research has focussed on post-cutting processes and application of anti-browning chemicals or
artificial coatings Baldwin et al., 1996; Gil et al., 1998. Post-cutting application of low O
2
or pure N
2
atmospheres have been shown to be useful in delaying apple slice softening and browning, but
these atmospheres can induce flavor problems due to accumulation of acetaldehyde, ethanol, ethyl
acetate and other volatiles Toivonen, 1997. In terms of chemical approaches, ascorbic acid vita-
min C has been widely used as an anti-browning additive Vamos-Vigyazo, 1981. However, ascor-
bic acid is quickly consumed in the anti-browning process, and thus provides only temporary protec-
tion Vamos-Vigyazo, 1981; Sapers, 1993. The fresh-cut industry would like to use approaches to
maintain fresh flavor, color and texture of cut fruit with little or no addition of chemical addi-
tives Anon., 1995, therefore non-chemical treat- ments are of interest.
It is generally accepted that O
2
plays a major role in apple browning Whitaker et al., 1998.
Reduced O
2
atmospheres have been shown to be beneficial in maintaining quality and extending
the shelf life of apple slices Nicoli et al., 1994; Gil et al., 1998 as well as pear and strawberry slices
Rosen and Kader, 1989. Low O
2
atmospheres inhibit color and texture changes of sliced
‘Granny Smith’ apples, however, they cause devel- opment of off-flavors Ke et al., 1991 and fer-
mentative deterioration Watada et al., 1998. Recent research on quality changes of several
fresh-cut fruits, has shown that low oxygen atmo- sphere storage has significant inhibitory effects on
the respiration and ethylene production as well as quality retention Qi and Watada, 1997. Using a
different approach, researchers have recently tested the use of superatmospheric O
2
levels on fruit. Solomos et al. 1997 applied pure O
2
to whole ‘Gala’ and ‘Granny Smith’ apples and this
treatment reduced both respiration rate and ethylene production rate to similar levels as low
O
2
treatment when applied at 18°C. It has also been found that pure O
2
atmosphere treatment for 4 weeks at 2°C inhibits both cut surface
browning and chilling injury in peaches Lu and Solomos, unpublished data.
Whole fruit have been shown to be more able to adapt to stresses such as chilling than cut pieces
of the same fruit Kang and Lee, 1997. Therefore it was hypothesized that pretreatment of whole
apple fruit prior to slicing could provide an ap- proach to modifying deterioration of tissue when
it is subsequently sliced and packaged rather than applying the treatment after the fruit had been
cut. The objective of this work was to investigate and compare the effects of 1 and 100 kPa O
2
pretreatments in whole apple fruit and their resid- ual effect on shelf life of subsequently cut slices.
Changes in cut surface color, tissue solute leak- age, firmness and the accumulations of acetalde-
hyde, ethanol and ethyl acetate were evaluated for apple slices that had been cut from whole apples
pretreated in 100 kPa O
2
, 21 kPa O
2
air control and 1 kPa O
2
and stored 14 days in modified atmosphere packages.
Two different experiments were conducted. The first experiment was designed to evaluate the ef-
fect of pretreatment duration of whole apples with 1, 21, and 100 kPa O
2
atmospheres on subsequent quality of packaged slices, made from the whole
apples, at 14 days of storage. The second experi- ment was designed to evaluate both quality and
physiological changes in packaged slices made from apples that had been pretreated for 12 days
with 1, 21, and 100 kPa O
2
atmospheres prior to slicing.
2. Materials and methods
2
.
1
. Apples Controlled atmosphere-stored ‘Spartan’ apples
were purchased from a local packing house in February from the Okanagan Valley, BC, and
stored for 14 days at 1°C in air prior to initiation of the experiments.
2
.
2
. Atmospheric pretreatments Three replicates of 15 apples were sealed in 3.8-l
glass jars at 1°C to which one of the following three atmospheric treatments were applied; 1 kPa
O
2
balance N
2
, air 21 kPa O
2
, and 100 kPa O
2
at a flow rate of 30 ml min
− 1
at 100 kPa total pressure. The gas streams were then humidified by
purging through gas washing bottles containing distilled water prior to being fed into the jars
containing the fruit. Five apples from each repli- cate of each treatment were removed after being
held for 5, 12 or 19 days.
2
.
3
. Slice preparation and modified atmosphere storage
After removal from atmospheric pretreatment, each apple was cut longitudinally into 12 slices
and the seeds and core removed using a sharp stainless steel paring knife. Three slices from each
of five fruit were then segregated into each repli- cate totalling 400 9 5 g in weight. Three replica-
tions were thereby selected randomly from each pretreatment after each of the three pretreatment
durations. Immediately after the cutting opera- tion, slices were vacuum-sealed into bags made of
40-mm-thick low density polyethylene OTR 2.28 fmol s
− 1
m m
− 2
Pa
− 1
at 23°C using a Swiss Vac Vacuum bag sealer Neue Transvac
Maschinen AG, Lucern, Switzerland and these were then stored at 1.0°C. The packaging film
OTR was approximately 0.95 fmol s
− 1
m m
− 2
Pa
− 1
at 1°C Dr A.L. Moyls, Agriculture and Agri-Food
Canada, Summerland,
BC, pers.
commun..
2
.
4
. Tissue fermentation product content Three slices from each treatment replication
were diced finely with a sharp stainless steel knife. Five grams of diced apple slice tissue were then
randomly selected and homogenized on ice in 15 ml of saturated CaCl
2
solution for 45 s with a Brinkman model PT 1035 Polytron Kinematica,
Switzerland on a speed setting of four. The resul- tant slurry was then centrifuged model RC-5,
Sorvall Products, Wilmington, DE at 5000 × g for 5 min. Three milliliters of the supernatant was
transferred to a 25-ml vial which was then closed with a septa seal. The vial was then incubated for
1 h at 37°C. A 0.5-ml sample from the headspace of the vial was taken using a gas-tight syringe and
measured using a gas chromatograph Varian model 3700, Mississauga, Ont. fitted with a flame
ionization detector and 2 m × 2 mm i.d. glass column packed with 6080 Carbopack B coated
with 50 Carbowax 20M Supelco, Oakville, Ont.. The column temperature was 80°C and the
nitrogen carrier gas flow was 20 ml min
− 1
. Ac- etaldehyde, ethanol and ethyl acetate standards
were prepared from authentic compounds Sigma, St Louis, MO in saturated CaCl
2
solution.
2
.
5
. Package headspace atmosphere analysis The package atmospheres were monitored for
CO
2
and O
2
levels every 2 – 3 days by withdrawing a 1-ml gas sample from the bags using a gas-tight
syringe. A 0.2-ml volume of this sample was then analyzed using a gas chromatograph Shimadzu
GC-14A, TekScience, Oakville, Ont. fitted with a TCD detector, using a dual column configuration
consisting of a 2.44 m × 3 mm o.d. stainless steel column packed with 80100 mesh molecular sieves
5A and a 1.88 m × 3 mm o.d. stainless steel column packed with 80100 mesh Porapak Q Su-
pelco, Oakville, Ont.. The gas chromatograph was calibrated using commercial primary gas
standards and O
2
corrected for argon, which co- elutes with O
2
Beveridge and Day, 1991. Ethylene was measured with the other 0.5-ml
sample using a gas chromatograph Varian model 3700, Varian Canada, Mississauga, Ont. fitted
with a flame ionization detector and 2 × 2 mm i.d. glass column packed with 6080 Carbopack B
which was coated with 5 Carbowax 20M Su- pelco, Oakville, Ont.. The column temperature
was 80°C and the nitrogen carrier gas flow rate was set at 20 ml min
− 1
. A 1 ml l
− 1
ethylene primary standard in N
2
was used to calibrate the gas chromatograph response.
2
.
6
. Experiment
1
This experiment was designed to evaluate the effect of pretreatment duration of whole apples
with 1, 21, and 100 kPa O
2
atmospheres on tissue fermentation product content and also on the cut
surface browning of packaged slices that were kept for 14 days at 1°C.
Three apples from each of three replicate jars from 1, 21, or 100 kPa O
2
atmospheres were removed on days 5, 12 and 19 of pretreatment.
Each apple was cut into 12 slices as described above. Three slices one from each apple in the
replicate were immediately tested for tissue fer- mentation products content. Five slices from each
apple in the replicate were placed into modified atmosphere packaging and held for 14 days at
1°C, as described above. At 14 days, the 15 slices in each package were removed and the color
evaluated with a Minolta chroma meter CR300, Minolta, Ramsey, NJ using the L values of the
Commision Internationale de l’Eclairage CIE color system Gil et al., 1998. Each of the 15 slices
in each package was evaluated separately and values were averaged to give a single value for
each of the three replicate packages of each treat- ment.
2
.
7
. Experiment
2
This experiment was designed to evaluate both quality and physiological changes in modified at-
mosphere packaged slices made from whole apples that had been pretreated for 12 days at 1°C with
1, 21, or 100 kPa O
2
atmospheres prior to slicing. Apples from each of three replicate jars from
the three atmosphere pretreatments 1, 21 or 100 kPa O
2
were removed after 12 days, sliced and sealed in modified atmosphere packages as de-
scribed above. A total of nine packages were made for each treatment of the three treatments i.e.
three replicates × three
destructive sampling
times. The package headspace CO
2
, O
2
and C
2
H
4
levels were determined every 2 – 3 days. The content of fermentation products was deter-
mined from three slices from each treatment repli- cation at each sampling time. Firmness was
measured on diced tissue from three slices from each treatment replication at each sampling time
with a model 4201 Instron instrument Canton, MA fitted with a Kramer shear press cell. A 50-g
sample of diced apple from above was placed into the shear press for analysis. The probe extension
distance was set as 90 mm and the rate of exten- sion was set at 100 mm min
− 1
. Results were expressed in Newtons N.
Five plugs weighing 5 g in total were cut, using a c 4 cork borer, from three apple slices from
each treatment replication at each sampling time at 2 and 1 days after the bags were opened on day
9 and 14 of storage, respectively. These cores were sectioned into 4-mm lengths and then rinsed twice
in distilled H
2
O. The plugs were then placed in 30 ml fresh distilled H
2
O and exposed twice to 2-min durations of light vacuum − 25 kPa to allow
rapid infusion of bathing solution into the tissue mass and enhance effusion of the solutes from the
plug tissue Lu and Ouyang, 1990. The sample was then shaken on an orbital shaker for 30 min.
The tubes containing the tissue in bathing solution were centrifuged for 5 min at 121 × g in a cen-
trifuge model RC-5, Sorvall Products, Wilming- ton, DE to deposit the tissue pieces at the bottom
of the tube. The absorbance of the supernatant was measured with a quartz cuvette at 280 nm.
Relative solute leakage was determined as previ- ously described Redmann et al., 1986; Lu and
Ouyang, 1990. This method measures leakage of ultraviolet light-absorbing compounds from tis-
sues, primarily free amino acids. Leakage of these compounds is considered to reflect relative mem-
brane leakage of cells within the tissue Redmann et al., 1986.
2
.
8
. Statistical analysis The data from both experiments were analyzed
as a randomized complete block design using the General Linear Models procedure SAS, Cary,
NC, with treatment and time as the independent variables. In all cases, treatment, time and the
interaction terms were all statistically significant P 5 0.05. Therefore the data are presented in
graphs to evaluate the simple effects and the variation is expressed as S.E. of the means for each
data point.
3. Results and discussion
