Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

THE ROLE OF POLYPHENOLS 273
compounds. The role of metal ions influencing the enzymatic browning has also been
investigated by several researchers. According to Aydemir (2004), Cu 2+ and Fe 3+ ions at
1 mM caused the activation of PPO, but at 10 mM concentration, both Cu 2+ and Fe 3+ ions
acted as poor inhibitors of PPO. Whereas Colak et al. (2007) and Kolcuoglu et al. (2007)
have recently found that Cu 2+ at 1 mM concentration was sufficient to inhibit PPO activity.
As well, the reported literature on the effects of different metal ions on the PPO activity is
varying. Interestingly, a strong correlation (r 2 = 0.92) between copper content of fruit and
PPO activity has been reported (Joshi et al., 2007). However, further investigations need to
be carried out to understand the role and concentration-dependent effect of metal elements
on PPO activity and enzymatic browning.
The pH optimum of POX varies with the enzyme source, the isoenzyme composition,
and the hydrogen donor substrate. In fruits it generally ranges from pH 4.0 to 6.5 (Vamos-
Vigyazo, 1981). The behavior of POX during different heating and cooling treatments is
most widely investigated due to the existence of different fractions of its heat resistance,
and part of its activity is restored during shorter or longer storage at room temperatures
following the thermal treatment.
12.7.2 Polyphenols and substrate specificity for PPO and POX
The substrate specificity of PPO varies in accordance with the source of the enzyme. The
extent to which naturally occurring phenolic substrates contribute to enzymatic browning
of individual fruits depends on the localization and concentration of phenolics as
well as on the color intensity of the macromolecular pigments obtained from the different
quinones (Vamos-Vigyazo, 1981). For example, total phenolic content among apple
cultivars is highly variable (Lee et al., 2003; Lata et al., 2005; Scalzo et al., 2005), which is
the cause of the differences in the browning intensity among cultivars (Russell et al., 2002).
A wide range of phenolic compounds is oxidized by PPO, and hence there is a high potential
for browning besides 3,4-dihydroxyphenylalanine (DOPA) and tyrosine (Baruah and
Swain, 1953; Sapers, 1993). The phenolic substrates of PPO in different fruits are given in
Table 12.3.
Among phenolic compounds, catechin and chlorogenic acids are the substrates with
a greater affinity for PPO enzyme activity (Janovitz-Klapp et al., 1990; Oszmianski and
Lee, 1990), whereas the flavonols appear to be less suitable as PPO substrates (Baruah and
Swain, 1953). However, in the presence of transfer substances such as chlorogenic acid
and catechin, flavonols glycosides are oxidized at measurable rate, probably through the
formation of dimers as a first step (Vamos-Vigyazo, 1981). However, in certain fruits and
vegetables, the main substrates of PPO are not catechin and chlorogenic acid (Marshall
et al., 2000). The principal phenolic substrate in banana, for example, was identified as
dopamine (3,4-dihydroxy phenylethylamine), while that in dates is 3-O-caffeoylshikimic
acid (dactylifric acid). The catechins oxidized more rapidly as compared to others; however,
the higher concentration of chlorogenic acid in apples is considered to play a decisive role
in acting as PPO substrate. Based on the degree of browning of 11 apple cultivars subjected
to bruising, Amiot et al. (1992) found that chlorogenic acid and catechins are the most
degraded phenolics as a result of enzymatic browning. However, in a recent study, it was
observed that “Eden TM ,” an apple cultivar that contains reasonable amount of chlorogenic
acid but very low level of catechin and epicatechin, did not brown as compared to “Empire”