if temperature increases Kader et al., 1989; Cameron et al., 1995. MA packaging has been
shown to prolong shelf life of green capsicums Capsicum annuum L. Bussel and Kenigsberger,
1975; Hughes et al., 1981; Meir et al., 1995; Lerdthanangkul and Krochta, 1996, but little has
been published on the possibilities and limitations of MA packaging for bell peppers.
Gas permeation rate through the film, pro- duct respiration, and CO
2
and O
2
levels within the MA package change dynamically with time and
temperature. MA packaging models have been developed, that describe the O
2
and CO
2
levels in polymeric film packages as affected by film
permeability, temperature and the rate of gas exchange of the stored product Beaudry et al.,
1992; Cameron et al., 1994; Merts, 1996; Hertog et al., 1997a, 1998. The general applicability
of such predictive models strongly depends on the completeness of information on how, at different
temperatures, gas exchange depends on the fruit’s internal atmosphere Banks et al., 1994 and on
how this, in its turn, depends on the gas condi- tions inside the package.
Although the effect of O
2
on fruit respiration rate is consistent in its general appearance and
can be successfully described applying Michaelis – Menten kinetics Chevillotte, 1973; Cameron et
al., 1994; Dadzie et al., 1996 the effect of CO
2
is more difficult to generalise due to the less consis-
tent responses reported. For capsicum, the possi- ble effect of CO
2
on respiration has not yet been reported.
Green capsicums have been reported to respond very favorably to seal-packaging techniques Ben-
Yehoshua et al., 1995. Since they do not exhibit marked ripening and climacteric processes Ben-
Yehoshua et al., 1995 they are expected to re- spond
to MA
packaging in
a stable
and predictable way.
In this study, we have characterised the effect of temperature on film permeability, respiration
rate and atmosphere compositions inside the package and inside the cavity of the packed cap-
sicums. We have used these data to develop a model that describes the complex system of MA
packed capsicums.
2. Materials and methods
2
.
1
. Film permeability The permeability of a 60 mm thick low-density
polyethylene LDPE film Transpak, Auckland, New Zealand at 0, 5, 10, 15, 20, 25, 30 and 35°C
9 0.5°C was determined using a continuous flow system Merts, 1996 with the film mounted
in a small PVC permeability cell. Eight samples from the outlet stream were analysed for each of
three replicate runs.
At steady state, the outflow of O
2
and CO
2
should equal their diffusion through the film re- sulting in:
F
out
· c
i out
· p
atm
R·T =
P
i film
· A
film
D X
film
· c
i in
− c
i out
· p
atm
1 were F
out
represents rate of outflow m
3
·s
− 1
, c
i out
concentration of gas i either O
2
or CO
2
in the outlet stream m
3
·m
− 3
, p
atm
atmospheric pres- sure Pa, R the universal gas constant 8.314
J·mol
− 1
·K
− 1
, T temperature of the air K, P
i film
permeability of
the film
to gas
i mol·s
− 1
·m·m
− 2
·Pa
− 1
, A
film
exposed area of the film m
2
, DX
film
thickness of the film m and c
i in
concentration of gas i in the inlet stream m
3
·m
− 3
. Solving for P
i film
results in: P
i film
= F
out
· c
i out
· DX
film
A
film
· c
i in
− c
i out
· 1
R · T 2
which was used for calculating film permeability to O
2
and CO
2
.
2
.
2
. Packaging experiment One hundred and twenty freshly harvested, ma-
ture, greenhouse grown green bell peppers Cap- sicum annuum L., cv. Tasty of uniform size were
obtained directly from a local grower in Palmer- ston North, New Zealand.
The initial respiration rate of each individual fruit at 20°C was determined by sealing them in a
1.1-l container for 1.5 h, producing an increase in CO
2
of up to about 0.5 kPa. Before sealing into packages, cannulae 14 gauge stainless steel
needles, cut down to 2-cm length were inserted
through the fruit wall into the cavity of the fruit. The connection between cannula and skin was
sealed gas-tight using epoxy adhesive 5 min cure; Areldite
®
, Ciba – Geigy, Auckland, New Zealand. Once the cannulae were attached, fruit were equi-
librated for 24 h overnight and steady state inter- nal gas conditions at 20°C in air were determined
by sampling 100 ml from the fruit cavities through the cannulae.
The fruit were randomly divided into four groups of 30, one lot for each of the four temper-
atures 0, 12, 20 and 30°C. Each fruit was indi- vidually
packed. Within
each temperature
treatment, 15 packages had soda lime added to remove all CO
2
, while the remaining 15 packages had no soda lime added to allow accumulation of
CO
2
. Within each soda lime treatment the 15 packages all had different areas of permeable film
exposed 0.806, 0.0018, 0.005, 0.015, 0.025, 0.034, 0.044, 0.054, 0.068, 0.088, 0.096, 0.108, 0.196,
0.240 or 0.486 m
2
. The remaining package area consisted of impermeable plasticised foil.
The cannulae on the fruit were connected to sampling ports on the outer surface of the pack-
ages using flexible tubing internal diameter: 0.5 mm, length: 7 cm. Through these, internal atmo-
spheres of the capsicum cavities could be moni- tored. Another sampling port was added to each
package for sampling the O
2
and CO
2
levels in the package. Silicone sealant Window and Glass sili-
cone, acid cured, Selleys Chemical Company Ltd., Auckland, NZ was used to seal all connections
on the surface of each package. Mesh was added in each package between fruit and film to ensure
the complete area of the package film being avail- able for gas exchange. To promote rapid equili-
bration
package volumes
were reduced
by reducing the headspace. At steady state, 100 ml
samples were taken to determine the gas condi- tions inside the cavity and inside the bag. Given
that, at steady state, gas exchange equals the diffusion through the film, O
2
consumption r
O
2
, mol·kg
− 1
·s
− 1
and CO
2
production r
CO
2
, mol·kg
− 1
·s
− 1
could be calculated as:
r
O
2
= P
O
2
film
· A
film
D X
film
· p
O
2
atm
− p
O
2
pkg
M 3
r
CO
2
= P
CO
2
film
· A
film
D X
film
· p
CO
2
pkg
− p
CO
2
atm
M 4
where: M is fruit mass kg, p
O
2
pkg
and p
O
2
atm
the oxygen partial pressures Pa and p
CO
2
pkg
and p
CO
2
atm
is the CO
2
partial pressures Pa in the package and in the surrounding atmosphere.
2
.
3
. Additional respiration measurements A second batch of freshly harvested, mature,
greenhouse-grown green bell peppers Capsicum annuum L., cv. Tasty was obtained from the same
local grower in Palmerston North, New Zealand. Respiration rates of 30 individual fruit were
determined at 20°C by measuring the accumula- tion of CO
2
. Subsequently, they were divided into three groups of ten fruit each, for measurements
at either 0, 12 or 30°C. The respiration rate of each fruit was again measured individually. The
times for which each fruit was sealed in the 1.1-l jar were 4.5, 2.5, 1.5 and 0.5 h at 0, 12, 20 and
30°C, respectively.
2
.
4
. Gas analysis All gas samples were analysed using an O
2
electrode Citicell CS type, City Technology Ltd., London, UK in series with a miniature infra-red
CO
2
transducer Analytical Development Com- pany, Hoddesdon, UK, with O
2
-free N
2
as car- rier gas flow rate 35 ml·min
− 1
. Output signals were analysed using HP integrators Hewlett
Packard, model 3396A. Commercially prepared standards were used for calibration of the gas
analysers. Ambient pressure data were collected with a pressure transducer Barigo Electronic Al-
timeter, Barigo Barometerfabrik GmBH, D-7730 Villingen Schwenningen.
2
.
5
. Data analysis All data collected were expressed according to
the units proposed by Banks et al. 1995. The data were analysed statistically with the iterative
non-linear regression routine of Statistical Analy- sis System SAS, 1992. The data on film perme-
ability were analysed using the temperature dependence according Arrhenius’ law from Eq.
10. The data from the packaging experiment were analysed together in one run, simultaneously
using temperature, film area, p
O
2
air
and p
CO
2
air
as independent variables and r
O
2
, r
CO
2
, p
O
2
cav
and p
CO
2
cav
as dependent variables using the model formula- tion from Eqs. 14, 15, 8 and 9 with the
temperature dependence according Arrhenius’ law Eq. 10 applied to r
O
2
max
, r
CO
2
max
, P
O
2
film
and P
CO
2
film
multi-response, multivariate, non-linear regres- sion analysis. The data on temperature depen-
dence of the additional respiration measurements were analysed using the temperature dependence
according to Arrhenius’ law Eq. 10. The refer- ence temperature for Arrhenius’s law was in all
cases fixed at 15°C 288.15 K. The non-linear equations were applied directly, without transfor-
mation to data or equations.
3. Model development
