Bananas - Crop Post-Harvest: Science and Technology - page 32

polythene bags has been limited by uncertainties about
the security of seals in individual cartons and the need to
remove the polyethylene bags and C 2 H 4 absorbent from
each carton for purposes of ripening the fruit (Wills et al .
1982). To avoid these problems, treatments that prolong
green-life, and that can be applied after harvest but before
transport to market, have been sought. Wills et al . (1982)
extended the green-life of bananas by exposing them to
low (less than 1 kPa) O 2 concentrations for 2 to 3 days
immediately after harvest. The internal O 2 concentration
equalled that used for treatment and then returned to
control levels within 24 hours of the fruit being returned
to air. Two days of exposure to low O 2 concentration
delayed ripening by 5 days, and 3 days of exposure delayed
ripening by 7-8 days in one experiment and 12 days in
another. Thus there was a net increase of 3-9 days in the
green-life of the fruit, which is significant but somewhat
less than that achieved by modified atmosphere packaging
(Table 3.1). The extension of green-life, since it occurred
after the fruit had been returned to air, was attributed to
the effect of the O 2 deficiency on C 2 H 4 synthesis sup-
ported by the observation that endogenous production of
C 2 H 4 was suppressed but the addition of exogenous C 2 H 4
hastened ripening of the treated fruit. In another
experiment, Wills et al . (1982) exposed fruit to a range of
O 2 concentrations between 0 and 21 kPa for 3 days and
found that the extension of green-life was inversely
proportional to the concentration of O 2 used. The increase
in green-life was 0.31 days/kPa O 2 as O 2 concentration
fell from 21 to 0 kPa. Wills et al . (1982) point out that a
threshold might be expected, especially since the K m O 2
for C 2 H 4 production is 2.2 kPa (Banks 1985a), rather than
a proportional increase in green-life over the whole range.
While short-term O 2 deficiency may interfere with C 2 H 4
synthesis on return of the fruit to air, it may also affect the
sensitivity of the fruit to C 2 H 4 through the synthesis of
C 2 H 4 binding sites. Oxygen concentrations in the range 21
to 100 kPa increase the rate at which banana fruit soften
and this is attributed to the effect of O 2 in increasing the
synthesis of C 2 H 4 binding sites in Cavendish cv 'Williams'
(Jiang and Joyce 2003).
Wills et al . (1990) extended the work of Wills et al . (1982)
by evaluating the effect of 3 days of nitrogen treatment
(low oxygen, < 0.5 kPa) on cv 'Williams' (AAA, Cavendish
subgroup) bananas grown in the tropics and subtropics and
in different seasons in those locations. Low O 2 treatment
extended green-life by about a week but caused some
burning of the peel where it had been previously ruptured.
The magnitude of the damage increased with the delay after
harvest in applying the low O 2 treatment.

Manipulation of shelf life
An increase in shelf life would make fruit available to
consumers for longer in retail shops and would extend the
time over which fruit might be consumed in the home.

Peel spotting
The development of brown spots on the peel of ripe bananas
indicates the end of shelf life in cultivars of the Cavendish
subgroup (AAA) (Wardlaw and Leonard 1940). In the AA
group cv 'Sucrier', spotting begins when the peel is slightly
more yellow than green (Trakulnaleumsai et al . 2006). For
consumers familiar with the 'Sucrier' cultivar, the spotting
indicates the fruit is ready for eating. However, consumers
more familiar with the Cavendish cultivars, perceive that
the 'Sucrier' fruit is likely to be over-ripe if peel spotting
has begun. This perception limits the ability of 'Sucrier'
fruit to penetrate markets where Cavendish cultivars domi-
nate, even though consumers are interested in greater
diversity of cultivars of bananas to increase their choices.
The fungal organism, Colletotrichum musae (Berk. and
Curtis) Arx, causes anthracnose, which exists in green
banana fruit as a latent infection in the peel and appears as
brown spots when the fruit ripens (Muirhead and Jones
2000). Trakulnaleumsai et al . (2006) point out the difficulty
of distinguishing between anthracnose infection and peel
spotting that may either have a physiological origin or indi-
cate senescence of the peel tissue. While anthracnose is very
common and causes brown spots on the peel of bananas, it
is unlikely that all brown spots will be due to anthracnose.
An additional difficulty is that fungicides applied to the peel
surface, whether pre- or post-harvest, are unlikely to control
a latent infection beneath the fruit surface.
Studies on peel spotting have examined the factors
that limit the rate of browning, such as the activity of
phenylalanine ammonia lyase (PAL), which converts
phenylalanine to free phenolic substances, which form
the substrate that polyphenol oxidase (PPO) converts to
quinines producing the browning reaction (Choehom
et al . 2004; Trakulnaleumsai et al . 2006; Maneenuam et al .
2007). In addition, environmental factors such as tempera-
ture (Trakulnaleumsai et al . 2006), vapour pressure deficit,
O 2 concentration (Maneenuam et al . 2007) and modified
atmospheres (Choehom et al . 2004) have been examined.
The association between the activity of the enzymes
PPO and PAL, free phenolics and the level of browning of
the peel seems to vary between experiments. Maneenuam
et al . (2007) propose a model that integrates the enzymatic
data from a number of experiments. The limiting factor in
browning is the supply of O 2 over the range of 5 to 90 kPa.
The activities of PPO and PAL and the amount of free