The influence of oxygen and carbon dioxide on the growth of prevalent Enterobacteriaceae andPseudomonasspecies isolated from fresh and controlled-atmosphere-stored vegetables

To obtain more insight into the specific impact of modified gas conditions on the composition of microflora of minimally processed vegetables, the prevalent bacteria on mungbean sprouts and cut chicory endive were determined during storage under controlled atmospheric (CA) conditions at 8°C. Enumeration of the total mesophilic counts, Enterobacteriaceae,Pseudomonasspecies, and lactic acid bacteria indicated that Enterobacteriaceae andPseudomonasspecies constituted the major populations found on these products before and after CA storage. Identification of the predominant species within these populations revealed that on fresh and CA-stored mungbean sprouts,Enterobacter cloacae, Pantoea agglomerans, Pseudomonas fluorescens, Ps. viridilividaandPs. corrugatawere the prevalent species. On chicory endive,Rahnella aquatilisand severalPseudomonasspecies were found on the fresh product, while after CA storage,Escherichia vulnerisandPs. fluorescenswere the main species. Growth of the predominant epiphytes was subsequently quantified in pure culture, using an agar model system at 8°C under 1.5 or 21% O₂with 0, 5, 20, or 50% CO₂. In general, these CA conditions did not strongly influence maximum population densities and lag times were not detected. For each of the strains, however, maximum specific growth rates were reduced at increased CO₂concentrations, independent of the 0₂concentration applied. This effect was more pronounced forPseudomonasspecies than forEnterobacteriaceae. Notably, the agar model study showed that individual species ofEnterobacteriaceaeorPseudomonasresponded similarly to the specific CA conditions applied. This did not correlate with the shift in predominant species observed on chicory endive. Our data underline the complexity of the ecological conditions to which micro-organisms on vegetables are subjected during storage under modified gas conditions.     

Prediction Equations for Gases in Flexible Modified-Atmosphere Packages of Respiring Produce are Different Than Those for Rigid Packages

Total pressure generally decreases in a sealed rigid package containing respiring produce, whereas total pressure is essentially constant and free volume decreases in a flexible package. We found that predicted O₂, CO₂, and N₂ partial pressures were different for similarly designed (same surface area, thickness, film permeabilities and produce mass) flexible and rigid packages at “quasi steady state” and steady state, respectively. Predicted O₂ and CO₂ partial pressures were slightly higher in a flexible package than in a rigid package and were a function of the film permeability ratio of N₂ to O₂ and of CO₂ to O₂. They also related to the ratio of product CO₂ production rate to O₂ uptake rate, target steady-state O₂ partial pressure, and respiration characteristics. Differences were slight for films such as low-density polyethylene.     

Measurement and Modeling of Respiration Rate of Guava (CV. Baruipur) for Modified Atmosphere Packaging

Several experiments were conducted at different storage temperatures for generating respiration data using close system method for respiration. A respiration rate model, based on enzyme kinetics and the Arrhenius equation was proposed for predicting the respiration rates of Guava as a function of O₂ and CO₂ concentrations and storage temperature. Temperature was found to influence the model parameters. In this model, the dependence of respiration rate on O₂ and CO₂ was found to follow the uncompetitive inhibition. The enzyme kinetic model parameters, calculated from the respiration rate at different O₂ and CO₂ concentration were used to fit the Arrhenius equation against different storage temperature. The activation energy and respiration pre-exponential factor were used to predict the model parameters of enzyme kinetics at any storage temperature between 0–30°C. The developed models were tested for its validity at 12°C and it was found to be in good agreement (the mean relative deviation moduli between the predicted and experimental respiration rates were found to be 8.95% and 8.02% for O₂ consumption and CO₂ evolution, respectively) with the experimentally estimated respiration rates.     

Modeling Respiration Rate of Strawberry (cv. San Andreas) for Modified Atmosphere Packaging Design

A model for strawberry (Fragaria × ananassa cv. San Andreas) respiration rate was determined as a function of O₂ and CO₂ concentrations and temperature. Strawberries were enclosed in containers under different gaseous compositions (0–24% O₂ and 0–15% CO₂) and temperatures (10, 19, 23°C). Respiration rate was determined as O₂ consumption and CO₂ production. Respiration rate data was fitted to Michaelis-Menten models, with temperature dependence according to Arrhenius type equation. Non–linear regression was applied to calculate model parameters. No CO₂ inhibition was verified, so a simple Michaelis–Menten model was selected (R² = 0.91).     

Use of passive and active modified atmosphere packaging to prolong the postharvest life of three varieties of apricot (Prunus armeniaca, L.)

The evolution of the atmospheric composition (O₂ and CO₂ and ethylene) passively produced by three varieties of apricots (Beliana, Rouge de Rousillon and Polonais) stored at 10 °C under four plastic films of different permeabilities was studied. The results corresponding to respiratory intensity (IR) revealed that, while the IR decreased during the storage period under modified-atmosphere packaging, it increased again with the opening of the bags and the exposure of the fruits to air. The modified atmosphere inside the bags (O₂ and CO₂) and the time necessary to reach equilibrium depended on the IR of the fruits and the permeability of the film. The ethylene concentration decreased from the second day onwards in all the cases studied; this decrease being more pronounced in the films of least permeability due to a combination of its diffusion through the film and the inhibition of its synthesis as the CO₂ concentration increased and O₂ concentration decreased. Partial deviations in the metabolism of the three varieties were detected in the two films of lowest permeability (6060 and 12100 cm³ of O₂ and CO₂ m⁻²/24 h⁻¹ atm⁻¹ at 25°C) due to anoxia. The use of the active modified atmosphere, replacing the initial atmosphere of the bags with an atmosphere enriched with CO₂ (20% CO₂ and 80% air), did not modify the gas composition at equilibrium, although it shortened the time necessary for it to be reached. For the Beliana variety a slight decrease in ethylene concentration was observed inside the bags with an active modified atmosphere in relation to that of the bags with a passive modified atmosphere. The accumulation of ethanol in the tissues was not higher under the active modified atmosphere than under the passive modified atmosphere. Received: 19 July 1999 / Revised version: 29 September 1999     

Modeling of Respiration Rate of Litchi Fruit under Aerobic Conditions

Respiration of the produce and permeation of gas through the packaging films are the processes involved in creating a modified atmosphere inside a package that will extend shelf life of agricultural perishables. Thus modeling respiration rate of the selected produce is crucial to the design of a successful modified atmosphere packaging system. Two different models based on regression analysis and enzyme kinetics were developed with the help of respiration data generated at temperatures 0, 5, 10, 15, 20, 25, and 30 °C for litchi fruit using the closed system method. Temperature was found to influence the model parameters. In the model, based on enzyme kinetics, the dependence of respiration rate on O₂ and CO₂ was found to follow the uncompetitive inhibition. The enzyme kinetic model parameters, calculated from the respiration rate at different O₂ and CO₂ concentration were used to fit the Arrhenius equation against different storage temperature. The regression coefficients values were used for the prediction of respiration rate using regression model. The activation energy and respiration pre-exponential factor were used to predict the model parameters of enzyme kinetics at any storage temperature. The developed models were tested for its validity at 2 °C. The models showed good agreement with the experimentally estimated respiration rate.     

Effect of CO2 modified atmosphere packaging on aflatoxin production in maize infested with Sitophilus zeamais

The weevil Sitophilus zeamais (Motschulsky), the maize weevil, is a pest of stored maize that can cause feeding damage and lead to the proliferation of toxigenic fungi. The application of modified atmospheres with a high concentration of CO₂ is an alternative method for the control of S. zeamais and the inhibition of fungal growth. The objectives of the study were to determine the effect of S. zeamais infestation, grain damage and grain moisture content on aflatoxin production by Aspergillus flavus on maize, and the impact of high CO₂ modified atmosphere packaging on pest infestation and aflatoxin production. Mycotoxin production was only recorded when maize was infested with S. zeamais and had A. flavus inoculum. However, production of mycotoxins was not recorded when the maize was mechanically damaged and stored at 18% moisture content, indicating that the biological activity of the insect was determinant in the production of mycotoxins. The high CO₂ modified atmosphere packaging tested (90% CO₂, 5% O₂ and 5% N₂) prevented mycotoxin production.     

Model for Fresh Produce Respiration in Modified Atmospheres Based on Principles of Enzyme Kinetics

A respiration model, based on enzyme kinetics, was proposed for predicting respiration rates of fresh produce as a function of O₂ and CO₂ concentrations. In this model, the dependence of respiration on O₂ was assumed to follow a Michaelis-Menten type equation (r = V_m[O₂]/{K_m+ [O₂]}), and the effect of CO₂ on respiration to follow an uncompetitive inhibition model (r = V_m[O₂]/{Km + (1 + [CO₂]/ Ki) [O₂]}). The model predictions agreed well with published data for a variety of commodities and with experimental data for cut broccoli. Fresh produce respiration rates (O₂ consumption or CO₂ evolution) at various O₂ and CO₂ concentrations, as well as transient and equilibrium gas concentrations within permeable packages, could be accurately predicted with the model equations.     

Optimization of processing of fresh‐cut pear

BACKGROUND: Successful processing of pears to render fresh‐cut fruits requires a thorough knowledge of their sensitivity to browning reactions, their respiratory metabolism, and their behaviour under modified atmosphere packaging. RESULTS: First, three different varieties of pear (Williams, Conference, Passacrassana) that had reached their commercial ripening stage were evaluated for suitability for minimal processing. Two antioxidant treatments were tested (treatment 1: 2% ascorbic acid + 1% citric acid + 1% CaCl₂; treatment 2: 2% ascorbic acid + 0.01% 4‐hexylresorcinol + 1% CaCl₂) to decide which one was the most effective against enzymatic browning. Finally, a modified atmosphere packaging was designed after two previous tests: measurement of respiratory activity of the peeled and cut pear at three temperatures (4, 15 and 25 °C); and evaluation of fruit tolerance to three different atmospheric compositions (21% O₂ + 10% CO₂; 2% O₂ + 0% CO₂; 2% O₂ + 10% CO₂). CONCLUSIONS: Conference pear was found to be the most suitable variety. Among the antioxidant treatments the one consisting of ascorbic acid, 4‐hexylresorcinol and CaCl₂ was proven to be the most effective against browning. The samples were packaged in a modified atmosphere with a composition of 10% O₂ + 10% CO₂ + 80% N₂. Copyright © 2008 Society of Chemical Industry     

Modelling respiration of packaged fresh-cut ‘Rocha’ pear as affected by oxygen concentration and temperature

Respiration rates were measured in fresh-cut ‘Rocha’ pear (Pyrus communis L.) stored at four temperatures (0, 5, 10 and 15 °C) and with oxygen partial pressures ranging from 0 to 18 kPa. Respiratory quotient and ethanol production were used to determine the fermentation threshold. The oxygen concentration effect on the respiration rate was accurately described using Michaelis–Menten kinetics, without non-competitive inhibition by CO₂, and the effect of temperature on the respiration rate was well modelled by exponential functions. The oxygen level at which respiration was half its maximum (apparent _Km,O₂$K_{{\text{m,O}}_2}$) was similar to or only slightly greater than the fermentation threshold. The narrow range of oxygen between _Km,O₂$K_{{\text{m,O}}_2}$ and the fermentation threshold, suggests that modified atmosphere packaging technology has a limited applicability toward extension of the shelf-life of fresh-cut ‘Rocha’ pear.     

Ripening and ethylene biosynthesis in controlled atmosphere stored apricots

 Ethylene synthesis and metabolism were studied in apricot fruits (Prunus armeniaca L. cv. Búlida, under controlled atmosphere (CA) conditions (20% CO₂ and 20% O₂; 20% CO₂ and 1% O₂; 0.03% CO₂ and 1% O₂) and at a temperature of 2  °C. In control apricots stored outdoors in air containing 20% O₂ and 0.03% CO₂, the evolution of the physical and chemical parameters characteristic of ripening (loss of texture, decrease in acidity and increase in soluble solids) was slow during the storage period. However, for apricots stored in CAs containing 1% O₂ and 20% or 0.03% CO₂, these parameters did not show significant changes during the storage period. Ethylene production was completely inhibited in apricots stored in the three CAs, while control fruits showed a maximum ethylene level after 2 weeks of storage. This inhibition was accompanied by lower levels of 1-aminocyclopropane-1-carboxylic acid, free and total, which were more pronounced in apricots stored in the CAs containing 20% CO₂. In apricots stored in the CAs containing 1% O₂, the ethanol content increased after 1 week of storage, and this caused a smell and taste that made the fruits unsuitable for consumption. Received: 16 October 1998     

Monitoring and modelling of headspace-gas concentration changes for shelf life control of a glass packaged perishable food

Gas concentrations in the headspace of hermetically sealed glass packages of seasoned soybean sprouts were monitored and related to their microbial quality, i.e., the change in aerobic bacterial count, in order to examine the potential for using package gas changes as a primary quality index for shelf life control. Aerobic bacterial count, CO₂ and O₂ concentrations were measured from packages stored at four different temperatures: 0, 5, 10 and 15 °C. The CO₂ concentration increased and the O₂ concentration decreased with microbial growth. The microbial growth and CO₂ concentration change were described by a logistic function to yield kinetic parameters, and their temperature dependence was analysed by a square-root model. The kinetic parameters for microbial growth and CO₂ production differed in their magnitude and temperature dependence. The lag time observed for the increase in CO₂ concentration could be used as a shelf life index that corresponds to the time to reach a given microbial limit (here, 10⁷ CFU/g) under different temperature conditions, particularly under conditions of temperature abuse.     

Growth/survival of natural flora and Aeromonas hydrophila on refrigerated uncooked pork and turkey packaged in modified atmospheres

To evaluate the growth/survival of natural flora and Aeromonas hydrophila on refrigerated normal low (pork) and high (turkey) pH meats packaged in modified atmospheres, A. hydrophila was inoculated onto fresh pork and turkey meat slices. Inoculated and control samples were packaged in modified atmospheres (100% N₂, 20/80 and 40/60 CO₂/O₂) or in air in plastic bags and kept at 1 and 7°C. Samples packaged in air showed a similar microbiological pattern to that usually observed in fresh meat stored aerobically. Packaging in modified atmosphere produced a strong inhibition of bacterial growth at 1°C, particularly in samples stored in CO₂/O₂enriched atmospheres. Aeromonas hydrophila grew on turkey and pork meat stored in 100% N₂at 1 and 7°C. Likewise, growth of this bacterium was detected on turkey stored in 20/80 CO₂/O₂at 7°C. No growth was observed in 40/60 CO₂/O₂in any meat at both temperatures assayed.     

Effect of Initial Headspace O2 Level on the Growth and Volatile Metabolite Production of Leuconostoc Mesenteriodes and the Microbial and Sensorial Quality of Modified Atmosphere Packaged Par‐Fried French Fries

This study evaluated the effect of residual O₂ level (0% to 5%) on microbial growth and volatile metabolite production on par‐fried French fries packaged in a modified atmosphere with 60% CO₂ (rest N₂) at 4 °C. The results obtained showed that the initial headspace (IH) O₂ level had an effect on growth of Leuconostoc mesenteroides on French fry simulation agar, whereby growth was slightly faster under 5% O₂. In terms of quantity, ethanol, 2‐methyl‐1‐propanol, and dimethyl disulphide were the most significant volatile metabolites produced by L. mesenteroides. The production of ethanol by L. mesenteroides was highest on simulation agar packaged under low IH O₂ levels (0% to 1%), indicating that the fermentative metabolism was induced under these conditions. In agreement with the results observed on the simulation medium, growth of native lactic acid bacteria was faster under an IH O₂ level of 5%. In addition, ethanol, 2‐methyl‐1‐propanol, and dimethyl disulphide were also quantitatively the most important volatile metabolites. However, in contrast, greater quantities of ethanol and dimethyl disulphide were produced on par‐fried French fries packaged under 5% O₂. This was attributed to the limited growth of the native flora on the par‐fried French fries under residual O₂ levels of 0% and 1%. Although some significant differences (P < 0.05) occurred between the French fries packaged in 0%, 1%, and 5 % residual O₂ during storage, all products were considered to be acceptable for consumption. The results of this study can be used to optimize the shelf‐life of packaged chill stored potato products.     

Storage of Spinach Under Low Oxygen Atmosphere Above the Extinction Point

The extinction point of spinach was <0.4% but above 0.2% O₂ at 0° and 5°C. Respiration rates were ～2.3 times greater at 5°C than at 0°C and were similar among cultivars. In 0.8% O₂ atmosphere, O₂ uptake of three cultivars was reduced by an average of 53% and CO₂ production was reduced by 35% relative to those stored in air. Deterioration of leaves was reduced by 30 to 54%, while weight loss, color and chlorophyll content were not affected by the 0.8% O₂ atmosphere. Thus O₂ could be allowed to be depleted to 0.8% in modified atmosphere packaging without quality loss due to anoxia.     

Respiration Rate of a Dry Coleslaw Mix Affected by Storage Temperature and Respiratory Gas Concentrations

The respiration rate of dry coleslaw mix at different concentrations of O₂ and CO₂ was determined in an open flow-through respirometer at 5°C. Product O₂ consumption rate fitted an enzyme kinetics model. V_m and K_m values of 22.72 mlkg^-1h^-1 and 1.083 %O₂, respectively were calculated. CO₂ had an inhibitory effect on respiration rate. The effect of CO₂ on respiration gave a good fit with an uncompetitive inhibition enzyme kinetics model (E = 5.11%). Product respiration rate increased with temperature by an Arrhenius type relationship. Activation energies for O₂ consumption and CO₂ evolution were 74.8 kJ/kg and 84.2 kJ/kg respectively. These results provide information essential for the design and optimization of modified atmosphere packs for this product.     

Mushroom Response to Postharvest Hyperbaric Storage

Vessels suitable for storing mushrooms [Agaricus bisporus (Lange) Singe] at absolute pressures to 35 atm were constructed from 43 liter (1A) gas cylinders and plumbed for conducting static or continuous-flow experiments. Neither pressurization nor gradual depressurization over 6 hr injured mushrooms. Pressure did not affect respiration, but significantly reduced moisture loss during storage. Mushrooms depleted O₂ to very low levels and showed high tolerance to CO₂. Pressure was used to safely increase CO partial pressure (p) 35-fold. At 0.035p, CO reduced mushroom browning but not respiration. A completely autonomous storage system with CO₂ removal and automatic O₂, replenishment was developed.     

Production and Regeneration of Principal Volatiles in Apples Stored in Modified Atmospheres and Air

In a series of exploratory experiments, storage of McIntosh apples (Malus domestica Borkh.) in modified atmospheres (MA) (5% CO₂+ 3% O₂ at 2.8°C) suppressed the development of headspace ethanol and acetaldehyde from that in apples stored in air at 0°C (RA). Acetaldehyde, ethanol, ethyl butyrate and hexanal production from intact fruit was further suppressed when the apples were stored in 1.5% CO₂+ 1.5% O₂ or 1.5% CO₂+ 1.0% O₂ at 2.8°C. Placement of fruit in RA following MA storage initially regenerated ethyl butyrate and hexanal in preference to ethanol and acetaldehyde. However storage of fruit in 1.5% CO₂+ 1.0% O₂ for 320 days completely suppressed the principal headspace volatiles and blocked their subsequent regeneration in RA.     

Modified Atmosphere Packaging of Fresh Beansprouts

Freshly harvested beansprouts displayed a respiration rate of about 1 mmol O₂ kg⁻¹ h⁻¹ at 10°C which was strongly dependent on temperature, a 10-fold increase being observed every 16·5°C (z=16·5°C, ie Q₁₀=4·4). This commodity is also characterised by a high initial microbial load (about 10⁷ cells g⁻¹). During storage at various temperatures from 1 to 20°C, oxygen uptake rates dramatically increased with time and this phenomenon was well correlated with the development of aerobic microorganisms which reached 10⁹ cells g⁻¹ after 2 days at 20°C or 9 days at 1°C. Beansprouts were packaged in films, with permeabilities ranging from 950 to 200000 ml O₂ m⁻² day⁻¹ atm⁻¹, and stored at 8°C. Due to plant and microbial metabolism, oxygen concentrations decreased steadily within all packs until the onset of plant tissue decay. The latter occurred after 5–6 days with the least permeable films but did not occur within when the film permeability was over 100000 ml O₂ m⁻² day⁻¹ atm⁻¹. However, such films favoured brown discolouration, exudation texture and breakdown. The orientated polypropylene film (OPP) induced anoxic condition within 2 days and favoured anaerobic metabolism and necrosis of the sprouts. In all packages there was a rapid development of aerobic microorganisms and lactic acid bacteria that resulted in the accumulation of acetate and lactate and a decrease in pH. Thus, it clearly appeared that tissue decay was enhanced by microbial activity. At 8°C, 0·24 m² of film per kg of sprouts provided the optimal atmosphere composition (ie 5% oxygen and 15% carbon dioxide) when a film permeability of 50000 ml O₂ m⁻² day⁻¹ atm⁻¹ was used. These conditions allowed a shelf-life of 4–5 days.