Postharvest Biology and Technology 18 2000 33 – 41
Internal atmosphere composition and skin permeance to gases of pepper fruit
Nigel H. Banks, Sue E. Nicholson
Food Technology Research Centre, Massey Uni6ersity, Pri6ate Bag
11
-
222
, Palmerston North, New Zealand Received 16 December 1998; accepted 2 September 1999
Abstract
Characterisation of internal atmosphere composition offers the potential to explain variability in responses of horticultural crops to modified atmosphere treatments and to quantify permeance of fruit skins to the respiratory
gases. In this paper, the theoretical basis by which fruit skin permeance can be calculated from other gas exchange variables is presented. Surface chambers close to equilibrium with the fruit’s internal atmosphere were used to
monitor internal atmosphere composition of sweet pepper Capsicum annum, cv. Reflex. Physical equilibration of chamber contents over wounded fruit surface was essentially complete in less than 4 h. However, physiological drift
in internal atmosphere composition meant that substantial changes continued to develop over more extensive periods. Removal of cuticle beneath the chamber was shown to be essential for equilibration of chamber contents within
physiologically meaningful periods. Samples of atmosphere removed destructively from the fruit cavity consistently contained more O
2
but less CO
2
than samples similarly removed from the fruit flesh. Levels of CO
2
were higher in samples removed directly from the flesh by syringe than in those taken from surface chambers, indicating potential
for an effect of the vacuum used to take direct removal samples on sample composition. Permeance of pepper cuticle to CO
2
was about ten times greater than that to O
2
244 and 24 pmols
− 1
m
− 2
per Pa, respectively. Removal of cuticle dramatically increased permeance of the fruit surface and hastened equilibration of surface chambers with the
fruit’s internal atmosphere. Surface chambers adhered over fruit surface from which the cuticle has been removed would be the most reliable means to assess composition of the atmosphere in immediate contact with the cells of
pepper tissue. © 2000 Elsevier Science B.V. All rights reserved.
Keywords
:
Carbon dioxide; Cuticle; Equilibration; Gas transfer; Oxygen; Respiration; Wounding www.elsevier.comlocatepostharvbio
1. Introduction
Much of the variability in responses of fruits and vegetables to modified atmospheres can be
explained if these are considered on the basis of internal, rather than external, atmosphere compo-
sition Burton, 1982; Banks et al., 1994; Dadzie et
Corresponding author. Tel.: + 64-6-3505551. E-mail address
:
s.e.nicholsonmassey.ac.nz S.E. Nichol- son
0925-521400 - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 5 2 1 4 9 9 0 0 0 6 4 - 2
al., 1996. Ways to characterise internal atmo- sphere composition that develops in response to
environmental manipulation are therefore of po- tential value to those attempting either to explain
or manage the effects of modified atmospheres on fruit and vegetable storage behaviour.
Proven methods to study internal atmosphere composition are not available for a wide range of
crop types. Rajapakse et al. 1990 reported satis- factory use of surface chambers for study of inter-
nal atmosphere composition in three fruit types apple, Asian pear and nectarine. In this study,
we evaluated the potential for use of such surface chambers for characterising internal atmosphere
composition of sweet pepper Capsicum annum, cv. Reflex and, by studying the kinetics of ex-
change between such chambers and the internal atmosphere, as a means of estimating skin perme-
ance of the fruit to O
2
and CO
2
. This is of particular interest in solanaceous fruits because
most of the fruit surface is devoid of pores Blanke, 1986; Blanke and Holthe, 1997. Such
fruits should therefore make ideal subjects for assessing whether fruit cuticles are differentially
permeable to the two respiratory gases, as pro- posed by Ben-Yehoshua et al. 1985. Differential
permeability of the cuticle to these two gases has been invoked as a means to explain the overall
differential permeability of fruit skins that devel- ops after treatment with surface coatings Banks
et al., 1997. In addition, the sweet pepper has an internal cavity, the contents of which might be
expected to be similar to the true internal atmo- sphere surrounding the cells within the tissue. If
cavity atmosphere and flesh internal atmosphere were shown to be identical, then characterising
internal atmosphere of sweet pepper, and perhaps other capsicum fruits, would be as simple as de-
termining composition of samples taken from the cavity within the fruit. On the other hand, if the
permeance of the cuticle on the outer fruit surface were low, this raises the interesting possibility that
the internal atmosphere of the flesh may be more modified than the cavity that lies anatomically
within it. In this study, we evaluated these propo- sitions through experiments that involved sam-
pling of internal atmosphere from chambers close to equilibrium with the internal atmosphere of the
fruit flesh and by direct removal from both the flesh and the inner cavity.
1
.
1
. Theoretical de6elopment The derivation of parameters associated with
equilibration of chambers of this type with the internal atmosphere with which they are in con-
tact has been published previously Banks and Kays, 1988. However, they are presented here in
the new units recommended by Banks et al. 1995 for the sake of clarity.
Exchange between the chamber and the internal atmosphere of gas j r
j ch,t
, mol s
− 1
at time t after sealing or flushing s can be expressed in two
ways: r
j
ch,t
= P
j
· A · p
j i
− p
j ch,t
1 where P
j
is the permeance of the skin to gas j mol s
− 1
m
− 2
per Pa, A
ch
is the area of skin beneath the chamber m
2
, p
j i
is the partial pressure of gas j beneath the skin surface Pa, p
j ch,t
= partial pres-
sure of gas j inside the chamber Pa at time t and: r
j
ch,t
= V
ch
R · T + 273.15 ·
dp
j ch,t
dt 2
where R is the gas constant 8.314 m
3
Pa mol
− 1
per K, T is the temperature °C, V
ch
is the chamber volume m
3
. Re-arranging and integrating provides:
lnp
j i
− p
j ch,t
= −
P
j
· A
ch
· R · T + 273.15 · t V
ch
+ k
1
3 where k
1
is the integration constant. If p
j ch,t
at t = 0, then k
1
= lnp
j i
, so that: ln
1 − p
j ch,t
p
j
i
= −
P
j
· A
ch
· R · T + 273.15 · t V
ch
4 or
p
j ch,t
= p
j i
· 1 − exp
− P
j
· A
ch
· R · T + 273.15 · t V
ch
5 Times required for 99 equilibration of cham-
ber contents t
0.99
, s with the internal atmosphere can be calculated from the values for the decay
constant k
2
= − P
j
· A
ch
· R · T + 273.15V
ch
, s
− 1
in the function used to derive fitted curves to plots of chamber atmosphere composition with
respect to gas j p
j ch,t
versus time after flushing with nitrogen.
p
j ch,t
= p
j i
· 1 − e
k
2
− t
6 t
0.99
= −
ln100 k
2
7 Permeance to gas j can be estimated from k
2
for the same gas as follows:
p
j
= k
2, j
· V
ch
A
ch
· R · T + 273,15 8
which, given the values of V
ch
, A
ch
and T used in this study for chambers over intact and wounded
cuticle equated
to −
5.44 × 10
− 6
· k
2,j
and −
2.27 × 10
− 5
· k
2,j
, respectively.
2. Materials and methods
