Surface coating effectively reduced early peel
spotting in 'Sucrier' banana. Surface coatings at 25%
and 30% also inhibited changes in colour, firmness,
and eating quality. this indicated an inhibition of
ripening. In contrast, 20% surface coating did not
affect these attributes, thus apparently did not impair
ripening. The coatings at 25% and 30% are not
feasible, as they induced off-flavour (30% wax) or reduced the eating quality (25% wax). Therefore,
further tests only used coating treatment at 20%,
besides the control.
Surface coatings seem to act like a plastic film.
It has been found previously to limit gas exchanges
of ripening banana (Amarante & Banks 2001). The
effect of surface coating on early peel spotting might
be the result of reduced flow of oxygen from the
atmosphere to the cells at the peel surface. this might
reduce the oxygen levels in the cells. Choehom et
al. (2004) reported that exogenous oxygen levels
lower than 5% were effective in controlling early
peel spotting in 'Sucrier' banana.
Browning or blackening in plant tissues seems
the result of oxidation of free phenolics. Part of this oxidation seems non-enzymatic, part is catalysed
by PPO (Martinez & Whitaker 1995). PPO plays
an important role in browning in several plant
tissues (Mayer & Harel 1991). However, despite the
difference in peel spotting, there was no difference in
PPO activity between control fruit and fruit treated
with 20% surface coating. Similarly, PPO activity
was not correlated with peel spotting in tests where
spotting was inhibited by oxygen levels below 5%
(Choehom et al. 2004). Weaver & Charley (1974)
also reported that there was no relationship between
PPO activity and browning of banana pulp. PPO
activity is measured in vitro, under conditions
whereby oxygen is not limiting the reaction rate.
Lack of correlation with PPO activity thus indicates
that the potential PPO activity is not limiting the
degree of blackening. The PPO reaction rate in vivo
will depend on at least two factors other than those
measured in vitro: the substrate concentration and
oxygen partial pressure.
Various isoforms of PPO may have different
substrate specificities. We used catechol as a
substrate. This compound is the substrate for PPO
in many tissues of fruits and vegetables (Vâmos-
Vigyâzo 1981). Therefore, we assume that catechol
also provided a suitable substrate to measure PPO
activity in banana peel. The main PPO substrate
depends on the commodity. For example, flavan-3-ol
monomers and dimers are the main PPO substrates
in litchi (Zhang et al. 2000), 3,4-dihydroxyphenyl
alanine (DOPA) is a substrate for PPO in potato
(Chen et al. 1991), /7-coumaryl and caffeoyl-tartaric
acid are substrate for PPO in grape, and chlorogenic
acid is a substrate for PPO in peach and nectarine
(Cheng & Crisosto 1995).
PAL promotes the synthesis of free phenolics
(Camm & Towers 1973). We observed a positive
relationship between early peel spotting and PAL
activity. Low PAL activity might be related with
low free phenolics levels, if the free phenolics
would accumulate. However, no consistent effect
was observed of the 20% surface coating on levels
of free phenolics.
It is concluded that surface coatings considerably
reduced early peel spotting in 'Sucrier' banana and
that this effect was accompanied by a lower PAL
activity. There was no relationship between peel
spotting and PPO activity measured in vitro. We
suggest that the delay of peel spotting, after coating,
might be related to reduced oxygen diffusion into
the peel.