Article pp.233-245 du Vol.24 n°3 (2004)

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ARTICLE ORIGINAL ORIGINAL PAPER

Effect of noble gases, nitrous oxide and nitrogen on the anaerobic metabolism and quality attributes

of mushroom (Agaricus bisporus L.) and sliced apple (Malus sylvestris Mil.)

Ozdemir I.S.¹*, P.-J. Varoquaux¹, F. Tournemelle¹, F. Yildiz²

RÉSUMÉ

Effet des gaz nobles sur le catabolisme fermentaire et sur les qualités des champignons (Agaricus bisporus L.) et des tranches de pomme (Malus sylvestris Mil)

Des champignons de Paris et des tranches de pommes ont été conditionnés sous des pressions partielles voisines de 99 kPa d’hélium, d’argon, de néon (dans le seul cas des champignons), de protoxyde d’azote et d’azote et con- servés à 10°C dans un système étanche. Dans des conditions anaérobiques, la production de gaz carbonique (Indice Fermentaire ou IF), la couleur et la fermeté, les concentrations en éthanol dans les tissus et dans l’espace de tête ont été mesurées. L’évolution des teneurs en sucres des tranches de pommes a été suivie.

Comparés à l’azote, les gaz nobles et le protoxyde d’azote ne diminuent pas significativement la vitesse du métabolisme anaérobie (taux de production d’éthanol et IF) ; ils n’affectent pas non plus la couleur ni la fermeté des champignons et des pommes tranchées. L’oxygène résiduel retenu dans les tissus est probablement responsable du léger brunissement, de la perte de fermeté et des sucres totaux. L’augmentation initiale de la production de CO₂ est probablement due à l’O₂ résiduel piégé dans les tissus qui a été estimé à 1,5 et 1,75 kPa respectivement pour les pommes tranchées et les champignons. Les quantités croissantes de gaz carbonique sont responsables du jaunissement et du maintien de la fermeté des champignons. Une corrélation élevée entre la teneur en éthanol dans les espaces de tête des récipients et celle des tissus végétaux a été mise en évidence.

1. Institut National de la Recherche Agronomique (INRA), UMR 408 Sécurité et Qualité des Produits d’Ori- gine Végétale, Domaine Saint-Paul, Site Agroparc, 84914 Avignon Cedex 9, France.

2. Middle East Technical University, Department of Food Engineering, Inönü Bulvari 06531 Ankara, Turkey.

* Corresponding author: Tel.: + 33 4 32 72 25 05; fax: + 33 4 32 72 24 92.

E-mail: [email protected]

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Mots clés

fermentation, éthanol, conditionnement sous atmosphère modifiée, facteur de qualité, physiologie végétale.

SUMMARY

Atmospheres containing 99 kPa of helium, argon, neon (for mushrooms only), nitrous oxide or nitrogen were flushed into airtight glass jars containing mushrooms or sliced apples to create anaerobic conditions at 10^oC. Ana- erobic CO₂ production rates (fermentative index, FI), color, firmness, tissue ethanol and headspace ethanol in the jars of both products were measured.

As compared to nitrogen, noble gases and nitrous oxide did not have any significant effect in reducing the rate of anaerobic catabolism (FI and ethanol production rate), color and firmness of either product. Increasing amounts of CO₂ were confirmed to be responsible for the yellow discoloration and firmness retention of mushrooms. High correlation coefficients were obtained between combined mean values of headspace ethanol, tissue ethanol and headspace CO₂ concentrations of mushrooms and sliced apples. Residual oxygen levels left in the jars and trapped in the tissues seem to be responsible for the initial rise in CO₂ production of sliced apples and mushrooms and were estimated as 1.5 and 1.75 kPa, respectively.

Key words

fermentation, ethanol, modified atmosphere packaging, quality attribute, plant physiology.

1 – INTRODUCTION

A major problem faced in the modified atmosphere packaging (MAP) of fruits and vegetables is the onset of anaerobic catabolism in packaged commodities inducing the generation of fermentative volatiles such as ethanol, acetaldehyde, ethyl acetate etc. (KADER et al., 1989). To overcome this problem, it is necessary to select packaging films with suitable permeabilities that are properly adjusted according to the respiration rate of the commodities to maintain the aerobic conditions inside the plant tissue (GOUBLE and VAROQUAUX, 1999).

However, commodities are sometimes stored anaerobically if the benefit outweighs the risk of a loss in quality (BEAUDRY, 1999). This was carried out by LEE et al. (1996) by creating an atmosphere with less than 0.5 kPa oxygen inside packages of prepared salads during shipping and the retail holding period. This resulted in a quality loss due to fermentation that was considered negligible compared with the high visual quality obtained. Thus, any solution that effectively hinders the anaerobic catabolism of plant tissues will be very beneficial for the preservation of produce in combination with the attainable advantages of anaerobiosis. Fresh sulfite-free pre-cut potatoes are currently packed in France under 99 kPa argon (VAROQUAUX, 1989).

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The results of studies concerning the effects of atmospheres containing noble gases and nitrous oxide on different quality attributes (physiological, biochemical, physical, microbial etc.) of fresh fruits and vegetables are varia- ble. DAY (2001) showed that the respiration rates of some selected prepared commodities were not significantly affected by high argon MAs. Similar results were also reported by BARRY-RYAN and O’BEIRNE (1998) for sliced apples, carrot discs and shredded lettuce under elevated (40%) argon atmosphere. Low oxygen controlled atmospheres (CA) (2%) containing 90%

argon, helium and nitrogen have been reported to be ineffective in delaying the accumulation of phenolics in fresh-cut lettuce and the loss of chlorophyll from broccoli florets (JAMIE and SALVEIT, 2002). SPENCER (1999) claimed that argon and the other noble gases neon, krypton and xenon reduced and controlled the activities of many enzymes in fruits and vegetables, thus effectively limiting respiration and enzymatic browning. Nitrous oxide, an anaesthetic used in medicine, has also been reported to delay climacteric rise in ethylene production rate of tomatoes (GOUBLE et al., 1995) and reduced the growth of some postharvest decay fungi (QADIR and HASHINAGA, 2001) and lowers the activity of cytochrome oxidase from pea leaves (CHERVIN and THIBAUD, 1992).

In this study, we investigated the effect of 99 kPa partial pressures of helium, argon, nitrogen, nitrous oxide and neon (for mushrooms only) on the anaerobic catabolism and quality attributes of sliced apples cv. “Golden Deli- cious” and mushrooms (Agaricus bisporus), which were selected as models because of the difference in their respiration rates and their sensitivity to enzymatic browning.

2 – MATERIALS AND METHODS

2.1 Plant materials

Apples cv. Golden Delicious were harvested from an orchard near Avignon, France. On arrival at the laboratory at INRA, the apples were sorted and stored in a cold room at 1 ˚C. The respiration rates in terms of oxygen consumption and carbon dioxide production of the sliced apples, RR_O2 and RR_CO2, at 10^oC (mean

± SD) were 0.27 ± 0.01 mmole.kg^-1.h^-1 and 0.28 ± 0.01 mmole.kg^-1.h^-1, respectively. The respiratory quotient (RQ), which is equal to RR_CO2/RR_O2, was 1.02 ± 0.01. Fermentation index (FI), CO₂ production rate under anaerobic conditions, was 0.19 ± 0.03 mmole.kg^-1.h^-1. Sliced apples had initial L*, a* and b* values of 83.6 ± 0.4, -2.7 ± 0.3 and 20.6 ± 1.4 respectively, and initial firmness was 11.3 ± 1.7 N. Apples of same batches were used for the replicate experiment.

Mushrooms (Agaricus bisporus L.) (white hybrids) were purchased from a local market near Avignon, France. On arrival, they were sorted for uniformity of appearance and size. Then, in a cold room at 10 ˚C, the stipes were cut 1 cm below the caps and immediately used for the experiments. The RR_O2 and RR_CO2 at 10 ^oC were 3.30 ± 0.03 mmole.kg^-1.h^-1and 2.84 ± 0.05 mmole.kg^-1.h^-1 with an RQ of 0.85 ±0.01. FI was measured as 0.21 ± 0.03 mmole.kg^-1.h^-1.

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Mushrooms had initial L* and b* values of 90.1 ± 1.7, and 14.5 ± 2.1, respectively, and an initial firmness of 14.5 ± 0.9 N.

2.2 Preparation of experimental sets

Apples with similar sizes were selected for the experiments. A slicer (Sirman, Perla 250, Padova, Italy) was used to simultaneously remove the core of the apple and cut the fruit longitudinally into 12 radial slices.

Approximately two hundred grams of prepared sliced apples or mushrooms were weighed precisely (± 0.1 g) and placed in 1.5 L airtight glass jars equipped with silicone septa in their lids for further gas analyses. In each jar about 15 apple slices or 17 mushrooms were needed to obtain 200 g of produce.

Prepared jars with their lids open, containing either sliced apples or mushrooms, were put into high-barrier plastic bags (Multi-layer coestruded film Cryo- vac FS 7100, 28×50 cm, P_O2(permeance to O₂) = 1 mL.m^-2.24 h^-1.atm^-1, P_CO2 (permeance to CO₂) = 2.5 mL.m^-2.24 h^-1.atm^-1) and placed in the filling port of a packing machine (Multivac A 300/16, Marne-la-Vallée, France) which was con- nected to the desired gas cylinder. Each jar was vacuumed to about 500 Pa pressure for 1 min. Then, the atmospheric pressure was restored by injecting 99kPa of the desired gas into the port. The open end of the plastic bags was sealed automatically and the lid of the jar was closed within the plastic bag, thereafter. The bags were then discarded. Atmosphere compositions were checked by gas chromatography for O₂, CO₂ and N₂ concentrations. All the jars were stored at 10 ± 1^oC in a temperature-controlled room during the experiments.

For each gas, a set of eight jars was prepared. Three jars of each set were analyzed twice a day, at 12-h intervals, for headspace gas composition (O₂, CO₂andethanol). Color and firmness were assessed every day for the mushrooms, starting from the second day of the experiments and every other day for the apples, starting from the third day of the experiments. Then, those samples were placed into plastic bags and stored in a freezer at –20˚C for biochemical analyses. The whole experiment was duplicated.

2.3 Headspace gas analysis

Headspace gas analyses were carried out on a gas chromatograph (MTI M200, Fremont, USA) consisting of two manifolds: one fitted with an MS-5A, 4-m capillary column thermostated at 80˚C and with helium as a carrier gas at a pressure of 110 kPa; the other fitted with a capillary Poraplot 4, 6-m column thermostated at 110˚C with argon as a carrier gas at a pressure of 193 kPa. Both manifolds were fitted with katharometric detectors. These analytical conditions permitted the elimination of argon in the oxygen peaks.

For nitrous oxide determination, a dilution method was used to avoid interference of nitrous oxide and carbon dioxide peaks. In this method, 5 mL of the sample taken with an airtight syringe from the headspace of the jar was injected in a 277 mL jar, with air inside. To quantify carbon dioxide in the sample, carbon dioxide in air within the jar must be taken into account.

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This initial concentration was multiplied by a dilution factor of 55.4 resulting in a contamination level ranging from 1.5 to 2.5 kPa. Moreover, the temperature of the first column was decreased from 80˚C to 30˚C for a better sepa- ration of these peaks.

Ethanol in the headspace of the jars was analyzed with a gas chromatograph (Intersmat IGC 121 FL) fitted with a Porapack Q DELSI, 3-m column with a diameter of 5 mm thermostated at 130˚C with nitrogen as a carrier gas at a pressure of 200 kPa and fitted with a flame ionization detector (FID) at 170˚C.

Injection temperature was 165˚C.

2.4 Respiration rate measurements

The RRs were determined at 10˚C by the glass jar technique (VAROQUAUX et al., 1996). Three jars were closed under air for initial RR measurements. RRs were calculated by linear regression from O₂ depletion and CO₂ production curves and expressed as mmole O₂ (CO₂).kg^-1.hr^-1. Mean values and standard deviations were measured on the three replicates.

2.5 Fermentative index determinations

Fermentative index was determined by the glass jar technique described above, but the headspaces of the jars were flushed with pure nitrogen. The gas composition within the jars was checked by gas chromatography to make sure the final partial pressure of nitrogen was at least 99 kPa. Gas samples were taken at 12-h intervals from the three jars per gas setting for the FI measurements under the tested gases.

2.6 Color measurement

Color was measured with a colorimeter (Minolta CM-1000, Minolta camera CO. LTD, Japan). The angle of view was 2˚ and illuminant used was D65. Meas- urements were made on one of the surfaces of all apple slices and on the center of all mushrooms caps. Results were expressed as CIE L*, a* and b* values and averaged.

2.7 Firmness measurement

Firmness measurements were performed with a computer driven firmness tester designed at INRA (DUPRAT et al., 1995). The indenter was a 4 mm diameter cylinder fitted with a hemispherical tip to avoid shearing of the plant tissues.

The maximum force (N) necessary to penetrate the tissue to a depth of 5 mm was recorded. All apple slices or mushrooms caps in the jars were tested and the results were averaged.

2.8 Determination of sugar and ethanol content in the fruit tissues Approximately 50 g of previously frozen sliced apples or mushrooms were weighed (± 0.1 g) and blended with 50 g of distilled water or 50 g of 4.5_✕10^–3 M potassium disulfite solution, respectively, in a Vorwerk blender for 30 sec at

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maximum speed and then centrifuged (10000_✕g for 10 min at 4 ˚C). Superna- tants were filtered through a 0.45 µm Acrodisc® (Gelman Sciences, USA) filter and injected into a Varian (Vista, 5000 series, Les Ulis, France) HPLC chromatograph fitted with a Polysphere® CHCA (0.65_✕30 cm) (Merck, Darmstadt, Ger- many) column thermostated at 85˚C. Sugars were eluted with DDI water at a flow rate of 0.5 mL.min^-1 and detected by differential refractometry (Knauer, Berlin, Germany). The instrument was calibrated with standards (Sigma): glucose, fructose, sucrose and ethanol (2/2/2 g and 5 mL.L^-1). The minimum sugar concentration detected by the differential refractometer was 0.2 mg.mL^-1.

2.9 Statistical Analysis

Three replicates were used per treatment and the whole experiment was duplicated. Data were analyzed by ANOVA and LSD (Tukey’s) test performed where appropriate. When ANOVA indicated that there were no significant difference among gas treatments, data for all treatments were pooled.

3 – RESULTS AND DISCUSSION

3.1 Sliced apples

3.1.1 Effects of applied gases on CO₂ production

As shown in Figure 1, the effect of anaerobic conditions in reducing CO₂ production rates of sliced apples was clear with significantly lower (LSD = 0.1004, p<0.01) FI values obtained under anaerobic conditions created by 99 kPa partial pressures of N₂, He, Ar and N₂O than for the CO₂ production rate under air after 4 days.

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Figure 1

Change in CO₂ partial pressures in the headspaces of jars with apple slices as a function of storage duration at 10˚C under the different atmospheres. Each data point represents the mean of triplicate measurements in the air curve and combined means of triplicate measurements under He, Ar, N₂ and N₂O atmospheres in the noble gas curve. Standard deviations are represented with vertical bars and they are not visible

when they are less than the size of the symbols.

However, there was no significant difference (p<0.05) among FI values of the sliced apples under the atmospheres of He, Ar, N₂O and N₂ during the same period. In all jars, during the initial 4 days, CO₂ production rates decreased from 0.29 ± 0.03 to 0.16 ± 0.03 mmole.kg^-1.h^-1 but CO₂ production rates of sliced apple under air stay unchanged in the same period. The similarity of CO₂ production rates indicates the coexistence of aerobic and anaerobic CO₂ production (RR_CO2 + FI) in anaerobic conditions during the initial 4 days. Aerobic CO₂ production might be induced due to oxygen trapped in the tissues and residual oxygen in jars which is difficult to remove reproducibly by vacuuming. The amount of residual oxygen in tissues can be estimated by calculating void volume in sliced apples by using its apparent density which was reported as 0.80 ± 0.02 g.cm^-3 and about 1 g.cm^-3 after vacuuming in water (YUWANA, 1997). In our conditions, these data correspond to about 40 mL void volume in apple tissue that contains at most 8 mL of oxygen corresponding to 0.5 kPa O₂ in the jars.

Therefore, total available oxygen in jars was estimated to be about 1.5 kPa including 1 kPa residual O₂ in the headspace of jars after vacuuming.

Our results are complementary to those of DAY (2001) and BARRY-RYAN and O’BEIRNE (1998) who reported that, under aerobic conditions, high Ar MA has a negligible effect on the respiration rates of fresh-cut produce. Thus, Ar was not found to be significantly effective in reducing both aerobic or anaerobic respiration of the fresh-cut produce, which is in contradiction with the findings of SPENCER (1996). The second clear conclusion is that N₂O was not found to be effective in reducing anaerobic respiration of the apple slices.

0 5 10 15 20 25

0 2 4 6 8 10

Storage duration (days) CO2 (kPa)

Noble gases Air

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3.1.2 Changes in ethanol and TSC contents in apple tissue

Tissue ethanol levels in sliced apple increased linearly during the experiment (figure 2). Sliced apples under anaerobic conditions had detectable ethanol content after a 3-day lag time (figure 2).

Figure 2

Change in the tissue ethanol content of sliced apples as a function of storage duration at 10˚C under the different atmospheres. Each data point represents the mean of triplicate measurements in the air curve and combined means of triplicate

measurements under He, Ar, N₂ and N₂O atmospheres in the noble gas curve.

Standard deviations are represented with vertical bars and they are not visible when they are less than the size of the symbols.

There was no significant difference (p<0.05) between the ethanol levels in tissues stored under the different atmospheres. The samples under air started to produce minute amounts of ethanol on day 11 of the experiment (0.0017 mg.(100 g.FW)^-1) (not visible on figure 2) when headspace oxygen amount decreased from 20.4 ± 0.1 to 0.35 ± 0.05 kPa. This value is close to the fermentation threshold reported by GRAN and BEAUDRY (1993) for this cultivar. Tissue ethanol concentrations were well correlated with the headspace ethanol and CO₂ concentrations (table 1).

y = 0.0252x - 0.0599 R² = 0.9935

0.00 0.05 0.10 0.15 0.20 0.25 0.30

0 2 4 6 8 10 12 14

Storage duration (days)

Ethanol (g/100 g.FW)

Noble gases Air

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Table 1

Correlation coefficients between tissue ethanol, headspace ethanol and headspace CO₂ contents of sliced apples and mushrooms.

This situation indicates that tissue and headspace ethanol levels reached an equilibrium after 3 days. The ratio between ethanol and CO₂ production rate was 57% in our experiments. BEAUDRY et al., (1993) found a ratio of 75% for blueberries and attributed this difference between the production rates of these fermentation products to carbon losses due to acetaldehyde volatilization and the interconversion of ethanol and acetaldehyde to other compounds (e.g. ethyl acetate). Change in the sugar content (fructose, sucrose and glucose) of sliced apples did not show a clear pattern under any conditions and were not correlated with measured fermentation products, ethanol (tissue and headspace) and CO₂. They fluctuated with small deviations around an average value during the storage period regardless of the atmospheres (data not shown).

3.1.3 Changes in color and firmness

There was no significant difference (p<0.05) among the L*, a* and b* values of the sliced apples under the applied atmospheres. Slices stored under air had significantly lower L* values (LSD = 0.7086, p<0.01) than slices stored under anaerobic atmospheres after day 6. Our results are in parallel with the results of JAMIE and SALTVEIT, 2002. They showed that low oxygen (2%) He and Ar CAs did not show any beneficial effect on color retention of lettuce leafs and broccoli florets compared to N₂ atmospheres.

Sliced apples stored under the various atmospheres almost maintained their initial firmness without any significant difference (p<0.05) among the different samples. After 11 days of storage, slices had the final firmness values of 10.6 ± 1.1, 10.0 ± 0.7, 10.6 ± 0.9, 11.0 ± 1.2 and 10.4 ± 1.0 N under atmospheres of argon, helium, nitrous oxide, nitrogen and air, respectively.

3.2 Mushrooms

3.2.1 CO₂ production

The applied gases (Ar, Ne, He and N₂O) had no marked effect on the FI values of the mushrooms compared with the reference N₂ (p<0.05) after day 1.

Mushroom Headspace ethanol Tissue ethanol Headspace CO₂

headspace ethanol 1

tissue ethanol 0.9914 1

headspace CO₂ 0.9891 0.9904 1

Sliced Apples

headspace ethanol 1

tissue ethanol 0.9880 1

headspace CO₂ 0.9963 0.9954 1

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Due to the high respiration rate of the mushrooms, O₂inside the jars filled with air was completely consumed within 24 hours and aerobic conditions switched to anaerobic (LOPEZ-BRIONES et al., 1992). CO₂ partial pressure in the headspace increased sharply from 0 to 19 kPa in the same period (figure 3).

Figure 3

Change in CO₂ partial pressures in the headspaces of the jars with mushrooms as a function of storage duration at 10°C under the different atmospheres. Each data

point represents the mean of triplicate measurements in the air curve and combined means of triplicate measurements under He, Ar, Ne, N₂ and N₂O atmospheres

in the noble gas curve. Standard deviations are represented with vertical bars and they are not visible when they are less than the size of the symbols.

The apparent Km of mushroom aerobic catabolism is very small (<0.1kPa O₂) (BEIT-HALACHMY and MANNHEIM, 1992, VAROQUAUX et al.,1999), therefore the CO₂ production in a closed system is almost constant until the onset of anaerobiosis.

As in the case of sliced apples, the short initial aerobic or partially aerobic phase of the CO₂ production curve can be explained by residual oxygen in the jars. How- ever CO₂ production rate dropped from 0.50 ± 0.14 to 0.18 ± 0.03 mmole.kg^-1.h^-1 more rapidly in mushrooms (1 day) than in sliced apples (3 days). Available residual oxygen for mushrooms within the jars (trapped in the tissues + headspace) was estimated as 1.75 kPa at most (apparent density, 0.7-0.8, 1.01-1.02 after vacuuming (VAROQUAUX et al., 1978)).

3.2.2 Ethanol production in the tissue

After day 3, a sharp linear increase was observed in tissue ethanol concentrations of mushrooms regardless of the atmospheres (figure 4).

0 5 10 15 20 25 30

0 1 2 3 4 5 6 7

Storage duration (days) Noble gases

Air

CO2 (kPa)

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Figure 4

Change in the tissue ethanol content of mushrooms as a function of storage duration at 10˚C under the different atmospheres. Each data point represents the combined means of triplicate measurements under the atmospheres of He, Ar, N₂, N₂O and air.

Standard deviations are represented with vertical bars and they are not visible when they are less than the size of the symbols.

There was no significant difference (p<0.05) among the tissue ethanol levels of mushrooms under the different applied atmospheres including air. According to VAROQUAUX and CHAMBROY (1994) a 10% CO₂ concentration in the atmosphere promotes ethanol production by mushrooms. This could account for the observed lag time. Tissue and headspace ethanol concentrations of mushrooms were equilibrated after day 3 with a high correlation coefficient. They were also well correlated with headspace CO₂ concentration (table 1).

3.2.3 Color and firmness

One of the most important quality parameters of mushrooms is their white- ness, which is measured by the L* value. There was no significant difference (p<0.05) among the L* and b* values of the mushrooms under any of the atmospheres. Final L* values were between 88 and 92 which are higher than the wholesalers acceptability limit reported as 80 for “white” and “cream” strains.

The b* values of the mushrooms increased linearly (slope 1.18, r²= 0.79) for 7 days, reflecting yellowing due to increasing levels of CO₂ inside the jars (LOPEZ-BRIONES et al., 1992).

Mushrooms almost maintained their initial firmness for 7 days under all applied atmospheres. Retention of initial firmness of mushrooms was due to the effect of increasing CO₂ concentration in the jars (LOPEZ-BRIONES et al., 1992) rather than any applied gas.

y = 0,7766x - 2,1156 R² = 0,9685

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0

0 2 4 6 8

Storage duration (days)

Ethanol (g/100 g.FW)

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4 – CONCLUSIONS

Noble gases and nitrous oxide do not protect plant tissues (common mushroom and apple slices) from detrimental effects of anoxia resulting in the production of ethanol and tissue necroses. In presence of even small partial pressure of oxygen these gases do not reduce enzymatic browning.

It has also been demonstrated that the ethanol concentration produced by fermenting mushrooms and apples in the head space of the container was highly correlated with the ethanol content of the plant tissues.

The previously reported antibrowning effect of noble gases and nitrous oxide could be attributed to their physical properties such as specific gravity (compared to air) which permits a more efficient purging of oxygen (1.38 and 1.53 g.L^-1 at ˚C and under atmospheric pressure, for argon and nitrous oxide respectively. The difference in density, solubility and viscosity of these gases could improve the efficiency of actively modified atmosphere obtained either by flushing or compensated vacuum.

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