Agriculture Reference
In-Depth Information
increased from 2 to 12 kPa along the fruit length, as the
fruit ripened. Perez and Beaudry (1998) concluded that
the 'hollow sphere' model of gas exchange was not appro-
priate for banana. To calculate O
2
gradients, Banks (1985a)
adopted a cylindrical model, but applied it only to the pulp.
However, the whole banana fruit consists of the peel and
the pulp, and a more comprehensive model would need
to take account of both tissues. The peel and the pulp can
be considered as two concentric cylinders. Then, the equa-
tions used to calculate O
2
profiles across roots, which have
layers of tissues arranged concentrically (Armstrong
et al
.
1991), can be adapted to the whole banana fruit. Assuming
the supply of O
2
to the fruit is radial, the O
2
deficit from the
outside of the peel to the centre of the fruit, ΔC
T
, is:
Leonard 1940) rising to 210 Pa/mm in ripe fruit (Brändle
1968). The maximum axial gradient in ripe fruit, assuming
a pulp length of 150 mm in the fruit used by Perez
and Beaudry (1998), was 80 Pa/mm, similar to the radial
gradient in green fruit obtained from the data of Wardlaw
and Leonard (1940). However, this axial gradient applies
only to the artificial situation where half the fruit is coated
with wax. In uncoated fruit, the axial gradient is minimal
at 7 Pa/mm. This small axial gradient supports the assump-
tion of using two concentric cylinders to estimate O
2
concentrations within the fruit. These radial gradients of
O
2
in banana pulp, although approximations for the data of
Wardlaw and Leonard (1940) and Perez and Beaudry
(1998) are of the same order as recorded in apples (Burton
1982) but are much greater than the theoretical radial
gradients of 5-13 Pa/mm calculated by Banks (1985a).
The difference between the calculated values and the
measurements indicates a greater resistance to diffusion of
O
2
through the tissues in reality, compared with the simpli-
fying assumptions of the model used by Banks (1985a).
In apples, gradients of O
2
partial pressure similar to those
measured in banana are associated with breakdown
disorders in fruit stored in controlled atmospheres. Thus,
O
2
gradients in the pulp of banana may contribute to the
negative impacts of controlled and modified atmospheres
on fruit physiology. The picture that we have of O
2
distri-
bution in the banana fruit at the moment is incomplete.
Studies are needed of the anatomy of the peel and pulp,
their separate respiration rates and the distribution of O
2
in
the tissues in green and ripening fruit. Work on the fruit
surface described by Banks (1984) needs to be comple-
mented with measurements of O
2
distribution within the
peel and pulp. This knowledge would support studies on
why the ripening of fruit coated with waxes tends to be
patchy, thus reducing their use in commerce.
Here we have focused on the concentration of O
2
in
fruit tissues. In modified atmosphere storage, there is an
increased concentration of CO
2
and C
2
H
4
may accumulate in
the tissues of fruit coated with impermeable substances
(Banks 1983). In addition, if areas of anaerobic tissue develop
in the fruit, then the products of alcoholic fermentation may
accumulate. To advance the technology of modified atmos-
phere storage of banana, it is important to undertake studies
on the individual components of the gas system before
attempting to build an integrated picture.
ΔC
T
= ΔC
S
+ ΔC
S1
+ ΔC
P
(Eqn 3.1)
where ΔC
S
is the
in situ
deficit across the peel caused by
the respiration rate of the peel, ΔC
S1
is the deficit across the
peel caused by the respiration of the pulp (through flow)
and ΔC
p
is the
in situ
deficit across the pulp caused by the
respiration rate of the pulp. The individual components of
equation 1 are:
ΔC
S
= [Q
S
r
P
2
/4D
S
] [(r
S
2
/r
P
2
) + 2 ln(r
P
2
/r
S
2
) − 1]
(Eqn 3.2)
ΔC
S1
= [Q
P
r
P
2
/2D
S
] [ln(r
S
/r
P
)]
(Eqn 3.3)
ΔC
P
= [Q
P
r
P
2
/4D
P
]
(Eqn 3.4)
where r
S
is the outer radius of the peel; r
P
is the radius of the
pulp; Q
S
is the respiration rate of the peel, per unit volume;
Q
P
is the respiration rate of the pulp, per unit volume; D
S
is
the diffusion coefficient of O
2
for the peel and D
P
is the
diffusion coefficient of O
2
for the pulp. These equations
apply only until the centre of the peel becomes anoxic.
An advantage of the miniaturized oxygen electrode used
by Brändle (1968) is that no assumptions need to be made
about the distribution of O
2
in the fruit because the electrode
measures O
2
concentration
in situ
. Micro-electrodes are
now available (Armstrong
et al
. 2000) and these would be
particularly useful for measuring O
2
concentrations in
fruit tissue. Data collected using electrodes will provide a
suitable validation of the models assumed by the use of
the cannula method for estimating O
2
concentration in
the pulp.
Estimates of the radial (beneath the peel to the fruit
centre) and axial gradients of O
2
partial pressure in the pulp
can be made from the data of Wardlaw and Leonard (1940),
Brändle (1968) and Perez and Beaudry (1998). The radial
gradient within a green fruit was 90 Pa/mm (Wardlaw and
Short-term oxygen deficiency
Bananas are usually transported long distances in cartons
containing 15 to 18 kg of fruit. The practical application
of modified atmosphere technology in bananas using
