Bananas - Crop Post-Harvest: Science and Technology - page 30

resistance to O 2 movement across the peel than through the
pulp, assuming that their respiration rates are similar (this
is the case for green fruit, on a fresh weight basis, but not
so in ripening fruit; Dominguez and Vendrell 1994) and
that the entry of O 2 into the fruit is mainly by a radial path-
way (from the peel to the fruit centre) through the stomata
(Banks 1984). Compared with green fruit, in ripe fruit the
concentration of O 2 is greatly reduced and the gradient
across the peel becomes very steep, up to 15 to 17 kPa
(Figure 3.2), supporting the view that the peel represents a
'barrier' to the movement of O 2 into the fruit. Within the
pulp of ripe fruit, there is a gradient of 2 to 3 kPa from just
beneath the peel to the centre of the fruit. The lower partial
pressures of O 2 in ripe fruit will be influenced by the
increased respiration rate of the pulp and leakage of solutes
and water into intercellular spaces that would decrease the
movement of gases through the tissues. However, the rela-
tive magnitude of the effects of respiration and blockage of
the intercellular spaces on O 2 concentration in ripening
banana has yet to be determined. Burton (1982) thought
that the O 2 concentrations of 3.5 to 0.2 kPa O 2 in the ripe
pulp, measured by Brändle (1968), may be too low because
of the method used. Brändle (1968) used a miniature Clark
electrode pushed into the fruit to measure O 2 concentration.
Burton (1982) considered that damage to the tissue caused
by the electrode and the need for O 2 to flow to the electrode
through this tissue would contribute to lower values than
might exist in the undisturbed tissues. However, the values
recorded by Brändle (1968) for ripe fruit are not dissimilar
to those measured by Wardlaw and Leonard (1940) using a
cannula method described by Wardlaw and Leonard
(1939). They recorded O 2 concentrations of 2 to 5 kPa at
the centre of ripe fruit that had been ripened at a relative
humidity of 70%.
Assuming the values for partial pressure of O 2 given by
Wardlaw and Leonard (1940) are indicative (Figure 3.2),
then the external O 2 partial pressure would need to fall to at
least 6 kPa for regions of anoxia to be induced in green fruit
at 29°C. The situation would be quite different for ripe fruit
where a small drop in O 2 partial pressure from 21 to 15 kPa
would be sufficient to place most of the pulp in anoxic
conditions. Anoxia causes an energy crisis in cells that then
switch to alcoholic fermentation. If anoxia is extended,
then permanent damage can occur.
Banks (1983) compared the cannula and syringe (direct
removal) methods for estimating internal O 2 concentra-
tions in banana fruit (Cavendish cv 'Valery') and found
little difference between the two methods. Over five days
of ripening, the internal concentration of O 2 was stable and
ranged between 11 and 15 kPa in control fruit, indicating

little change as the fruit changed from green to full yellow.
Wardlaw and Leonard (1940) found that the concentration
of O 2 in the centre of the fruit throughout ripening differed
according to the relative humidity at which the fruit were
ripened. During 5 days of green-life at 29°C, O 2 concentra-
tion fell gradually from 15 kPa to 8 kPa just before the
climacteric rise in respiration. In fruit ripened at high
humidity, the concentration of O 2 in the fruit returned to
about 15 kPa after the climacteric rise in respiration and
then declined gradually to 2-4 kPa in over-ripe fruit at 15
days. If the bananas were ripened at low humidity, the con-
centration of O 2 declined exponentially during the whole
two weeks of storage and ripening, with no rise occurring
after the climacteric peak in respiration. The impact of
these differences in internal O 2 concentrations on fruit
quality needs to be determined. More recently, Perez and
Beaudry (1998) used cannula methods to estimate changes
in the internal concentration of O 2 in ripening bananas.
They found that while green, the internal O 2 concentration
was 18-20 kPa for the first 7 days but then began to fall and
reached about 12 kPa at 11 days coinciding with the peak
of internal C 2 H 4 concentration. Wills et al . (1982) meas-
ured the internal O 2 concentration in ripening bananas
(Cavendish cv 'Williams') by placing the fruit in water,
extracting gas under vacuum and then analysing the
extracted gas. With this method, they observed changes in
internal O 2 concentration similar to those recorded by
Perez and Beaudry (1998) who used the cannula method.
The internal O 2 concentration fell from 18-20 kPa when
the fruit were green to 9-12 kPa in ripe fruit. Thus, in fruit
exposed to air, in one experiment (Banks 1983) there was
no significant change in internal O 2 concentration during
ripening, but in another three experiments (Wardlaw and
Leonard 1940; Wills et al . 1982; Perez and Beaudry 1998)
the internal O 2 concentration was almost halved as the fruit
changed from green to ripe.
Perez and Beaudry (1998) point out that use of the
cannula method to estimate internal O 2 concentrations in
fruit assumes a 'hollow sphere' model of gas movement
in the fruit. This model is based on the skin being the main
site of resistance to O 2 entry into the fruit and that there is
little constraint on O 2 movement internally. In this case, no
gradients in O 2 concentrations would be expected inside
fruit. They tested this model for banana by coating, with
wax, various fractions of the fruit and measuring the
gradients in internal O 2 concentrations using cannulae sited
at five positions along the fruit length. They found internal
gradients of less than 1 kPa along the fruit length in
uncoated fruit but up to 12 kPa in fruit that was half coated
with wax. In addition, the gradients in the coated fruit