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
3
.
1
. Experiment
1
Pretreatment with 100 kPa O
2
resulted in maintenance of higher L values, indicating lower
cut surface browning in apple slices stored in modified atmosphere packages for 14 days as
compared with the other two pretreatments Fig. 1. Slices made from apples pretreated with 1 kPa
O
2
browned more than those pretreated at 100 kPa O
2
but less than those pretreated in air. Low oxygen has been shown previously to inhibit cut
surface browning in apples Gil et al., 1998, however 100 kPa O
2
pretreatment appeared to be even more effective in reducing cut surface brown-
ing. After 19 days pretreatment, apple slices made from fruit pretreated with 100 kPa O
2
began to show more browning than those pretreated for
shorter durations. It appeared that the longer 19-day pretreatment duration was deleterious to
apple tissue. A deleterious effect of high O
2
has been demonstrated for apples with longer dura-
tion exposures Solomos et al., 1997. This inter- pretation was supported by the fact that ethanol,
acetaldehyde and ethyl acetate remained negligi- ble in the 100 kPa O
2
treatment until day 19 of pretreatment, then tissue levels increased signifi-
cantly Fig. 2. It is interesting to note that the levels of fermentation products in the air pretreat-
ment were similar to those in the 100 kPa pre- treatment. Since a reduced effect on preventing
cut surface browning was seen after 12 days of pretreatment in the 100 kPa O
2
, subsequent exper- imentation was performed only on apples pre-
Fig. 2. Accumulation of fermentation products in whole apple tissue during pretreatment in 1, 21, or 100 kPa O
2
atmospheres for 5, 12, or 19 days. S.E. of the means are indicated by
vertical lines through a data point n = 3. Points not showing error values are hidden by the symbols for the data point.
Fig. 1. Browning of slices cut from apples pretreated in 1, 21, or 100 kPa O
2
atmospheres for 5, 12, or 19 days measured as L values after 14 days at 1°C in modified atmosphere pack-
ages. A lower L value indicates greater degree of cut surface browning. S.E. of the means are indicated by vertical lines
n = 3.
treated for 12 days prior to slicing, packaging and storage.
3
.
2
. Experiment
2
The atmospheres of the packages containing slices from the apples pretreated with air were
modified to a greater extent than those from the 1 and 100 kPa O
2
pretreatments Fig. 3. This sug- gests that both the 1 and 100 kPa O
2
pretreat- ments reduced respiration rates of the apple slices
and this reduction was maintained throughout the 14 days at 1°C. Such a response is consistent with
reports for response to low O
2
by apple slices Gil et al., 1998 and whole apples to high O
2
Solo- mos et al., 1997. The CO
2
concentration in pack- ages containing slices from apples pretreated with
100 kPa O
2
was relatively stable between day 2 and day 14 of storage, whereas the CO
2
concen- trations in packages containing slices from the
two other pretreatments increased linearly. The ethylene concentrations in the packages from the
1 and 100 kPa O
2
treatments were lower than in the packages containing slices from the air pre-
treatment for the first week Fig. 3, but after 9 days of storage at 1°C, the concentrations of
ethylene were similar for slices from all three treatments. The reduction of ethylene production
with low O
2
exposure in fruit slices has been demonstrated previously Gil et al., 1998. Whole
apples have responded similarly to high O
2
Solo- mos et al., 1997. These results indicate that both
1 and 100 kPa O
2
pretreatments may have resid- ual effects on respiration and ethylene production
of slices made from pretreated apples and this leads to development of different package atmo-
spheres. Inhibition of respiration by low and high oxygen has been reported previously Turner and
Quartley, 1956; Haugaard, 1968; Kader, 1986. However, there are differences in the basis of how
low and high O
2
exposure inhibit respiration. Low O
2
atmospheres have been shown to inhibit respi- ration through a shift to anaerobic respiration
Kader, 1986, where glycolysis predominates and the Krebs cycle is essentially shut down. In con-
trast, high O
2
directly interrupts the operation of the Krebs cycle without inducing anaerobic respi-
ration Turner and Quartley, 1956; Haugaard, 1968.
The tissue content of anaerobic volatiles was consistently highest in the slices from apples pre-
treated in 1 kPa O
2
Fig. 4. This difference was generated during the pretreatments Fig. 2 and
maintained after slicing and storage in modified atmosphere packaging at 1°C. In general, the
anaerobic volatiles were highest during the stor- age at 1°C for apple slices made from fruit pre-
treated with 1 kPa O
2
. The package atmospheres for the slices from the 1 kPa O
2
pretreatment were not modified to the point where induction of
anaerobic metabolism would be expected Fig. 3, Ke et al., 1991. Therefore the higher levels of
fermentation products in the slices from 1 kPa O
2
pretreatment were likely associated with a residual response to the pretreatment rather than the
package atmospheres during storage. In addition, the package atmospheres for slices from apples
exposed to the air pretreatment were more highly modified, but significant levels of these volatiles
did not accumulate during storage at 1°C.
Collectively, the results suggest that the slices from apples pretreated with 1 kPa O
2
should have off-flavors. An informal organoleptic evaluation
confirmed this to be the case data not shown. This appears to be contradictory to results of Gil
et al. 1998, using ‘Fuji’ apples in their studies. Different cultivars of apples have different flavor
profiles Benoit Girard, Agriculture and Agri- Food Canada, Summerland, BC, pers. commun.
and also respond to low oxygen atmospheres to different degrees, and this is dependent on sugar
Fig. 3. Package CO
2
, O
2
, and ethylene concentrations for slices cut from apples pretreated in 1, 21, or 100 kPa O
2
atmo- spheres for 12 days and kept at 1°C for 2 weeks. S.E. of the
means are indicated by vertical lines through a data point n = 3. Points not showing error values are hidden by the
symbols for the data point.
Fig. 4. Tissue fermentation product concentration for slices cut from apples pretreated in 1, 21, or 100 kPa O
2
atmospheres for 12 days and kept at 1°C for 2 weeks. S.E. of the means are
indicated by vertical lines through a data point n = 3. Points not showing error values are hidden by the symbols for the
data point.
sure of apple slices Gil et al., 1998. The high oxygen pretreatment was superior for retaining
lightness of the cut surface. This superior effect of the high oxygen treatment in reducing browning
has been seen previously in peaches Lu and Solomos, unpublished data.
Firmness declined for all treatments during the 14 days in storage at 1°C Fig. 5. However, slices
from the 100 kPa O
2
pretreatment retained greater firmness than slices from the other two
pretreatments. This difference increased with longer storage durations. Slices from apples pre-
treated with 1 kPa O
2
were significantly firmer than those pretreated in air on day 14. Low
oxygen has been shown to increase firmness reten- tion in whole apples Kader, 1986 but no other
work has demonstrated a firmness effect of low oxygen on apple slices. The high oxygen pretreat-
ment appears to be much more effective in induc- ing firmness retention than the low oxygen
pretreatment.
Fig. 5. Cut surface L values and firmness for slices cut from apples pretreated in 1, 21, or 100 kPa O
2
atmospheres for 12 days and kept at 1°C for 2 weeks. S.E. of the means are
indicated by vertical lines through a data point n = 3. Points not showing error values are hidden by the symbols for the
data point.
content Ke et al., 1991. Higher sugar content tends to mask the detection of anaerobic products
by taste panelists Ke et al., 1991. The apples used in this work had been stored for 4 months,
and as such may have lost a significant amount of sugars. Therefore it is not surprising that low
oxygen treatments may or may not produce dis- cernible off-flavors, depending on the cultivar and
age of the apple in question.
The CIE L value was highest for slices from the 100 kPa O
2
pretreatment throughout the 14 days of storage at 1°C Fig. 5. Slices from the 1
kPa O
2
pretreatment retained their lightness better than the air pretreated slices i.e. they browned
less, with this effect becoming more pronounced at later sampling times. This is consistent with
results obtained by others using low oxygen expo-
Fig. 6. Solute leakage for tissue from slices cut from apples pretreated in 1, 21, or 100 kPa O
2
atmospheres for 12 days and kept at 1°C for 9 and 14 days. S.E. of the means are
indicated by vertical lines n = 3.
enriched O
2
data not shown much like the effect of hot water dips on inducing antioxidant protec-
tive systems in zucchini Wang, 1994.
4. Conclusions
