Agriculture Reference
In-Depth Information
25
W & L Green
W & L Ripe
Brändle Ripe
20
15
10
5
0
-5
0
5
10
15
20
Depth beneath peel (mm)
Figure 3.2
Profiles of oxygen partial pressure in green and ripe banana fruit. The data 'W & L green' and
'W & L ripe' are from Wardlaw and Leonard (1940) and 'Brändle ripe' from Brändle (1968). The value of 21 kPa
above the peel has been added to show the drop in oxygen partial pressure across the peel. The zero point
on the x-axis is the border between the peel and the pulp. The W & L data at 18 mm are for the 'centre' of the
fruit and were placed there for comparison with the data of Brändle (1968).
summarises both the status of our understanding of how
to measure gas concentrations and the implications of
the findings of scientific studies conducted over the past
few decades.
It is worthwhile looking at the O
2
concentrations and
profiles within green and ripe fruit so that the concentration
of O
2
within tissues can be anticipated, if the external con-
centration is known. One concern in modified atmosphere
storage is that if the external concentration of O
2
is too low,
then areas of O
2
deficiency will develop within the fruit
leading to problems in ripening and poor quality. On the
other hand, if O
2
concentration is too high, then C
2
H
4
synthesis will continue, as O
2
is required for this process.
To extend green-life, the internal O
2
partial pressure would
need to be less than 2.2 kPa (approximately 2.2%), which
is the K
m
O
2
for the production of C
2
H
4
in banana pulp,
according to the methods used by Banks (1985a). This
argument assumes there is a threshold concentration of O
2
below which green-life will be extended and another above
which ethanolic fermentation can be avoided. However,
Wills
et al
. (1982) show that the impact of short-term O
2
concentrations on extending green-life is proportional to
the concentration of O
2
applied (see later). Hence, the idea
of using O
2
concentration as a 'switch' to turn ethylene
synthesis off does not seem applicable.
The O
2
concentration at any point in the fruit will be
influenced by the rate of respiration of the tissues, the
gradient to an external source and the resistance to gaseous
diffusion provided by the intercellular spaces and the
anatomy of the tissues. In addition, the contribution of
photosynthesis by green chlorophyll in the peel to O
2
concentration in the fruit that may be in the light needs to be
evaluated. Dynamic changes in metabolic activity and in the
diffusion pathways of gases during ripening mean that
internal O
2
concentrations decrease rapidly in ripening fruit.
Burton (1982) summarized the measurements of the
concentration gradients of O
2
in the pulp of banana fruit
made by Wardlaw and Leonard (1940) (AAA, cv 'Gros
Michel') and Brändle (1968) (AAA, Cavendish subgroup)
(Figure 3.2). In green fruit in air (21 kPa O
2
), the partial
pressure of O
2
beneath the peel was 17 kPa and in the centre
of the fruit was 15.5 kPa. This gradient implies a greater
