Agriculture Reference
In-Depth Information
phenolics do not limit the browning reactions. However,
dopamine, a free phenolic compound, decreases more
rapidly where browning is more intense and it might be
used as a substrate. Since PPO and PAL are not limiting in
the case of peel spotting, Trakulnaleumsai
et al
. (2006)
point out that control of browning by modulating these
enzymes lacks promise.
Peel spotting develops more rapidly at high temperature
(25-27°C) than at lower temperatures, and 12°C prevents
peel spotting without causing chilling injury to the fruit
(Trakulnaleumsai
et al
. 2006). However, 12°C is too cool
for the peel to yellow and the pulp to ripen and so fruit can-
not be continuously held at 12°C. Moreover, fruit held at
12°C develops peel spotting when returned to higher tem-
peratures. The absence of peel spotting at 12°C, but at O
2
concentrations that are well able to support spotting, raises
some questions. It is unlikely that this temperature is too
cold for enzyme activity because changes in PPO and PAL
activity are correlated with the appearance of browning in
fruit of 'Sucrier' that are stored at 6°C or 10°C and that
experience chilling injury (Nguyen
et al
. 2003, 2004).
When fruit are chilled, the membranes in the tissues lose
their integrity, which is a prerequisite for damage, and
browning is a common symptom of this damage. At 12°C,
in the absence of chilling injury in cv 'Sucrier', no peel
spotting occurs, and so at this temperature the prerequisite
rupturing of cells may not have been achieved and so
brown spotting of the peel is suppressed. This implies a
rupturing of the cells or membranes in the peel for spotting
to proceed. Coincidentally, the optimum temperature for
the growth of anthracnose is 27-30°C, and there is little
growth below 15°C (Muirhead and Jones 2000).
Browning reactions occur when enzymes and substrate
that are normally separate within cells are brought together.
This is often associated with changes in membrane integ-
rity and so studies on lesion development in the peel of
banana, such as have been done with superficial blemishes
(Williams
et al
. 1989), may provide a useful way forward
for identifying the cause and subsequently managing peel
spotting.
100% between treatments and occurred when the fruit
were full yellow, was less in mature fruit that had been
ripened at lower temperatures (17.5°C compared with 25°C)
and that had received C
2
H
4
treatment to initiate ripening.
Oxygen deficiency to extend shelf life
Klieber
et al
. (2002) investigated N storage of ripe bananas
(Cavendish cv 'Williams') for 6 to 24 hours to extend shelf
life, based on the work of Wills
et al
. (1990) in extending
green-life. The fruit were either more green than yellow or
more yellow than green at the start of the treatment, and
O
2
concentrations were limited to < 1.0 kPa. Fruit were
returned to air after treatment. N storage did not extend the
shelf life, and the treated fruit displayed browning of the
peel. Klieber
et al
. (2002) point out that the ripe fruit
respond differently to short-term O
2
deficiency on return to
air than do green fruit. Ripe fruit continue their ripening
process when returned to air after exposure to < 1 kPa O
2
,
but green fruit show a delay in ripening (Wills
et al
. 1982).
However, when Wills
et al
. (1982) exposed green fruit to
< 1 kPa O
2
for 6 to 20 hours, the same time span as used by
Klieber
et al
. (2002), they observed no effect of the treat-
ment on green-life on return of the fruit to air. To delay the
ripening of green fruit, exposure to low O
2
for 2 to 3 days
was needed. Thus ripe and green fruit need not differ in
their response to short-term O
2
deficiency when exposure
is less than a day. In contrast to the results of Wills
et al
.
(1982), Yi
et al
. (2006) extended the green-life of cv
'Brazil' (AAA group) bananas by 6 days by exposing the
fruit to 0.05 kPa O
2
for just 9 hours, considerably less than
the 2 to 3 days observed by Wills
et al
. (1982). A lesser or
greater exposure time, within the range of 0 to 24 hours,
had less effect. A difficulty in the interpretation of these
responses is that we do not know what is happening inside
the fruit during the treatment phase, in terms of either O
2
concentration or the metabolism of the pulp or peel.
These treatments, in another context, would be termed
'anoxic shock' (Gibbs and Greenway 2003), which is the
sudden exposure of plant tissue to anoxia. The effect that
'anoxic shock' has on tissues differs from that when the
tissue is exposed more gradually to the O
2
deficiency.
There is a need to separate the effects of sudden imposition
from gradual imposition of O
2
deficiency and to decide
what might be desirable outcomes for the quality of the
produce. Separating 'anoxic shock' from more gradual
development of O
2
deficiency might not be easy in very
bulky organs. As the banana fruit ripens over a period of
days, the internal O
2
concentration decreases and regions
of O
2
deficiency in the pulp would develop gradually. In
this situation, it is likely that regions of anoxia may be
Finger drop
When bananas are ripened in hands, the individual fruit
may break at the pedicel when ripe. Some cultivars of
banana are more susceptible to this condition than others,
and it can be very important if fruit are ripened on bunches
and then transported. Paull (1996) summarized work on
this problem and undertook investigations into factors
associated with finger drop in cv 'Prata Anã' (AAB, Pome
group) bananas. Finger drop, which varied from 0% to
