Bananas - Crop Post-Harvest: Science and Technology - page 27

should be above that which can cause chilling injury.
Commonly, 13°C is the lowest temperature used to trans-
port bananas as the probability of chilling injury occurring
increases below 13°C. Temperature influences the rate of
metabolism of the fruit and the Q 10 is 2.0 to 2.5 (Gane
1936). At temperatures above 24°C the peel of the banana
does not change to yellow as rapidly as it does at lower
temperatures. Plantain cultivars de-green at high tempera-
tures, while banana cultivars of the Cavendish group do not
(Seymour et al . 1987). Above 30°C, the peel remains green
during ripening. However, the ripening of the pulp is
hastened by high temperature, producing fruit that are
over-ripe but green in appearance. In the market, this is
known as 'soft-green' fruit. Banana fruit ripened at 30°C to
40°C does not de-green, but soften. This softening is medi-
ated by the effect of the increased temperature on increased
synthesis of new C 2 H 4 binding sites (Jiang et al . 2002).
Temperatures of 3°C or 8°C, which cause chilling injury,
reduce the synthesis of C 2 H 4 binding sites and this prevents
the fruit from ripening, a common observation in chilled
banana fruit (Jiang et al . 2004). If temperature influences
the rate of synthesis of C 2 H 4 binding sites, then in practice
this would mean the faster that fruit can be cooled after
harvest, the longer would be the expected green-life
because the fruit would be less sensitive to any ethylene
present in the post-harvest chain.

They point out that their data support the conclusion of
Peacock (1972) that there is no threshold concentration
of exogenous C 2 H 4 for the initiation of ripening in banana
and that endogenous C 2 H 4 is active during fruit growth and
after harvest. This differs from the views of others who
propose a threshold concentration of C 2 H 4 needed to initi-
ate ripening. In a number of different studies, summarized
by Acedo and Bautista (1993), the threshold varied over a
100-fold range from 0.015 to 1.0 μL C 2 H 4 L −1 , attributed to
differences in fruit maturity, cultivars and methods used. In
support of the 'no threshold' argument of Peacock (1972),
Wills et al . (2001) found linear relationships between the
time to ripen and the logarithm of the C 2 H 4 concentration
over the range of 0.005 to 10 μL C 2 H 4 L −1 . The slopes of
these lines indicate the sensitivity of the fruit to ethylene,
which may in turn be influenced by temperature. At 20°C,
banana was more sensitive to increased C 2 H 4 concentration
than other fruit, such as kiwifruit, custard apple, mango
and tomato. The linear relationship between time to ripen
and the logarithm of C 2 H 4 concentration, over a very wide
range of C 2 H 4 concentrations, does not support the 'thresh-
old' concept. In practice, where C 2 H 4 is used to ripen fruit
for commercial purposes, then a 'threshold' may be a use-
ful idea, as it would indicate the minimum concentration of
C 2 H 4 needed to achieve commercial objectives.
Wills et al . (1999) measured C 2 H 4 concentrations in
cartons of hard green bananas arriving at markets in
Sydney, Australia. Concentrations ranged from below
detectable limits up to 0.28 μL C 2 H 4 L −1 . Fifteen per cent of
the cartons contained air with C 2 H 4 concentrations above
0.1 μL L −1 . There was no link between the source of the
fruit and C 2 H 4 concentrations, even though some fruit had
travelled 3000 km from tropical North Queensland (Lat
17°S) and others had travelled 500 km from subtropical
Northern NSW (Lat 32°S). The C 2 H 4 detected in the
cartons may have come from the bananas themselves, and
mechanical damage to the fruit may have been a
contributing factor. Wounding the peel stimulates C 2 H 4
production (McGlasson 1969), and the amount of
C 2 H 4 produced from a wound is less if the peel of the green
fruit is harder (Chillet and de Lapeyre de Bellaire 2002).
Combining these observations on the concentration of
C 2 H 4 in cartons with their experimental data, Wills et al .
(1999) concluded that if C 2 H 4 concentrations in cartons
could be reduced, then bananas could be transported without
the need for refrigeration. This is an interesting conclusion
since the work of Inaba and Nakamura (1988) includes the
response of pre-climacteric bananas to a range of C 2 H 4 con-
centrations at different temperatures, including the range
likely to be used in refrigerated transport. However, the

Sensitivity to ethylene
Ethylene gas stimulates ripening of banana fruit and is
used commercially to begin the ripening process. The time
taken for the fruit to respond to the exogenous application
of C 2 H 4 is extended at low temperature and if the concen-
tration of exogenous C 2 H 4 is low. Inaba and Nakamura
(1988) showed that at 25°C the minimum time to ripen
(fruit more yellow than green) ranged from 5 h at 1000 μL
C 2 H 4 L −1 to 15 h at 0.1 μL C 2 H 4 L −1 but these times extended
to 18 and 50 h, respectively, at 15°C. More recently,
Wills et al . (1999) showed that C 2 H 4 concentrations below
0.1 μL L −1 , the lower limit used by Inaba and Nakamura
(1988), were capable of inducing ripening in bananas.
Wills et al . (1999) used air scrubbed of C 2 H 4 as their
control and assumed that it had a C 2 H 4 concentration
of 0.001 μL L −1 , below the detection limit of their instru-
mentation. The upper limit of their treatments was 1.0 μL
C 2 H 4 L −1 . Ethylene reduced green-life from a range of
27 to 44 days in the control fruit to 3.2 to 3.6 days at
1.0 μL C 2 H 4 L −1 . Variation within a C 2 H 4 concentration was
caused by differences in the cultivar and source of the fruit.
Their data show a linear decrease in the time taken to ripen
with the logarithm of the increasing C 2 H 4 concentration.