Initial O2 Concentration Research Articles

Effect of nitrate on the degradation of bisphenol A by UV/H2O2 and ozone/H2O2 oxidation in aqueous solution

The goal of the present study was to investigate the effect of nitrate on the removal of bisphenol A (BPA) by conducting bench-scale UV/H2O2 and O3/H2O2 operations under a variety of reaction time and initial concentrations of H2O2 and NO3-–N. Also, this study evaluated the advanced oxidation process processes and determined the optimal conditions, for removing BPA. The practical reaction time and initial H2O2 concentration for the more than 85% removal of BPA proved to be 2.0 min and 150 mg/L, respectively, in the presence or absence of nitrate. The removal efficiency of BPA was improved by the hydroxyl radicals produced from nitrate photolysis in the absence of H2O2. In the presence of H2O2, however, nitrate hindered the BPA removal by acting as OH scavengers under the operating conditions described above. Also, H2O2 acts as free-radicals scavenger at concentrations above optimum required. In O3/H2O2 process with nitrate addition, more than 75% of the BPA was removed under the conditions of an initial O3 concentration of 7.5 mg/L and H2O2 concentration of more than 150 mg/L.

Pulse Laser Photolysis of Aqueous Ozone in the Microsecond Range Studied by Time-Resolved Far-Ultraviolet Absorption Spectroscopy

Chemical dynamics of an ozone (O3) pulse-photolytic reaction in aqueous solutions were studied with pump-probe transient far-ultraviolet (FUV) absorption spectroscopy. With a nanosecond pulse laser of 266 nm as pump light, transient spectra of O3 aqueous solutions (78-480 μM, pH 2.5-11.3) were acquired in the time range from -50 to 50 μs in the wavelength region from 190 to 225 nm. The measured transient spectra were linearly decomposed into the molar absorption coefficients and the concentration-time profiles of constituted chemical components with a multivariate curve resolution method. From the dependences of the time-averaged concentrations for 20 μs of the constituted chemicals on the initial concentration of O3, it was found that the transient spectra involve the decomposition of O3 and the formation of hydrogen peroxide (H2O2) and a third component that is assigned to hydroxyl radical (OH) or perhydroxyl radical (HO2). Furthermore, the pH dependence of the time-averaged concentration of the third components indicates that HO2 is more probable than OH as the third component. The time-averaged concentration ratio of each chemical component to the initial O3 concentration depends on the pH conditions from -0.95 to -0.60 for O3, 0.98 to 1.2 for H2O2, 0.002 to 0.29 for OH, and 0.012 to 0.069 for HO2.

Pyrophoric tendency of hydrogen sulfide corrosion product in oil tanks under aerobic condition

The study simulated the sulfidation reaction of Fe2O3 and H2S in oil tanks containing sulfur under aerobic condition. To evaluate the pyrophoric tendency of H2S corrosion product, the oxidation reaction was taken under different temperatures after the sulfidation reaction and the temperature changes were measured during the oxidation process. The results showed that, with the increase of initial O2 concentration in the mixture of sulfidation gas, the oxidation possibility of the sulfidation product in the sulfidation process increased, then the final amount of sulfidation product in the oxidation process decreased, and the pyrophoric reactivity of H2S corrosion product decreased. The higher the initial temperature at which the oxidation of the sulfidation product took place, the faster the temperature of the oxidation reaction rose, and the greater the pyrophoric reactivity of H2S corrosion product.

Efficient Desulfurization in a New Scheme of Oxyfuel Combined with Partial CO2 Removal from Recycled Gas and MILD Combustion

A new scheme of oxyfuel combustion combined with partial removal of CO2 from recycled gas and MILD combustion was proposed. Through experiments and theoretical analysis including the Monte Carlo method, the characteristics of in-furnace desulfurization in this new scheme was investigated. It was found that as the initial O2 concentration decreased, the gas recirculation ratio, SO2 concentration, and global efficiency of in-furnace desulfurization increased. On the other hand, the gas recirculation ratio, SO2 concentration, and global efficiency of in-furnace desulfurization increased as the CO2 removal ratio increased. The practical residence time of SO2 in oxyfuel-MILD coal combustion increased to about five to thirteen times as long as that of conventional pulverized coal combustion. The contributions of oxyfuel combustion, partial removal of CO2 from recycled gas, and MILD combustion, to the high desulfurization efficiency, were almost the same importance. At CO2 removal ratio = 11% and initial O2 conc...

Autoxidation and acetylene-accelerated oxidation of NO in a 2-phase system: Implications for the expression of denitrification in ex situ experiments

Nitric oxide (NO) is known to induce denitrification in model organisms, but the quantitative effect of NO and its concentration dependency has not been assessed for denitrification in soils. NO is chemically unstable in the presence of oxygen due to autoxidation, and the oxidation of NO is accelerated by acetylene (C2H2) which is commonly used as an inhibitor of N2O reductase in denitrification studies. As a first step to experimentally explore the role of NO in soil denitrification, we investigated NO oxidation kinetics for a closed “two phase” system (i.e. liquid phase + headspace) typically used for denitrification experiments with soil slurries, both with and without acetylene present. Models were developed to adequately predict autoxidation and acetylene-accelerated oxidation. The minimum oxygen concentration in the headspace ([O2]min, mL L−1) for acetylene-accelerated NO oxidation was found to increase linearly with the NO concentration ([NO], mL L−1); [O2]min = 0.192 + [NO] * 0.1 (r2 = 0.978). The models for NO oxidation were then used to assess NO oxidation rates in denitrification experiments with batches of bacterial cells extracted from soil. The batches were exposed to low initial oxygen concentrations in gas tight serum flasks (with and without C2H2), and monitored for O2, NO, N2O and N2 production while depleting the oxygen and switching to anoxic respiration. Acetylene effectively scavenged NO from the cultures until oxygen concentration reached below ∼0.19 mL L−1, and the estimated rate of acetylene-accelerated NO oxidation was more than sufficient to explain an observed reduction of the N2O production induced by acetylene. When [O2] reached below 0.19 mL L−1, the NO concentrations increased and stabilized at the same level as in the treatments without acetylene, but the rate of denitrification was much lower than without acetylene. The results indicate that the early accumulation of 10–20 nM NO during oxygen depletion has a significant effect on the expression of denitrification in soil communities. This warrants a greater interest in NO as a regulator of denitrification in soils and shows that the acetylene inhibition method may be problematic even for intentionally anoxic incubations, unless precautions are taken to secure initial O2 concentrations below 0.19 mL O2 L−1.

EFFICIENT DESULFURIZATION IN O2/CO2 COMBUSTION: DEPENDENCE ON COMBUSTION CONDITIONS AND SORBENT PROPERTIES

Based on experiments on desulfurization, CaSO4 decomposition, and a system approach using theoretical analysis, efficient in-furnace desulfurization in O2/CO2 combustion was investigated. The influence of combustion conditions and sorbent properties on system desulfurization efficiency was clarified. The global desulfurization efficiency was found to increase with O2 purity. The global desulfurization efficiency in a dry recycle was higher than that in a wet recycle. The global efficiency of in-furnace desulfurization decreased with initial O2 concentration. As the temperature increased, the global desulfurization efficiency increased first and then decreased due to the decomposition of CaSO4. In the temperature range investigated, the global desulfurization efficiency in O2/CO2 coal combustion was much higher than that of conventional coal combustion in air. The global desulfurization efficiency decreased with sorbent size. When the particle radius decreased to one quarter, the global desulfurization efficiency doubled, becoming as high as 80%. The global desulfurization efficiency was very different among the three sorbents investigated, whether in O2/CO2 combustion or in conventional air combustion. The global desulfurization efficiency increased in the order of Ca(OH)2, scallop, and limestone in O2/CO2 combustion, but in the order of scallop, Ca(OH)2, and limestone in conventional air combustion. Nevertheless, all three sorbents demonstrated much higher desulfurization efficiency in O2/CO2 combustion than in conventional air combustion.

Characterization and reactivity with NO/O2 of the soot formed in the pyrolysis of acetylene–ethanol mixtures

Characterization analyses and soot–O2 and soot–NO interaction experiments have been performed for soot samples obtained in the pyrolysis of acetylene–ethanol mixtures at different temperatures from 1275 to 1475K. The objective of these analyses is to address the influence of soot formation conditions on soot properties and structure, as well as on its capability to interact with gaseous compounds.The characterization techniques used are: elemental analysis, transmission electron microscopy (TEM), Brunnauer–Emmett–Teller (BET) surface area analysis, Raman spectroscopy and X ray diffraction (XRD). The characterization of soot samples is useful to increase the database on soot composition and structure and may help to find a dependence of those characteristics with soot formation conditions and the fuel from which soot is formed. From these data it can be observed a certain degree of graphitization for the soot samples formed at higher temperatures and/or from fuel mixtures with a higher content in ethanol.The interaction of soot with NO and O2 is investigated in order to analyze the capability of soot to interact with gas reactants. Soot samples formed at the highest temperatures are less reactive towards O2 and NO than those formed at lower temperatures. Soot samples appear to be more reactive when the fuel mixture presents a lower initial volume of ethanol. These observations can be related to the higher C/H ratio, associated to slightly higher degree of ordering, for the soot samples formed in such conditions. Experimental results have also demonstrated that soot samples are more reactive towards O2 than NO, although the initial concentration of O2 is lower.

Influence of Inlet Dilution of Reactants on Premixed Combustion in a Recuperative Furnace

This paper presents a numerical study by RANS modeling that investigates the effect of external dilution on the premixed combustion occurring in a recuperative furnace. Calculations are performed using the detailed GRI-Mech 3.0 mechanism to ensure the accuracy of the modeling. Results of the in-furnace flow, temperature, and concentrations of OH, O2, CO2 and NOx are provided. It is found that the external dilution with the inert gas CO2 plays a significant role in establishing the premixed MILD (Moderate or Intense Low-oxygen Dilution) combustion. Externally diluting the reactant mixture not only reduces the initial concentration of O2 but also ensures a stronger internal dilution by recirculation of more hot combustion products. Importantly, the latter effect is more significant for achieving the MILD regime. There is a critical mass fraction of the diluent CO2 present, below which MILD combustion cannot occur. While the traditional premixed flame produces much more NOx than the MILD combustion, the emission of NOx appears to result most from the thermal-NO route and least from the N2O route no matter which mode occurs. Moreover, the present simulation demonstrates that the MILD mode occurs over a wider range of initial reactant conditions for premixed combustion than for diffusion combustion.

Impact of CO2, N2 or Ar diluted in air on the length and lifting behavior of a laminar diffusion flame

Air-side dilution on laminar methane/air diffusion flames in the buoyant regime was extensively studied. CO2, important for EGR and fire safety, was chosen as a diluent. It was complemented by N2 and Ar. Flame stability, its external aspect and inner structure were quantified by complementary experiments and simulations. Two flame-response categories were highlighted, depending on whether mechanisms relied on several effects or mainly on pure dilution. In the former case, physico-chemical properties of diluents were essential. The greatest influence was detected with CO2, the least one with Ar. No critical value of the initial molar O2-concentration (XO2,i) was found to control quantities involved in stability (lifting limit, standoff distance of the attached flame), soot formation and flame-tip opening. It was found a critical flow-rate ratio from which flame-liftoff always occurred, crucial to describe lifting process induced by dilution. The reaction, naturally weak at the flame tip, led to an apparent open-tip with diluents. However, simulation showed the reaction, even weakened, persisted. On the contrary, pure dilution controlled flame length behavior. When XO2,i was decreased, length increased according to a unique evolution whatever the diluents. To measure flame length, several techniques were applied: OH Planar Laser Induced Fluorescence (Lf,OH), soot-Laser Induced Incandescence (Lf,LII), direct flame emission (Lf,luminous and Lf,peak). Length Lf,luminous was unable to correctly characterize flame lengthening. Lf,peak was the closest to the calculated stoichiometric length. So, the method giving Lf,peak appeared as the easiest and the most accurate one. Comparison between our data and correlations found in literature indicated Roper’s correlation was the most efficient to predict length evolution.

Research on Plasma Chemistry Reactions in C3H6/NO/O2/N2 Mixture Gases

A DBD-type (dielectric barrier discharge) NTP (non-thermal plasma) reactor was designed and an experimental system was established, in order to study the mechanism of plasma chemistry reactions in a gas mixture of C3H6/NO/O2/N2. This paper is focused on a. the effects of the initial concentrations of both NO and O2 upon NO conversion rate and the variations of NOx and O3 concentrations versus SED (specific energy density) when mixed gases of NO/O2/N2 passed the NTP reactor; b. the effects of the initial concentration of C3H6 upon NO oxidation and reduction reactions in mixed gases of NO/N2/C3H6 and NO/N2/O2/C3H6. The conclusions drawn are as follows. a. When the mixed gases of NO/O2/N2 passed the NTP reactor, NO was simultaneously involved in oxidation and reduction reactions with the former dominant and NO2 concentration increased at the NTP reactor outlet. The NO conversion rate decreased with the increase of the initial concentration of NO and O2, the O3 concentration firstly increased and then decreased with the increase in SED value. b. NO reduction reaction was very weak and energy consumption was very high in a mixed gases of NO/N2. However, when C3H6 was added, the NO conversion rate was significantly increased. When C3H6 was added to the mixed gases of NO/O2/N2, the NO conversion rate was significantly increased and the energy consumption was considerably reduced.

An Observational Study to Assess Changes in Arterial Blood Gas Values During Untreated Porcine Ventricular Fibrillation

Background. Cardiocerebral resuscitation (CCR) is reportedly superior to cardiopulmonary resuscitation (CPR) for out-of-hospital cardiac arrest (OHCA) even though active ventilation is not initially provided. Understandably, concerns have been raised regarding the withholding of positive pressure ventilation (PPV) during CCR because of the longstanding belief that respiratory gas exchange is a critical action during resuscitation. Objective. In this observational study, we sought to quantify the effect of prolonged untreated ventricular fibrillation (VF) on arterial pH, partial pressure of carbon dioxide (pCO2), and partial pressure of oxygen (pO2) values in a swine model of witnessed cardiac arrest to begin exploring the validity of these concerns. Methods. Both included studies were approved by the institutional animal care and use committee (IACUC). Eighty-three animals (25–35 kg) were instrumented under general anesthesia. Baseline characteristics were recorded. An arterial blood gas (ABG) sample was drawn from each animal via femoral catheter just prior to electrical induction of VF. After 8 minutes of untreated VF in one study (study 1 [n = 30]) and 10 minutes of untreated VF in the other study (study 2 [n = 53]), a second ABG sample was drawn. All samples were processed immediately using an i-STAT portable whole blood analyzer. Baseline characteristics of animals in the two studies were assessed using descriptive statistics. For the second ABG sample in each study, the mean pH, pCO2, and pO2 values, with 95% confidence intervals (95% CIs), were determined. The paired ABG results for each animal were then compared and the average pH, pCO2, and pO2 proportions, with 95% CIs, for each study were calculated. Results. The baseline characteristics of the animals in the two studies were similar. After 8 and 10 minutes of untreated VF cardiac arrest, the pH values were 7.35 (95% CI = 7.32, 7.37) and 7.37 (95% CI = 7.36, 7.38), the pCO2 increased to 44.1 mmHg (95% CI = 41.1, 47.1) and 52.7 mmHg (95% CI = 51.0, 54.4), and the pO2 decreased to 44.8 mmHg (95% CI = 42.2, 47.4) and 45.5 mmHg (95% CI = 43.3, 47.6), respectively. Conclusions. Using our swine model of witnessed cardiac arrest with prolonged untreated VF, the arterial pH remained essentially unchanged and the pCO2 increased to 1.42 times baseline after 10 minutes, while almost half of the initial O2 concentration in the blood at the beginning of resuscitation remained.

Carbon and oxygen controls on N2O and N2 production during nitrate reduction

Abstract Here we provide evidence that the form of carbon compound and O2 concentration exert an inter-related regulation on the production and reduction of N2O in soil. 6.7 mM d -glucose, 6.7 mM D-mannitol, 8 mM L-glutamic acid or 10 mM butyrate (all equivalent to 0.48 g C l−1) were applied to slurries of a sandy loam soil. At the start of the experiment headspace O2 concentrations were established at ∼2%, 10% and 21% O2 v/v for each C treatment, and 2 mM K15NO3 (25 atom % excess 15N) was applied, enabling quantification of 15N–N2 production, 15N–(N2O-to-N2) ratios and DNRA. The form of C compound was most important in the initially oxic (21% O2 v/v) soils, where addition of butyrate and glutamic acid resulted in greater N2O production (0.61 and 0.3 μg N2O–N g−1 soil for butyrate and glutamic acid, respectively) than the addition of carbohydrates (glucose and mannitol). Although, there was no significant effect of C compound at low initial O2 concentrations (∼2% O2 v/v), production of 15N–N2 was greatest where headspace O2 concentrations were initially, or fallen to, ∼2% O2 v/v, with greatest reduction of N2O and lowering 15N–(N2O-to-N2) ratios (∼0–0.27). This may reflect that the effect of C is indirect through stimulation of heterotrophic respiration, lowering O2 concentrations, providing sub-oxic conditions for dissimilatory nitrate reduction pathways. Addition of carbohydrates (glucose and mannitol) also resulted in greatest recovery of 15N in NH4+ from applied 15N–NO3−, indicative of the occurrence of DNRA, even in the slurries with initial 10% and 21% O2 v/v concentrations. Our 15N approach has provided the first direct evidence for enhancement of N2O reduction in the presence of carbohydrates and the dual regulation of C compound and O2 concentration on N2O production and reduction, which has implications for management of N2O emissions through changing C inputs (exudates, rhizodeposition, residues) with plant species of differing C traits, or through plant breeding.

Effect of High-Oxygen Atmospheres on the Antioxidant Potential of Fresh-Cut Tomatoes

The effect of different initial in-package O2 and CO2 concentrations (2.5 kPa of O2 + 5 kPa of CO2, 10 kPa of O2 + 5 kPa of CO2, and 21, 60, and 80 kPa of O2) on the antioxidant potential of fresh-cut tomatoes was investigated. Changes in individual phenolic compounds, individual carotenoids, vitamin C, and antioxidant capacity as well as in O2, CO2, and ethylene headspace concentrations inside packages were assessed for 21 days at 4 degrees C. High-oxygen and passive atmospheres induced higher production of carotenoids and phenolic compounds. The degradation of the initial content of vitamin C was highly promoted by the presence of oxygen. Lower hydrophilic antioxidant capacity (DPPH assay) was obtained in tomato slices stored under 80 kPa of O2, whereas the antioxidant capacity of the lipophilic fraction was enhanced with oxygen availability inside headspace packages. Therefore, 2.5 kPa of O2 + 5 kPa of CO2 atmospheres not only reduced the formation of carotenoids but also maintained vitamin C in fresh-cut tomatoes.

Influence of the Decrease in Oxygen during Malaxation of Olive Paste on the Composition of Volatiles and Phenolic Compounds in Virgin Olive Oil

The sensory and health properties of virgin olive oil (VOO) are highly related to its volatile and phenolic composition. Oxygen control in the pastes during malaxation may be a new technological parameter to regulate enzymatic activities, such as polyphenoloxidase, peroxidase, and lipoxygenase, which affect the phenolic and volatile composition of VOO. In this work, we monitored CO2 and O2 concentrations during industrial-scale olive paste malaxation with various initial O2 concentrations within the malaxer headspace. Results show that the O2 concentration in the malaxer headspace did not affect CO2 production during processing, whereas a strong influence was observed on the changes of the phenolic composition of olive pastes and VOOs, with high correlation coefficient for the total phenols (R = 0.94), especially for oleuropein and demethyloleuropein derivatives (R = 0.81). In contrast, aroma production during malaxation was minimally affected by the O2 concentration in the malaxer headspace.

Low-temperature selective catalytic reduction of NO with NH3 based on MnO x -CeO x /ACFN

MnO x -CeO x /ACFN were prepared by the impregnation method and used as catalyst for selective catalytic reduction of NO with NH3 at 80°C-150°C. The catalyst was characterized by N2-BET, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). The fraction of the mesopore and the oxygen functional groups on the surface of activated carbon fiber (ACF) increased after the treatment with nitric acid, which was favorable to improve the catalytic activities of MnO x -CeO x /ACFN. The experimental results show that the conversion of NO is nearly 100% in the range 100°C-150°C under the optimal preparation conditions of MnO x -CeO x /ACFN. In addition, the effects of a series of performance parameters, including initial NH3 concentration, NO concentration and O2 concentration, on the conversion of NO were studied.

A method to estimate net community metabolism from profiles of dissolved O2 and N2

A method to estimate net community metabolism (NCM) in natural waters using vertical profiles of water temperature, salinity, dissolved O2, gas tension, and calculated dissolved N2 is presented. The method utilizes the disparate biological activity of dissolved O2 and N2 to estimate metabolism at different depths in the water column. For well-mixed surface waters, N2 saturation levels are assumed to be the result of a quasi steady state balance of net warming or cooling and air–water gas exchange. Dissolved O2 levels are assumed to maintain a similar balance, subject to net biological activity, and NCM is then calculated based on the difference between N2 and O2 saturation levels and the estimated timescale required to equilibrate the layer with the atmosphere. For deeper stratified layers of water that warmed after layer formation in isolation from the atmosphere, the temperature at formation is calculated using the measured N2 concentration and an assumed N2 saturation level of 100% at formation. By assuming that initial N2 and O2 saturation levels were equal, the initial O2 concentration is calculated based on solubility relationships. NCM of the deeper waters is then estimated based on this information and knowledge of the general seasonal heating cycle of the waters. Daily mean water temperature and dissolved gas levels are used in the calculations. The method was assessed using profile measurements collected at Long Pond, Plymouth, Massachusetts, USA, on 23 August 2002. Oxygen was supersaturated relative to N2 by approximately 4% in the 0–6 m deep epilimnion, and undersaturated relative to N2 by approximately 7% in the stratified water at 9 m depth. The estimated 4-day average NCM for the epilimnion was 140 ± 70 mgC m−2day−1. For waters at 9 m depth, the temperature at formation was calculated to be 6.58 °C, and the estimated 100-day average NCM was −2.5 ± 0.6 mgC m−3 day−1. An independent estimate of −4.6 ± 0.9 mgC m−3 day−1 was derived from the measured O2 decline at 9 m depth over the same period of 2003.

Enhanced dechlorination of chloroethenes by granular sludge under microaerophilic conditions

This study investigates an innovative dechlorination process using anaerobic granular sludge that was partially exposed to oxygen. The exposure supported a synchronously anaerobic and aerobic bioconversion process that combined reductive dechlorination with aerobic co-oxidation in a sludge granule. Experimental results showed that the highest dechlorination rates of tetrachloroethene, trichloroethene, cis-dichloroethene and vinyl chloride were 6.44, 2.98, 1.70 and 0.97 nmol/gVS day, at initial O2 concentrations of 10, 100, 5 and 0%, respectively. Strictly anaerobic conditions favored the dechlorination of vinyl chloride while absolutely aerobic conditions were preferred for trichloroethene dechlorination. Microaerophilic conditions are suggested to ensure the overall biodegradation of the chlorinated ethenes present in groundwater as a mixture.

High-Temperature Liquid Water: A Viable Medium for Terephthalic Acid Synthesis

We report new information concerning the effect of oxygen concentration and catalyst concentration and identity [MnBr2, CoBr2, ZrBr4, and Mn(OAc)2] on the partial oxidation of p-xylene in dense water from 250 to 380 degrees C. Water is a more environmentally benign replacementfor the acetic acid solvent used commercially. We used a 440 mL Hastelloy batch reactorfor all experiments and monitored O2 consumption and product (including COx) formation. Increasing the catalyst concentration at 300 degrees C significantly increased terephthalic acid yields. MnBr2 was the most active catalyst of those we assessed. Increasing the initial O2 concentration beyond a modest excess did not significantly alter the terephthalic acid yield, but it increased the CO2 yield. Injecting supplemental 02 midreaction, however, did cause the terephthalic acid yield to increase. The highest terephthalic acid yields (>80%) occurred at 300 degrees C, [p-xylene]0 = 0.02 M, [O2]0 = 0.10 M, [Br] = 0.014 M, and t = 5-15 min. These yields are the highest reported to date from this reaction in high-temperature liquid water. Moreover, under these conditions and t=15 min, COx yields were below 2% and reaction intermediates were not detected.

Effect of initial oxygen concentration and film oxygen transmission rate on the quality of fresh‐cut romaine lettuce

AbstractModified atmosphere packaging (MAP) is widely used to maintain the quality of fresh‐cut produce by matching the oxygen transmission rate (OTR) of the packaging film to the respiration rate of the packaged product. The effect of the interaction between film OTR and the initial headspace O2 on quality of fresh‐cut vegetables has not previously been reported. Romaine lettuce leaves were sliced, washed, dried and packaged with film OTRs of 8.0 and 16.6 pmol s−1 m−2 Pa−1, and with initial headspace O2 of 0, 1, 2.5, 10 and 21 kPa. Packages were hermetically sealed and stored at 5 °C for up to 14 days. For samples packaged in 16.6 OTR film, increasing the initial headspace O2 concentration delayed O2 depletion within the packages, hastened the onset and increased the intensity of discoloration, and inhibited the development of CO2 injury, acetaldehyde and ethanol accumulation, off‐odors and electrolyte leakage. With 8.0 OTR‐packaged lettuce pieces, ≤1 kPa initial headspace O2 treatments induced an essentially anaerobic environment within the packages and increased acetaldehyde and ethanol accumulation and off‐odor development. Increasing the initial O2 concentration above 1 kPa in 8.0 OTR packages transiently increased O2 concentrations and reduced fermentative volatile production, off‐odors, electrolyte leakage and CO2 injury. Regardless of initial headspace O2 concentration, all 16.6 OTR‐packaged samples had severe discoloration after 14 days of storage. Quality was better maintained in 8.0 OTR‐packaged lettuce pieces as the initial headspace O2 concentration was increased. A 21 kPa initial O2 treatment of 8.0 OTR‐packaged lettuce maintained good quality throughout storage and had the best overall quality score. Copyright © 2005 Society of Chemical Industry

Effect of Superatmospheric Oxygen Packaging on Sensorial Quality, Spoilage, and Listeria monocytogenes and Aeromonas caviae Growth in Fresh Processed Mixed Salads

Atmospheres with O2 levels higher than 70 kPa have recently been suggested as an innovation to modified atmosphere packaging (MAP) for fresh processed vegetables to maintain sensory quality and safety. In the present work, mixed vegetable salad collected from a commercial processing plant and stored with the MAP technique was studied. Two gas mixtures were actively generated by using an initial O2 concentration of 95 kPa and combined with two plastic films. The low-barrier film permeability for O2 was 1,629 mlO2/m2 × 24 h × atm with 30μm of thickness (Hyplast, Hoogstraten, Belgium) and the O2 permeability of the high-barrier film was 2 mlO2/m2 × 24h × atm with 150μm of thickness (Euralpack, Wommelgen, Belgium) at 23°C. As control, active conventional MAP with application of 3 to 5 kPa of O2 and 6 to 8 kPa of CO2 was used. Packaged salads were stored up to 8 days at 4°C and at temperatures simulating chilled distribution chain conditions. Microbial safety and sensory quality, as well as the survival of inoculated Listeria monocytogenes and Aeromonas caviae, were monitored. The effect of superatmospheric O2 on the growth of aerobic microflora was variable. Under superatmospheric conditions, lactic acid bacteria and members of Enterobacteriaceae were inhibited. Nevertheless, growth of yeast and A. caviae seem to be stimulated by superatmospheric O2, whereas growth of psychrotrophic bacteria and L. monocytogenes was not affected. The overall visual appearance (mainly color) of the mixed vegetable salads was better maintained and the shelf life prolonged when packaged under O2 concentrations greater than 50 kPa.