Growth of Listeria monocytogenes Scott A, serotype 4 and competitive spoilage organisms in raw chicken packaged under modified atmospheres and in air

Effect of modified atmosphere packaging on the growth and survival of listeria in raw minced beef / Efecto del envasado en atmósferas modificadas sobre el crecimiento y la supervivencia de listeria en carne picada de ternera

A study on the behaviour of Listeria monocytogenes and L. innocua in raw minced meat packaged under modified atmospheres was carried out. Three gas atmospheres were tested with various CO2 concentrations (100% CO2; 65% CO2, 25% O2, 10% N2; 20% CO2, 80% O2). Packages containing minced meat were inoculated or uninoculated with L. monocytogenes and L. innocua and were stored at 4 °C for 18 d. Samples were taken every 3 d, and the development of the bacterial species, pH and water activity ( aw) were monitored. The 100% CO2 atmosphere was the most effective for the inhibition of growth of both species; pH influenced microbial inhibition but low pH values were not the most important factor in the inhibition of Listeria, the direct effect of the CO2 was necessary for that inhibition. Water activity values did not change during storage. None of the gas mixtures were bactericidal. The numbers of L. innocua recovered from all the modified atmos pheres tested were always lower than those of L. monocytogenes.

Effect of Lactic Acid Bacteria on Beef Steak Microbial Flora Stored Under Modified Atmosphere and on Listeria Monocytogenes in Broth Cultures

Beef steaks were inoculated with one or other of two protective strains of lactic acid bacteria, the bacteriocinogenic Lactobacillus sakei CTC 372 or the uncharacterised Lactobacillus CTC 711. They were stored under modified atmospheres (20–40% CO2). Inoculation of meat with both strains inhibited the growth of the spoilage bacteria. Neither CO2 in the pack atmosphere, inoculation with protective strains, nor a combination of both, affected formation of metmyoglobin or the development of off-odours. The formation of metmyoglobin in meat pigments and the sensory odour scores were compatible to those of fresh meat which had not undergone either oxidative deterioration or microbial spoilage. Listeria monocytogenes were inhibited in broth by meat surface microbiota containing either of the protective strains. With an initial population of 5.6 log cfu/mL, after 7 days incubation at 3°C, Listeria monocytogenes were recovered at log mean population of 2.8 log cfu/mL when neither protective strain was present. At 8°C, the population of Listeria monocytogenes recovered were reduced by about 2.5 or 1.5 log cfu/mL in the presence of Lactobacillus sakei CTC 372 or Lactobacillus CTC 711, respectively. At 25°C, the population of Listeria monocytogenes recovered from broth containing either protective strain were about 5 log cfu/mL less than the population recovered from broth containing Listeria monocytogenes only.

Influence of in-package cold plasma treatment on microbiological shelf life and appearance of fresh chicken breast fillets

The effect of in-package cold plasmas (CP) was studied on microbiological shelf life and surface lightness of fresh chicken fillets (pectoralis major). Chicken fillets were packaged in food trays in air or modified atmosphere (MA) gas (O2:CO2:N2 = 65:30:5) and stored at 4 °C after exposed to an in-package cold plasma (80 kV for 180 s) treatment. Populations of mesophiles, psychrophiles, and pseudomonas spp. were measured as indicators for microbiological shelf life and CIELAB L(∗) values as an indicator for raw meat appearance. Results show that regardless of microbial type, there were no significant differences in microbial counts between the control and CP treated chicken fillets packed in air. However, in the MA packages, microbial counts were consistently lower than the non-treated control during refrigerated storage. Regardless of CP treatment, the microbial counts on the samples packed in air were much higher than in MA. They were more than 6 logs cfu/g in air compared to fewer than 4 logs cfu/g in MA after 7 d storage and fewer than 6 logs cfu/g after 14 d storage. Regardless of CP treatment and gas composition in package, there were no significant differences in the surface L(∗) value between the fillets pre-treatment and those after storage at 4 °C. These results demonstrate that the effects of in-package CP treatments on microbiological shelf life of fresh chicken fillets depend upon headspace composition in packages. When fresh chicken fillets are packed in air, CP treatment has no effect on microbiological shelf life. MA packages with high O2 and CO2 significantly extend shelf life and CP treatment with MA can at least double shelf life of fresh chicken meat (more than 14 days). Regardless of headspace composition, in-package CP does not have negative effects on chicken meat appearance.

Behavior of Escherichia coli O157:H7 and Listeria monocytogenes during fermentation and storage of camel yogurt

In addition to its nutritional and therapeutic properties, camel milk has the ability to suppress the growth of a wide range of foodborne pathogens, but there is a lack of information regarding the behavior of these pathogens in products such as yogurt produced from camel milk. The objective of the current study was to investigate the behavior of Listeria monocytogenes and Escherichia coli O157:H7 during manufacture and storage of camel yogurt. Camel milk inoculated with L. monocytogenes and E. coli O157:H7 was fermented at 43° C for 5h using freeze-dried lactic acid bacteria (LAB) starter cultures (Streptococcus thermophilus and Lactobacillus bulgaricus) and stored at 4 or 10 °C for 14 d. Camel milk inoculated with L. monocytogenes and E. coli O157:H7 without starter culture was also prepared. During fermentation, the numbers of L. monocytogenes and E. coli O157:H7 increased 0.3 and 1.6 log cfu/mL, respectively, in the presence of LAB, and by 0.3 and 2.7 log cfu/mL in the absence of LAB. During storage at 4 or 10 °C, L. monocytogenes increased 0.8 to 1.2 log cfu/mL by 14 d in camel milk without LAB, but in the presence of LAB, the numbers of L. monocytogenes were reduced by 1.2 to 1.7 log cfu/mL by 14 d. Further, E. coli O157:H7 numbers in camel milk were reduced by 3.4 to 3.5 log cfu/mL in the absence of LAB, but E. coli O157:H7 was not detected (6.3 log cfu/mL reduction) by 7d in camel yogurt made with LAB and stored at either temperature. Although camel milk contains high concentrations of natural antimicrobials, L. monocytogenes was able to tolerate these compounds in camel yogurt stored at refrigerator temperatures. Therefore, appropriate care should be taken during production of yogurt from camel milk to minimize the potential for postprocess contamination by this and other foodborne pathogens.

The glutamate decarboxylase acid resistance mechanism affects survival of Listeria monocytogenes LO28 in modified atmosphere-packaged foods

AIMS

The contribution of the glutamate decarboxylase (GAD) acid resistance system to survival and growth of Listeria monocytogenes LO28 in modified atmosphere-packaged foods was examined.

METHODS AND RESULTS

The survival and growth of the wild-type LO28 and four GAD deletion mutants (DeltagadA, DeltagadB, DeltagadC, DeltagadAB) in packaged foods (minced beef, lettuce, dry coleslaw mix) during storage at 4, 8 and 15 degrees C were studied. Survival and growth patterns varied with strain, product type, gas atmosphere and storage temperature. In minced beef, the wild-type LO28 survived better (P < 0.05) than the GAD mutant strains at 8 and 15 degrees C. In both packaged vegetables at all storage temperatures, the wild-type strain survived better (P < 0.05) than the double mutant DeltagadAB. The requirement for the individual gad genes varied depending on the packaged food. In the case of lettuce, gadA played the most important role, while the gadB and gadC genes played the greatest role in packaged coleslaw (at 15 degrees C).

CONCLUSIONS

This work demonstrates that elements of the GAD system play significant roles in survival of L. monocytogenes LO28 during storage in modified atmosphere-packaged foods.

SIGNIFICANCE AND IMPACT OF THE STUDY

A better understanding of how L. monocytogenes behaves in modified atmosphere-packaged foods, and how it responds to elevated carbon dioxide atmospheres.

Efficacy of lactic acid against Listeria monocytogenes attached to poultry skin during refrigerated storage

AIMS

The aim of this study was to evaluate the effect of lactic acid washing on the growth of Listeria monocytogenes on poultry legs stored at 4 degrees C for 7 days.

METHODS AND RESULTS

Fresh inoculated chicken legs were dipped into either a 0.11, 0.22 mol l(-1) or 0.55 mol l(-1) lactic acid solution for 5 min or distilled water (control). Surface pH values, sensorial characteristics and L. monocytogenes, mesophiles and pychrotrophs counts were evaluated after treatment (day 0) and after 1, 3, 5 and 7 days of storage at 4 degrees C. Legs washed with 0.55 mol l(-1) lactic acid for 5 min showed a significant (P < 0.05) inhibitory effect on L. monocytogenes compared with control legs, being about 1.74 log units lower in the first ones than in control legs after 7 days of storage. Sensory quality was not adversely affected by lactic acid, with the exception of colour.

CONCLUSIONS

Treatments with 0.55 mol l(-1) lactic acid reduced bacterial growth and preserved reasonable sensorial quality after storage at 4 degrees C for 7 days. However, it was observed a reduction in the colour score within 1 day post-treatment with 0.55 mol l(-1) lactic.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study demonstrates that, while lactic acid did reduce populations of L. monocytogenes on poultry, it did not completely inactivate the pathogen. The application of lactic acid may be used as an additional hurdle contributing to extend the shelf-life of raw poultry.

Changes in growth, rRNA content, and cell morphology of Listeria monocytogenes induced by CO2 up- and downshift

Cell morphology, rRNA content, and growth were examined for Listeria monocytogenes LO28 and EGD, respectively, grown in brain-heart infusion (BHI) and on slices of sausage at 10 degrees C in 100% CO2, 100% N2, and air. In CO2, filamentous cells were formed by both strains on sausage slices and by L. monocytogenes EGD in BHI. Filamentation was not induced by anaerobiosis only. Fluorescent in situ rRNA hybridization (FISH) of cells grown in BHI showed that the L. monocytogenes EGD filaments consisted of chains of individual slightly elongated cells. The rods formed by L. monocytogenes LO28 had the same size in air and CO2. Septation and cell division were induced in the filaments after a CO2 downshift (i.e., exposure to air). In BHI, the number of colony forming units increased rapidly when L. monocytogenes EGD grown in CO2 was exposed to air whereas the number of L. monocytogenes LO28 remained almost unchanged. On sausage slices, the number of colony forming units also increased rapidly for both strains in response to CO2 downshift. Large variations in rRNA content of individual cells were observed in the tested scenarios. The results demonstrate the risk of underestimating the number of infectious units under circumstances where filamentation may occur. Furthermore, the study illustrates the lack of residual inhibitory effect of CO2 in this type of products after opening.

CO2- and anaerobiosis-induced changes in physiology and gene expression of different Listeria monocytogenes strains

Although carbon dioxide (CO(2)) is known to inhibit growth of most bacteria, very little is known about the cellular response. The food-borne pathogen Listeria monocytogenes is characterized by its ability to grow in high CO(2) concentrations at refrigeration temperatures. We examined the listerial responses of different strains to growth in air, 100% N(2), and 100% CO(2). The CO(2)-induced changes in membrane lipid fatty acid composition and expression of selected genes were strain dependent. The acid-tolerant L. monocytogenes LO28 responded in the same manner to CO(2) as to other anaerobic, slightly acidic environments (100% N(2), pH 5.7). An increase in the expression of the genes encoding glutamate decarboxylase (essential for survival in strong acid) as well as an increased amount of branched-chain fatty acids in the membrane was observed in both atmospheres. In contrast, the acid-sensitive L. monocytogenes strain EGD responded differently to CO(2) and N(2) at the same pH. In a separate experiment with L. monocytogenes 412, an increased isocitrate dehydrogenase activity level was observed for cells grown in CO(2)-containing atmospheres. Together, our findings demonstrate that the CO(2)-response is a partly strain-dependent complex mechanism. The possible links between the CO(2)-dependent changes in isocitrate dehydrogenase activity, glutamate metabolism and branched fatty acid biosynthesis are discussed.

The effect of growth atmosphere on the ability of Listeria monocytogenes to survive exposure to acid, proteolytic enzymes and bile salts

Four isolates of Listeria monocytogenes from food, human and environmental sources were grown separately in broth (pH 6.0 at 8 degrees C) under atmospheres of air, 100% N(2), 40% CO(2):60% N(2) or 100% CO(2). Exponential and stationary phase cells were harvested to determine if growth atmosphere and growth phase influenced this pathogen's ability to survive exposure to an acid environment coupled with proteolytic enzymes, and the activity of bile salts. In general, isolates were more resistant to the acid environment than the bile salts environment and stationary phase cells were significantly more resistant to both environments than exponential phase cells. Irrespective of prior growth atmosphere, none of the isolates when in exponential phase remained detectable following full exposure to the acid environment (110 min at 37 degrees C) or the bile environment (3 h at 37 degrees C). With the exception of one isolate grown under the atmosphere of 40% CO(2):60% N(2), all isolates when in stationary phase were detectable following full exposure to the acid environment but death rates varied significantly. Stationary phase cells of all isolates grown under 40% CO(2):60% N(2) and 100% CO(2) were highly susceptible to the bile salts environment: cells were not detectable after a 2-min exposure whereas stationary phase cells grown under air or 100% N(2) were recovered following full exposure to the bile environment. Survival curves were characterised by a population decline of at least 3 log(10)/ml (from an initial level of 7 log(10) CFU/ml) in the first 15 min; thereafter a constant population number of approximately 4 log(10)/ml was maintained over the remaining exposure period. No survival was observed when stationary phase cells of L. monocytogenes FRRB 2538 grown in air and 100% N(2) were subjected to the acid environment followed by immediate exposure to the bile salts environment. The results showed that growth atmosphere and growth phase could influence survival of this pathogen against conditions that imitate the extremes of the most important nonspecific defence mechanisms against microbial infection: the acid environment of the stomach coupled with the activity of proteolytic enzymes, and the activity of bile salts in the small intestine.

Behaviour of Listeria monocytogenes in packaged fresh mushrooms (Agaricus bisporus)

AIMS

The aim of this study was to evaluate the potential of Listeria monocytogenes to grow in mushrooms packaged in two different types of PVC films when stored at 4 degrees C and 10 degrees C.

METHODS AND RESULTS

Mushrooms were packed in two polymeric films (perforated and nonperforated PVC) and stored at 4 degrees C and 10 degrees C. The carbon dioxide and oxygen content inside the packages, aerobic mesophiles, psychrotrophs, Pseudomonas spp., Listeria monocytogenes, faecal coliforms, Escherichia coli, anaerobic spores and major sensory factors were determined. The mushrooms packaged in nonperforated film and stored at 4 degrees C had the most desirable quality parameters (texture, development stage and absence of moulds). Listeria monocytogenes was able to grow at 4 degrees C and 10 degrees C in inoculated mushrooms packaged in perforated and nonperforated films between 1 and 2 log units during the first 48 h. After 10 d of storage, the populations of L. monocytogenes were higher in mushrooms packaged in nonperforated film and stored at 10 degrees C.

CONCLUSIONS

MAP followed by storage at 4 degrees C or 10 degrees C extends the shelf life by maintaining an acceptable appearance, but allows the growth and survival of L. monocytogenes.

SIGNIFICANCE AND IMPACT OF THE STUDY

According to this study additional hurdles must be studied in order to prevent the growth of L. monocytogenes.

Influence of modified atmospheric storage, lactic acid, and NaCl on survival of sublethally heat-injured Listeria monocytogenes

The effect of package atmosphere on survival of uninjured and sublethally heat-injured Listeria monocytogenes, inoculated onto tryptose phosphate agar containing 0.85% lactic acid and 2% NaCl (TPALAS) was investigated. Inoculated TPALAS plates were packaged in air, 100% N2 (N2), 30% CO2-70% N2 (CO2-N2), and vacuum and stored at 4 and 20 degrees C for up to 31 days. Recovery of L. monocytogenes from TPALAS was influenced by the injury status (i.e., injured and uninjured) of the inoculum, storage atmosphere (air, N2, CO2-N2, and vacuum), storage temperature (4 and 20 degrees C), and recovery media [tryptose phosphate agar (TPA) and modified Oxford agar (MOX)] (P <0.05). Overall, storage at 4 degrees C supported greater survival than storage at 20 degrees C (P< 0.05). Uninjured L. monocytogenes stored at 4 degrees C was recovered on TPA better than sublethally heat-injured L. monocytogenes stored at 40 degrees C (P < 0.05). Recovery of sublethally heat-injured L. monocytogenes stored at 4 degrees C followed the order N2 > CO2-N2 > air > vacuum (P < 0.05), whereas recovery of uninjured L. monocyrogenes stored at 4 degrees C followed the order N2 > CO2-N2 > vacuum > air (P < 0.05). Air and vacuum atmospheres supported greater survival of uninjured and heat-injured L. monocytogenes than N2 and CO2-N2 atmospheres at 20 degrees C (P < 0.05). Recovery of sublethally heat-injured L. monocytogenes stored at 20 degrees C followed the order vacuum > air> CO2-N2 = N2 (P <0.05), whereas recovery of uninjured L. monocytogenes stored at 20 degrees C followed the order vacuum > air> CO2-N2 > N2 (P<0.05). Uninjured L. monocytogenes stored under N2 at 4 degrees C was recovered best, whereas sublethally heat-injured L. monocytogenes stored under N2 at 20 degrees C was recovered poorest (P < 0.05). Factors such as package atmosphere and storage temperature, involved in the production, storage, and distribution of fermented foods must be thoroughly evaluated when determining strategies for control and detection of L. monocytogenes in such products.

Growth of Listeria monocytogenes and Yersinia enterocolitica colonies under modified atmospheres at 4 and 8 degrees C using a model food system

The growth of Listeria monocytogenes and Yersinia enterocolitica colonies was studied on solid media at 4 and 8 degrees C under modified atmospheres (MAs) of 5% O2: 10% CO2: 85% N2 (MA1), 30% CO2: 70% N2 (MA2) and air (control). Colony radius, determined using computer image analysis, allowed specific growth rates (mu) and the time taken to detect bacterial colonies to be estimated, after colonies became visible. At 4 degrees C both MAs decreased the growth rates of L. monocytogenes by 1.5- and 3.0-fold under MA1 (mu = 0.02 h(-1)) and MA2 (mu = 0.01 h(-1)), respectively, as compared with the control (mu = 0.03 h(-1)). The time to detection of bacterial colonies was increased from 15 d (control) to 24 (MA1) and 29 d (MA2). At 8 degrees C MA2 decreased the growth rate by 1.5-fold (mu = 0.04 h(-1)) as compared with the control (mu = 0.06 h(-1)) and detection of colonies increased from 7 (control) to 9 d (MA2). At 4 degrees C both MAs decreased the growth rates of Y. enterocolitica by 1.5- and 2.5-fold under MA1 (mu = 0.03 h(-1)) and MA2 (mu = 0.02 h(-1)), respectively, as compared with the control (mu = 0.05 h(-1)). At 8 degrees C identical growth rates were obtained under MA1 and the control (mu = 0.07 h(-1)) whilst a decrease in the growth rate was obtained under MA2 (mu = 0.04 h(-1)). The detection of colonies varied from 6 (8 degrees C, aerobic) to 19 d (4 degrees C, MA2). Refrigerated modified atmosphere packaged foods should be maintained at 4 degrees C and below to ensure product safety.

Predictive food microbiology for the meat industry: a review

Predictive food microbiology (PFM) is an emerging multidisciplinary area of food microbiology. It encompasses such disciplines as mathematics, microbiology, engineering and chemistry to develop and apply mathematical models to predict the responses of microorganisms to specified environmental variables. This paper provides a critical review on the development of mathematical modelling with emphasis on modelling techniques, descriptions, classifications and their recent advances. It is concluded that the role and accuracy of predictive food microbiology will increase as understanding of the complex interactions between microorganisms and food becomes clearer. However the reliance of food microbiology on laboratory techniques and skilled personnel to determine process and food safety is still necessary.

The combined affects of modified atmosphere, temperature, nisin and ALTA 2341 on the growth of Listeria monocytogenes

A cocktail of seven Listeria monocytogenes isolates of food, human and environmental origin was used to assess the antilisterial activity of the bacteriocins nisin and ALTA 2341 in combination with various atmospheres: air, 100% N2, 40% CO2:60% N2, or 100% CO2. Buffered tryptone soya broth (pH 6.0) was used as the growth medium and incubation was at 4 degrees C (21 days) or 12 degrees C (7 days), or when temperature fluctuated between these values for defined periods. It was observed that atmosphere alone influenced the growth rate of L. monocytogenes, with 100% CO2 exerting the greatest inhibition. A 5 log population increase was observed in all atmospheres after 7 days at 12 degrees C. At 4 degrees C a 4-5 log population increase was observed in air, 100% N2 and 40% CO2:60% N2 within 21 days. Growth was prevented by 100% CO2. In the presence of nisin (400 IU/ml), an increase in the lag phase was observed before growth (5 log population increase after 7 days) in all atmospheres at 12 degrees C. This effect was enhanced at 4 degrees C where a maximum 2 log population increase was observed in all atmospheres except 100% CO2, in which growth was prevented. Increasing the concentration of nisin to 1250 IU/ml prevented L. monocytogenes growth in all atmosphere combinations at 4 and 12 degrees C. Two concentrations of ALTA 2341 were also tested. In the presence of 0.1% ALTA 2341 and at 12 degrees C, a 3-5 log population increase was observed in all atmospheres with the exception of 100% CO2, which prevented L. monocytogenes growth. At 4 degrees C, growth was observed in the combination of 0.1% ALTA 2341 and 100% N2 only (3 log population increase). Use of a higher concentration of ALTA 2341 (1.0%) resulted in a population decrease below the detection level within 24 h in all atmosphere/temperature combinations. Re-growth occurred in the presence of 1.0% ALTA 2341 in all atmospheres at 12 degrees C, and in combination with air or 100% N2 at 4 C. When the effectiveness of either nisin or ALTA 2341 and atmosphere was tested against L. monocytogenes as temperature fluctuated for periods between 4 and 12 degrees C, only the combination of 100% CO2 and 1.0% ALTA 2341 prevented growth. Cells surviving exposure to nisin or ALTA 2341 were recovered from 28 of the 32 combinations tested that contained bacteriocin. Nisin survivors remained sensitive to the bacteriocin. ALTA 2341 survivors had become resistant to the bacteriocin.

Concentration of carbon dioxide in the water-phase as a parameter to model the effect of a modified atmosphere on microorganisms

The effect of modified atmosphere packaging can mainly be attributed to the bacteriostatic action of CO2. The dissolved CO2 in the water-phase of a food product is strongly dependent on several intrinsic and extrinsic parameters and will determine the effectiveness of a modified atmosphere packaging configuration. The effect of pH, gas/product ratio, initial %CO2 in the gas-phase, lard content and storage temperature on the amount of dissolved CO2 was screened in a preliminary experiment. The initial CO2-concentration in the gas-phase and the gas/product ratio turned out to be the two major factors determining the amount of dissolved CO2. The initial pH also determined significantly the final CO2-concentration in the broth. Temperature and lard content were shown to have only a minor effect on the amount of dissolved CO2 compared to the above mentioned parameters. This demonstrates the importance of the packaging configuration in the effectiveness of a modified atmosphere. In a second step, a model was constructed to predict the amount of dissolved carbon dioxide in modified BHI-broth as a function of the gas/product ratio, the initial CO2-concentration and the temperature by means of Response Surface Methodology (RSM). A second equation was also derived based on Henry's law and was shown to be a powerful tool in the quantification of the effect of intrinsic and extrinsic parameters on the CO2-solubility in food products. The possibility of the use of the concentration of dissolved CO2 in the water-phase as a determinative factor for the inhibitory effect of modified atmospheres was examined on Pseudomonas fluorescens. Growth curves at 7 degrees C of P. fluorescens in different packaging configurations (initial %CO2 and gas/product ratio) resulting in equal amounts of dissolved CO2 were compared. P. fluorescens was shown to be similarly inhibited by equal amounts of dissolved CO2-concentrations, independent of the packaging configuration. This demonstrates the potential of the application of the concentration of dissolved CO2 in the water-phase as a parameter to characterise a modified atmosphere and its inhibition of certain microorganisms.

Predictive model of the effect of CO2, pH, temperature and NaCl on the growth of Listeria monocytogenes

The growth responses of L. monocytogenes as affected by CO2 concentration (0-100% v/v, balance nitrogen), NaCl concentration (0.5-8.0% w/v), pH (4.5-7.0) and temperature (4-20 degrees C) were studied in laboratory medium. Growth curves were fitted using the model of Baranyi and Roberts, and specific growth rates derived from the curve fit were modelled. Predictions for specific growth rate, doubling time and time to a 1000-fold increase could be made for any combination of conditions within the matrix. Predictions of growth from the model were compared with published data and this showed the model to be suitable for predicting growth of L. monocytogenes in a range of foods packaged under a modified atmosphere.

Predictive modeling of the growth of Listeria monocytogenes in CO2 environments

The effects of pH (5.5, 6.5), temperature (4, 7 and 10 degrees C) and carbon dioxide (10, 30, 50, 70 and 90%) on the growth and/or survival of a five strain mixture of Listeria monocytogenes were examined in brain heart infusion broth. All three variables had a major influence on the growth characteristics of the organism. As expected, both the lag time and generation time increased as the CO2 level increased, and as pH and temperature decreased. Growth over a 30-day period was observed at all parameter combinations tested, except at pH 5.5, 4 degrees C in the presence of either 50, 70 or 90% carbon dioxide. Two primary models, the Gompertz and Baranyi equations, were compared for their ability to describe the growth of L. monocytogenes. In general, the Gompertz model predicted both longer lag and shorter generation times, compared to the Baranyi model. The Baranyi model appeared to fit the overall data better than the Gompertz model. However, these differences were often small. Response surface models were developed for predicting the effects and interactions of the three independent variables on the growth and/or survival of L. monocytogenes in the different modified atmospheres. Results demonstrate the importance of strict temperature control for maintaining the advantages of food shelf life extension in enriched carbon dioxide environments. The information obtained in this study could be used as a guide to manufacturers of modified-atmosphere packaged foods, especially when designing products in which this organism may be a concern.

Predictive models of the effect of temperature, pH and acetic and lactic acids on the growth of Listeria monocytogenes

The combined effect of temperature (1-20 degrees C), pH (4.5-7.2) and acetic acid (0-10,000 mg/l; model 1) or lactic acid (0-20,000 mg/l; model 2) on growth of Listeria monocytogenes in laboratory media was studied. Growth curves at various combinations of temperature, pH and acid concentration were fitted by the model of Baranyi and Roberts (1994), and specific growth rates derived from the curve fit were modelled. Predictions of growth from the models were compared with data in the literature, and this showed the models to be suitable for use in predicting growth of L. monocytogenes in a range of foods including meat, poultry, fish, egg and milk and dairy products. The two models are compatible, i.e. they give similar predictions for cases when no acid is present.

Effects of carbon dioxide on the fate of Listeria monocytogenes, of aerobic bacteria and on the development of spoilage in minimally processed fresh endive

Minimally processed fresh broad-leaved endive (Cichorium endivia L.) were stored at 3 and 10 degrees C in modified atmospheres containing air, 10% CO2/10% O2, 30% CO2/10% O2, and 50% CO2/10% O2. The effects of these modified atmospheres on the fate of both aerobic bacteria and three strains of Listeria monocytogenes, was investigated. Increases in CO2 concentrations significantly reduced the growth of the aerobic microflora. The best preservation of the visual quality occurred on endive leaves stored in 10% CO2/10% O2, whereas leaves stored in 30% CO2/10% O2 and 50% CO2/10% O2, and to a lesser extent in air, showed extensive spoilage after storage. Listeria monocytogenes was slightly affected at 3 degrees C by the modified atmospheres, as compared to air. At 10 degrees C, results varied between replicate experiments, but L. monocytogenes generally grew better as the CO2 concentration was increased. The three test strains behaved in a similar way. In conclusion, among the modified atmospheres tested, a modified atmosphere containing 10% CO2/10% O2 resulted in improved visual quality of minimally processed fresh endive, without a marked effect on the growth of the aerobic microflora or of L. monocytogenes.

Factors affecting the growth of Listeria monocytogenes on minimally processed fresh endive

The influence of various factors on the fate of Listeria monocytogenes on cut leaves of broad-leaved endive has been studied. Factors considered were temperature, characteristics of the leaves (age, quantity and quality of the epiphytic microflora) and characteristics of the L. monocytogenes inoculum (concentration, strain). The increases in numbers of L. monocytogenes were lower than those of the aerobic mesophilic microflora at 3 degrees, 6 degrees, 10 degrees and 20 degrees C. Doubling times of the populations of L. monocytogenes were in the same order of magnitude as those of aerobic bacteria at 10 degrees and 20 degrees C, but longer at 3 degrees and 6 degrees C. There were positive significant correlations between growth of L. monocytogenes and populations of aerobic bacteria, and between growth of L. monocytogenes and extent of spoilage on the leaves. Of 225 bacteria isolated from the leaves, 84% were identified as fluorescent pseudomonads; there was no difference in the species isolated from leaves that showed a low growth of L. monocytogenes and leaves that showed a high growth of L. monocytogenes. Populations of L. monocytogenes increased faster during the first 2 and 4 d of storage at 10 degrees C on leaves inoculated with 10-10(3) cfu g-1 than on leaves inoculated with about 10(5) cfu g-1, but the population reached after 7 d was lower. The behaviour of L. monocytogenes was similar among the three strains tested.

The microbiology of minimally processed fresh fruits and vegetables

Minimally processed fresh (MPF) fruits and vegetables are good media for growth of microorganisms. They have been involved in outbreaks because of the consumption of products contaminated by pathogens. They are also sensitive to various spoilage microorganisms such as pectinolytic bacteria, saprophytic Gram-negative bacteria, lactic acid bacteria, and yeasts. Contamination of MPF fruits and vegetables occurs at every stage of the food chain, from cultivation to processing. Polluted environments during cultivation or poor hygienic conditions in processing increase the risk of contamination with foodborne pathogens. Although MPF fruits and vegetables may harbor psychrotrophic microorganisms such as fluorescent pseudomonads or Listeria monocytogenes, good control of refrigeration temperature limits growth of spoilage and pathogenic microorganisms. Modified atmospheres are often efficient to maintain or improve visual organoleptic quality of MPF fruits and vegetables, but their effects on microorganisms are inconsistent. Chemical disinfection can partially reduce the initial bacterial contamination; irradiation seems to be more efficient. The applications of legislations and quality assurance systems to control contamination, survival, and growth of foodborne pathogens in MPF fruits and vegetables are discussed.

Growth of Listeria monocytogenes and Clostridium sporogenes in cottage cheese in modified atmosphere packaging

Low fat cottage cheese (pH 5.14) was inoculated with three strains of Listeria monocytogenes, serotypes 1a and 4b, an isolate from a dairy processing plant, and Clostridium sporogenes ATCC 3584. The cheese was packaged with or without added dissolved CO2 in polystyrene tubs overwrapped with or without high barrier heat shrink film and stored at 4, 7, and 21 degrees C for up to 63 d. The concentration of CO2 in the container headspace was 35% (vol/vol). The CO2 concentration in that headspace declined by one-third over the 63 d of storage at 4 degrees C. Clostridium sporogenes failed to grow under any condition applied in this study. In the conventionally packaged cottage cheese, L. monocytogenes increased from 10(4) to 10(7) cfu/g after lag phases of 28 and 7 d at 4 and 7 degrees C, respectively. In contrast, L. monocytogenes failed to grow in cottage cheese packaged with CO2 and stored at 4 degrees C up to 63 d and increased from 10(4) to 10(5) cfu/g in products packaged with CO2 at 7 degrees C. These data suggest that the addition of CO2 to cottage cheese to extend shelf-life does not represent an increased Listeria or botulism hazard but that cottage cheese could be a vehicle for listeriosis.

Listeria monocytogenes, a food-borne pathogen

The gram-positive bacterium Listeria monocytogenes is an ubiquitous, intracellular pathogen which has been implicated within the past decade as the causative organism in several outbreaks of foodborne disease. Listeriosis, with a mortality rate of about 24%, is found mainly among pregnant women, their fetuses, and immunocompromised persons, with symptoms of abortion, neonatal death, septicemia, and meningitis. Epidemiological investigations can make use of strain-typing procedures such as DNA restriction enzyme analysis or electrophoretic enzyme typing. The organism has a multifactorial virulence system, with the thiol-activated hemolysin, listeriolysin O, being identified as playing a crucial role in the organism's ability to multiply within host phagocytic cells and to spread from cell to cell. The organism occurs widely in food, with the highest incidences being found in meat, poultry, and seafood products. Improved methods for detecting and enumerating the organism in foodstuffs are now available, including those based on the use of monoclonal antibodies, DNA probes, or the polymerase chain reaction. As knowledge of the molecular and applied biology of L. monocytogenes increases, progress can be made in the prevention and control of human infection.