Directory UMM :Data Elmu:jurnal:P:Postharvest Biology and Technology:Vol18.Issue2.Mar2000:

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Effect of 1 and 100 kPa O

atmospheric pretreatments of

whole ‘Spartan’ apples on subsequent quality and shelf life

of slices stored in modified atmosphere packages

Changwen Lu, Peter M.A. Toivonen *

Agriculture and Agri-Food Canada,Pacific Agri-Food Research Centre,Summerland,BC,Canada V0H1Z0 Received 3 March 1999; accepted 30 September 1999

Abstract

Whole ‘Spartan’ apples were exposed to 1 kPa O2, air (21 kPa O2) and 100 kPa O2at 1°C for 5, 12 and 19 days.

Subsequent to the pretreatment, apples were cored, sliced and 40095 g of slices were sealed in 40-mm-thick low

density polyethylene film bags having a moderate O2transmission rate (OTR2.28 fmol s−1m m−2Pa−1at 23°C)

and held for up to 2 weeks at 1°C. Cut surface browning and flesh softening were inhibited for packaged slices by pretreating the whole fruit with 100 and 1 kPa O2as compared with those from the air pretreatment. However the

slices from the 100 kPa O2- and air-pretreated apples contained a much lower content of fermentation products

associated with off-flavors compared with the slices from apples pretreated in 1 kPa O2. Packages containing apple

slices from the air pretreatment developed a more highly modified atmosphere than packages containing apples slices from the other two pretreatments, suggesting that both 1 and 100 kPa O2pretreatments suppressed the respiration

rate of apple slices. Slices from apples pretreated with 1 kPa O2 developed more cut surface browning and greater

tissue solute leakage and enhanced accumulations of acetaldehyde, ethanol and ethyl acetate in apple slice tissue as compared with slices cut from the 100 kPa O2-pretreated apples. © 2000 Elsevier Science B.V. All rights reserved.

Keywords:Fermentation products; Pure oxygen; Browning; Solute leakage; Respiration; Firmness

www.elsevier.com/locate/postharvbio

1. Introduction

Fresh-cut products comprise approximately 10% of all fresh produce sold in North America and it is expected by the year 2000 that 25% of all produce sold will be in fresh-cut form (Vance

Publishing, 1996). Fresh-cut, packaged and ready-to-eat vegetables already have a significant market share and recently processors have begun placing more emphasis on the development of the fresh-cut fruit market. Unfortunately, it is difficult to consistently assure reasonable shelf life in fresh-cut fruit products due to the fact that many fruits must be processed at their optimal ripening qual-ity and therefore are at the brink of senescence (Gorny and Kader, 1996). Most of the current

This article is Centre Contribution Number 2022. * Corresponding author. Tel.: +1-250-4946386; fax: + 1-250-4940755.

E-mail address:[email protected] (P.M.A. Toivonen)

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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 O2 or pure

N2 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 O2 plays a major

role in apple browning (Whitaker et al., 1998).

Reduced O2 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 O2 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 O2 levels on

fruit. Solomos et al. (1997) applied pure O2 to

whole ‘Gala’ and ‘Granny Smith’ apples and this treatment reduced both respiration rate and ethylene production rate to similar levels as low

O2 treatment when applied at 18°C. It has also

been found that pure O2 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 O2

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 O2, 21 kPa O2 (air control)

and 1 kPa O2 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 O2atmospheres 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 O2 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 O2(balance N2), air (21 kPa O2), and 100 kPa O2

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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 40095 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 CaCl2 solution for 45 s with a

Brinkman model PT 10/35 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 60/80 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 CaCl2 solution.

2.5. Package headspace atmosphere analysis

The package atmospheres were monitored for

CO2and O2levels 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 80/100 mesh molecular sieves

5A and a 1.88 m×3 mm o.d. stainless steel

column packed with 80/100 mesh Porapak Q

(Su-pelco, Oakville, Ont.). The gas chromatograph was calibrated using commercial primary gas

standards and O2 corrected for argon, which

co-elutes with O2 (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 60/80 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 N2) 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 O2 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.

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Three apples from each of three replicate jars

from 1, 21, or 100 kPa O2 atmospheres were

Minolta, Ramsey, NJ) using theL* 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. Experiment2

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 O2atmospheres prior to slicing.

Apples from each of three replicate jars from the three atmosphere pretreatments (1, 21 or 100

kPa O2) 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 CO2, O2and C2H4

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 c4 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 H2O. The plugs were then placed in 30

ml fresh distilled H2O 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

(P50.05). 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 O2 resulted in

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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

O2 browned more than those pretreated at 100

kPa O2 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 O2pretreatment 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 O2 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 O2 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 O2 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 O2, 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 O2atmospheres

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 O2atmospheres for 5, 12, or 19 days) measured as

L* values after 14 days at 1°C in modified atmosphere pack-ages. A lowerL* 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 O2 pretreatments (Fig. 3). This

sug-gests that both the 1 and 100 kPa O2

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 O2by apple slices (Gil

et al., 1998) and whole apples to high O2

(Solo-mos et al., 1997). The CO2concentration in

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100 kPa O2 was relatively stable between day 2

and day 14 of storage, whereas the CO2

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 O2 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 O2 exposure in fruit slices has been

demonstrated previously (Gil et al., 1998). Whole

apples have responded similarly to high O2

(Solo-mos et al., 1997). These results indicate that both

1 and 100 kPa O2 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 O2exposure inhibit respiration. Low

O2atmospheres have been shown to inhibit

respi-ration through a shift to anaerobic respirespi-ration (Kader, 1986), where glycolysis predominates and the Krebs cycle is essentially shut down. In con-trast, high O2 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 O2 (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 O2. The package atmospheres

for the slices from the 1 kPa O2pretreatment 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 O2

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 O2should 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 CO2, O2, and ethylene concentrations for slices

cut from apples (pretreated in 1, 21, or 100 kPa O2

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.

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Fig. 4. Tissue fermentation product concentration for slices cut from apples (pretreated in 1, 21, or 100 kPa O2 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 O2 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 O2 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 surfaceL* values and firmness for slices cut from apples (pretreated in 1, 21, or 100 kPa O2atmospheres 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 O2 pretreatment throughout the 14

days of storage at 1°C (Fig. 5). Slices from the 1 kPa O2pretreatment 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

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expo-Fig. 6. Solute leakage for tissue from slices cut from apples (pretreated in 1, 21, or 100 kPa O2atmospheres 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 O2(data not shown) much like the effect

of hot water dips on inducing antioxidant protec-tive systems in zucchini (Wang, 1994).

4. Conclusions

Cut surface browning, flesh softening, and off-flavor were significantly inhibited in ‘Spartan’ ap-ple slices via pretreatment of the whole fruit with

100 kPa O2 before cutting. All three pretreatment

durations (5, 12 and 19 days) with 100 kPa O2

maintained the best quality in terms of low fer-mentation product accumulation, low cut surface browning and greatest flesh firmness retention over 14 days of storage at 1°C in modified atmo-sphere packages. Apple slices from apples

pre-treated with 1 kPa O2had an intermediate degree

of browning and retention of firmness but had very high levels of fermentation product accumu-lation which could be detected in an informal tasting. The inhibition of browning in the 100 kPa pretreatment could be associated with retention of cellular integrity, whereas there is another mecha-nism for inhibition of browning for slices made

from apples pretreated in 1 kPa O2. The use of

100 kPa O2 as a pretreatment prior to slicing of

apples may, at some point in time, reduce the dependence on antioxidant additives to inhibit browning of the fresh-cut apple slices. However, it

must be kept in mind that pure O2 is highly

flammable and therefore must be managed care-fully and with due caution. The safety issue must be worked out before such a treatment can be-come commercially viable.

References

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Baldwin, E.A., Nisperos, M.O., Chen, X., Hagenmaier, R.D., 1996. Improving storage life of cut apple and potato with edible coating. Postharvest Biol. Technol. 9, 151 – 163. Beveridge, T., Day, N.B., 1991. Respiration of sweet cherries

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Browning is associated with loss of membrane integrity that can occur during tissue deterioration and senescence (Toivonen, 1992). To test whether this was the case for these apple slices, solute leakage was determined on slices that had been removed from the packages for 1 – 2 days (Fig. 6). The leakage was greatest for apple tissue from

slices made from apples pretreated in 1 kPa O2,

and the lowest was in the slices from apples

pretreated with 100 kPa O2. The low leakage rate

associated with the low degree of cut surface browning of slices from apples pretreated in 100

kPa O2 was expected. However, a high leakage

associated with intermediate browning in the

slices pretreated with 1 kPa O2 was unexpected.

The slices from the high oxygen pretreatment, having lower membrane leakage would be able to maintain the separation of polyphenol substrates, which are localized in the vacuole, from polyphe-nol oxidase which is localized in the cytoplasm and various organelles (Lieberman and Biale, 1956). In the case of the slices from the low oxygen pretreatment, the reduction in browning in comparison with the air treatment must be associated with some inhibitory activity on the

PPO – polyphenol – O2 interaction required to

pro-duce tissue browning. The mechanism of this re-sponse is not presently understood. The lower membrane leakage in the slices from the 100-kPa pretreatment may be ascribed to the induction of antioxidant protectant systems for the membrane in response to the oxidative stress induced by

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Gorny, J.R., Kader, A.A., 1996. Fresh-cut fruit products. In: Cantwell, M. (Ed.), Fresh-cut Products: Maintaining Qual-ity and Safety. Postharvest Horticulture Series No. 10. Postharvest Outreach Program, University of California, Davis, CA, pp. 14-1 – 14-11.

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Three apples from each of three replicate jars from 1, 21, or 100 kPa O2 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 theL* 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. Experiment2

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 O2atmospheres prior to slicing.

Apples from each of three replicate jars from the three atmosphere pretreatments (1, 21 or 100 kPa O2) 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 CO2, O2and C2H4

levels were determined every 2 – 3 days.

expressed in Newtons (N).

Five plugs weighing 5 g in total were cut, using a c4 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 H2O. The plugs were then placed in 30

ml fresh distilled H2O 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 (P50.05). 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 O2 resulted in

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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 O2 browned more than those pretreated at 100

kPa O2 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 O2pretreatment 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 O2 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 O2 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 O2 treatment until day 19 of

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 O2atmospheres

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 O2atmospheres for 5, 12, or 19 days) measured as L* values after 14 days at 1°C in modified atmosphere pack-ages. A lowerL* 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 O2 pretreatments (Fig. 3). This

sug-gests that both the 1 and 100 kPa O2

et al., 1998) and whole apples to high O2

(Solo-mos et al., 1997). The CO2concentration in

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100 kPa O2 was relatively stable between day 2

and day 14 of storage, whereas the CO2

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 O2 treatments were lower than in

demonstrated previously (Gil et al., 1998). Whole apples have responded similarly to high O2

(Solo-mos et al., 1997). These results indicate that both 1 and 100 kPa O2 pretreatments may have

O2atmospheres have been shown to inhibit

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 O2 (Fig. 4). This difference was

for the slices from the 1 kPa O2pretreatment 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 O2

Collectively, the results suggest that the slices from apples pretreated with 1 kPa O2should have

Fig. 3. Package CO2, O2, and ethylene concentrations for slices

cut from apples (pretreated in 1, 21, or 100 kPa O2

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Fig. 4. Tissue fermentation product concentration for slices cut from apples (pretreated in 1, 21, or 100 kPa O2 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.

Firmness declined for all treatments during the 14 days in storage at 1°C (Fig. 5). However, slices from the 100 kPa O2 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 O2 were significantly firmer

Fig. 5. Cut surfaceL* values and firmness for slices cut from apples (pretreated in 1, 21, or 100 kPa O2atmospheres 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.

The CIE L* value was highest for slices from the 100 kPa O2 pretreatment throughout the 14

days of storage at 1°C (Fig. 5). Slices from the 1 kPa O2pretreatment 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

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expo-Fig. 6. Solute leakage for tissue from slices cut from apples (pretreated in 1, 21, or 100 kPa O2atmospheres 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 O2(data not shown) much like the effect

of hot water dips on inducing antioxidant protec-tive systems in zucchini (Wang, 1994).

4. Conclusions

Cut surface browning, flesh softening, and off-flavor were significantly inhibited in ‘Spartan’ ap-ple slices via pretreatment of the whole fruit with 100 kPa O2 before cutting. All three pretreatment

durations (5, 12 and 19 days) with 100 kPa O2

100 kPa O2 as a pretreatment prior to slicing of

apples may, at some point in time, reduce the dependence on antioxidant additives to inhibit browning of the fresh-cut apple slices. However, it must be kept in mind that pure O2 is highly

flammable and therefore must be managed care-fully and with due caution. The safety issue must be worked out before such a treatment can be-come commercially viable.

References

Anon., 1995. Firm debuts fresh-cut cantaloupe. Fresh-cut 3(6), 6 – 7, 36.

determined in sealed impermeable containers. Can. Inst. Food Sci. Technol. J. 5, 213 – 217.

Gil, M.I., Gorny, J.R., Kader, A.A., 1998. Responses of ‘Fuji’ apple slices to ascorbic acid treatment and low-oxygen atmospheres. HortScience 33, 305 – 309.

and the lowest was in the slices from apples pretreated with 100 kPa O2. The low leakage rate

associated with the low degree of cut surface browning of slices from apples pretreated in 100 kPa O2 was expected. However, a high leakage

associated with intermediate browning in the slices pretreated with 1 kPa O2 was unexpected.