (Joshi et al., 2007). The site of substitution
of mono- and dihydroxy phenols is also an important factor from the aspect of the action
of PPO. Monophenols are hydroxylated only if they have a parasubstituted CH 2 group,
and p-substituted 3,4-dihydroxy phenols are oxidized at higher rates than 2,3-dihydroxy
phenols (Baruah and Swain, 1953).
On the other hand, POX is highly specific to the peroxide substrate, and its main substrate
is H 2 O 2 . POX has low specificity for hydrogen donor substrates. POXs are able to
oxidize hydroxycinnamic derivatives and flavans. They can also oxidize flavonoids, which
are not PPO substrates but are found degraded in brown fruits due to progressing of coupled
oxidation reactions.
In addition to serving as PPO substrates, some phenolic compounds can also act as
inhibitors of PPO. For example, certain flavonoids, cinnamic acid derivatives, and coumarins
are shown to inhibit PPO activity. Quercetin is not a preferred substrate for PPO, but act as a
competitive inhibitor of PPO (Xie et al., 2003). Another flavonol, kaemferol, has also been
found to inhibit PPO, presumably through a mechanism of chelating copper in the enzyme
(Kubo and Kinst-Hori, 1999). Natural and synthetic naphthoquinones were found to inhibit
POX activity. Some of them showed a stimulating effect in a certain concentration range
(Vamos-Vigyazo, 1981).
12.7.3 Enzymatic browning of minimally processed fruits
Enzymatic browning has direct influence on the color, flavor, and sensory attributes of fresh
as well as processed fruit products. Minimally processed fruits are also called as fresh-cut,
lightly processed, partially processed, or ready-to-eat products, designed to keep the produce
fresh without losing the convenience and nutritional quality, and also to have an increased
shelf life (Lattanzio, 2003). In general, the processing steps of these products include