Growth/survival of psychrotrophic pathogens on meat packaged under modified atmospheres

Combined Effect of Organic Acids and Modified Atmosphere Packaging on Listeria monocytogenes in Chicken Legs

The combined effect of organic acid (citric, propionic or acetic acid) treatment and modified atmosphere packaging (MAP) on the growth of L. monocytogenes in chicken legs kept at 4 °C for 10 days was evaluated. Chicken legs were inoculated with L. monocytogenes and washed with either 2% citric, 2% propionic or 2% acetic acid solution or distilled water (control). Legs were packaged under the following conditions: air, vacuum, 80% N2/20% CO2, 60% N2/40% CO2 or 40% N2/60% CO2. The greatest L. monocytogenes growth reductions after treatment were observed in chicken legs washed with propionic acid (2.14 log units lower compared to control legs). The lowest growth rates of L. monocytogenes were found in samples washed with acetic acid and packaged in atmospheres containing CO2. An extended shelf life was observed in legs packaged in 40% N2/60% CO2, but these packaging conditions did not reduce L. monocytogenes growth. Consequently, it is necessary to design measures in order to control this bacterial pathogen. Washing of chicken with 2% propionic acid or 2% acetic acid can decrease L. monocytogenes counts in chicken packaged in MAP.

Evaluation of the microbiological safety and sensory quality of a sliced cured-smoked pork product with protective cultures addition and modified atmosphere packaging

The aim of this study was to evaluate the effect of two protective lactic acid bacteria cultures combined with modified atmosphere packaging on the survival/growth of Listeria innocua 2030c (as a surrogate for Listeria monocytogenes) and on sensory attributes of ready-to-eat 'lombo' over storage time. Sliced 'lombo', a traditional cured-smoked pork loin, was inoculated with L. innocua 2030c, Lactobacillus sakei ST153 (isolated from 'salpicão') and BLC35 culture (with Lactobacillus curvatus, Staphylococcus xylosus and Pediococcus acidilactici; CHR Hansen) as protective cultures. Samples were packed in two modified atmosphere packaging conditions (20% CO₂/80% N₂ and 40% CO₂/60% N₂) and stored at 5 ℃ for 124 days. Both cultures led to a reduction of 1-2 log CFU/g of L. innocua 2030c after 12 h; however, at the end of storage only Lb. sakei ST153 maintained this antilisterial effect, which was more evident at 40% CO₂/60% N₂. The influence of cultures addition and modified atmosphere packaging conditions on the sensory characteristics of the product were not significant. Thus, Lb. sakei ST153 combined with modified atmosphere packaging is a strong candidate to be used in a biopreservation strategy maintaining the traditional sensory quality of cured-smoked pork products and increasing their safety with respect to Listeria spp.

Effects of vacuum and high-oxygen modified atmosphere packaging on physico-chemical and microbiological properties of minced water buffalo meat

OBJECTIVE

In this study, the effects of vacuum (VP) and high-oxygen modified atmosphere (80% O2+20% CO2) packaging (HiOx-MAP) on physico-chemical and microbiological properties of minced water buffalo meat were investigated.

METHODS

After minced meat preparation, samples were packaged under VP and HiOx-MAP and stored at 2°C±0.5°C for 14 days. Samples taken on certain days were subjected to total aerobic mesophilic bacteria, total aerobic psychrotrophic bacteria, lactic acid bacteria, Pseudomonas, Enterobacteriaceae and yeast-mold counts as well as pH, color (L*, a*, and b*) and thiobarbituric acid reactive substances (TBARS) analyses.

RESULTS

In minced water buffalo meat packaged under HiOx-MAP, TBARS value exceeded 1 mg malondialdehyde/kg on the 4th day of the storage. In VP samples, TBARS value remained close to initial TBARS value during storage. According to the findings, a* value was determined to be high in the HiOx-MAP samples within initial days of the storage. However, no significant changes in a* value were observed in VP samples during storage. In contrast, the mean value of L* was detected as higher in HiOx-MAP sample than VP samples. The count of psychrotrophic bacteria increased more than that of mesophilic bacteria during storage. The growth of Enterobacteriaceae and Pseudomonas was delayed in both the packaging methods. However, lactic acid bacteria exhibited more growth in VP samples compared to MAP samples.

CONCLUSION

The lipid oxidation proceeded faster than expected in minced water buffalo meat packed with HiOx-MAP method. This situation adversely affected the a* value. On the other hand, similar microbiological results were obtained in both packing methods.

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.

Shelf life determination of sliced Portuguese traditional blood sausage--Morcela de Arroz de Monchique through microbiological challenge and consumer test

Morcela de Arroz (MA) is a ready-to-eat blood and rice cooked sausage produced with pork, blood, rice, and seasonings, stuffed in natural casing and cooked above 90 °C/30 min. It is commercialized whole, not packed, with a restricted shelf life (1 wk/0 to 5 °C). The objective of this work was to establish sliced MA shelf life considering both the behavior of L. monocytogenes through a microbiological challenge test (MCT) and the consumer acceptability of MA stored: vacuum packed (VP), modified atmosphere packed (MAP: 80% CO2/20% N2 ), and aerobic packed (AP). The MCT was conducted inoculating ±3 log CFU/g of L. monocytogenes cell suspension on the MA slices. Packaged samples were stored at 3 ± 1 °C and 7 ± 1 °C until 20 d. At 3 ± 1 °C, L. monocytogenes behavior was not affected by packaging or storage time. At 7 ± 1 °C, the pathogen increased nearly 1 log CFU/g in the first 4 d. L. monocytogenes populations in AP were higher (P < 0.05) than in MAP. The pathogen may grow to hazardous levels in the 1st days if a temperature abuse occurs. Considering the acceptability by the consumers, the shelf life of MA stored at 3 ± 1 °C was 4.4 d for AP, 8.1 d for VP, and 10.4 d for MAP. The sensory shelf life established based on sensory spoilage is shorter than the shelf life to maintain the population of L. monocytogenes in safe levels.

Monitoring of chicken meat freshness by means of a colorimetric sensor array

A new optoelectronic nose to monitor chicken meat ageing has been developed. It is based on 16 pigments prepared by the incorporation of different dyes (pH indicators, Lewis acids, hydrogen-bonding derivatives, selective probes and natural dyes) into inorganic materials (UVM-7, silica and alumina). The colour changes of the sensor array were characteristic of chicken ageing in a modified packaging atmosphere (30% CO(2)-70% N(2)). The chromogenic array data were processed with qualitative (PCA) and quantitative (PLS) tools. The PCA statistical analysis showed a high degree of dispersion, with nine dimensions required to explain 95% of variance. Despite this high dimensionality, a tridimensional representation of the three principal components was able to differentiate ageing with 2-day intervals. Moreover, the PLS statistical analysis allows the creation of a model to correlate the chromogenic data with chicken meat ageing. The model offers a PLS prediction model for ageing with values of 0.9937, 0.0389 and 0.994 for the slope, the intercept and the regression coefficient, respectively, and is in agreement with the perfect fit between the predicted and measured values observed. The results suggest the feasibility of this system to help develop optoelectronic noses that monitor food freshness.

Survival of Campylobacter jejuni and Salmonella enterica Typhimurium in vacuum-packed, moisture-enhanced pork

The abilities of Campylobacter jejuni and Salmonella enterica Typhimurium to survive in vacuum-packaged, moisture-enhanced pork stored at 4 or 10°C were examined. Pork loins were surface inoculated with either C. jejuni or Salmonella Typhimurium and then moisture enhanced to a target of 10 or 20%. The enhanced pork loins were sliced 1 cm thick and vacuum packaged. A pork loin without moisture enhancement was sliced and vacuum packaged as a control. Samples were collected, plated, and the numbers of surviving organisms were determined periodically during storage at 4 and 10°C. The numbers of C. jejuni or Salmonella Typhimurium in samples with different moisture enhancement levels were similar (P > 0.05). No significant differences (P > 0.05) in C. jejuni counts were observed between samples at 10°C and those at 4°C. In contrast, the numbers of Salmonella Typhimurium in samples at 10°C had significantly (P < 0.05) increased (0.41 log CFU/g) from those at the refrigerated temperature of 4°C. Vacuum storage at 4 and 10°C for 28 days did not result in dramatic reductions in the mean numbers of C. jejuni and Salmonella Typhimurium. Our findings indicate that vacuum packaging under chilled conditions will not add substantially to safety for moisture-enhanced pork. Strict hygienic practices or the implementation of decontamination technologies is recommended.

Development of a new method for the detection of lactic acid bacteria capable of protecting ham against Enterobacteriaceae

AIMS

Challenge trials seem to be the best assessment approach to evaluate the potency of food protective cultures. However, this method is time consuming and often difficult to implement. Here, we describe the development of the 'sequential culturing method', a new method for the screening of strains as protective cultures.

METHODS AND RESULTS

The sequential culturing method is based on the simulation, in a meat simulation medium (named BHI5L200), of the inhibition of Enterobacteriaceae by Lactobacillus, observed previously in situ. Results obtained with this sequential culturing method were in good agreement with those of the challenge test on sliced cooked ham and confirmed the antagonistic potency of Lactobacillus. The results obtained from the screening of 187 lactic acid bacteria (LAB) indicated that Lactobacillus sakei, Lactococcus lactis diacetylactis and Carnobacterium spp. were strong inhibitors of Enterobacteriaceae whereas Pediococcus spp., Leuconostoc spp., Weisselia spp. and other species of Lactobacillus and Lactococcus, did not possess the same inhibitory capacity.

CONCLUSIONS

Sequential culturing method appeared to be a useful tool to rapidly select LAB cultures which are good candidates for bioprotection of meat.

SIGNIFICANCE AND IMPACT OF THE STUDY

Sequential culturing method and simulating media could efficiently mimic challenge test experiments in the selection of potential protective culture for all types of food, on the condition to have the appropriate simulating media, corresponding to the food for which protective cultures were searched.

Effect of different concentrations of carbon dioxide and oxygen on the growth of pathogenic Yersinia enterocolitica 4/O:3 in ground pork packaged under modified atmospheres

The influence on Yersinia enterocolitica counts of a gradual increase of carbon dioxide concentrations (percentage by volume in air) during packaging and storage of ground pork meat artificially contaminated with this pathogen was evaluated. Ground meat was packaged under customary conditions using modified atmospheres with various carbon dioxide percentages (0, 30, 50, 70, and 100% CO2 by volume; for atmospheres of less than 100% CO2, the rest of the gas was O2). The packs were stored at 2 degrees C for 12 days. During the entire storage time, counts of Y. enterocolitica were determined by the spread plate method for direct plate counts (DPCs). Microbiological shelf life of the stored ground pork also was assessed by total mesophilic aerobic bacterial plate counts (APCs). Y. enterocolitica counts were not significantly different (P > or = 0.05) in the ground pork packaged under the various CO2-enriched atmospheres. The growth of Y. enterocolitica was nearly entirely inhibited in all tested modified atmospheres containing the protective CO2. However, in ground pork packaged with 100% oxygen, there was a significant decrease (P < or = 0.05) in the DPC for Y. enterocolitica from 4.30 log CFU/g (day 0) to 3.09 log CFU/g at the end of the storage time (day 12). The decrease was presumably due to the marked increase in APC seen only in those packages stored under 100% O2. Packaging with high CO2 concentrations had significant inhibitory effect (P < or = 0.05) on the growth of mesophilic aerobic bacteria.

Modified-atmosphere storage under subatmospheric pressure and beef quality: I. Microbiological effects1

The microflora was studied in beef stored in stainless steel containers kept under reduced pressure (20 to 30 kPa) in a modified atmosphere (70% N2 + 30% CO2 or pure CO2) at 3 to 4 degrees C and 0 to 1 degrees C at a headspace:meat volume ratio of 2:1. Samples were obtained at weekly intervals, 1 to 3 times. Total colony counts (TCC) for Pseudomonas spp. and Brochothrix thermosphacta were generally 1 to 2 log10 cfu greater than in the control group of vacuum-packaged beef cuts stored at the same temperatures. In containers with the 70% N2 + 30% CO2 atmosphere at 20 to 30 kPa and 3 to 4 degrees C, substantial growth of Pseudomonas sp. was observed (median of 6 log10 cfu/cm2 at d 21 of storage compared with 3 log10 cfu/cm2 for vacuum-packaged beef). Pseudomonas counts were lower when the container system was held at 0 to 1 degrees C, especially when combined with the pure CO2 atmosphere. As expected for CO2-enriched atmospheres, B. thermosphacta was the dominant spoilage bacterium, in the same log10 order as the TCC. Lowering the storage temperature and changing the atmosphere to pure CO2 resulted in a reduction of 1 log10 for TCC (median values after 2 wk of storage). Although pathogenic bacteria such as Campylobacter, Salmonella, and Listeria monocytogenes were not detected in any sample, further studies are necessary to evaluate potential growth risks. The results demonstrate that CO2-enriched and O2-depleted atmospheres under low pressure have a limited effect on reducing bacterial growth, probably because the antibacterial activity of CO2 is proportional to the effective concentration of this gas in the headspace. At pressures of 20 to 30 kPa, a headspace with pure CO2 would still contain only approximately 20 to 30% CO2.

Changes in the spoilage-related microbiota of beef during refrigerated storage under different packaging conditions

The microbial spoilage of beef was monitored during storage at 5 degrees C under three different conditions of modified-atmosphere packaging (MAP): (i) air (MAP1), (ii) 60% O2 and 40% CO2 (MAP2), and (iii) 20% O2 and 40% CO2 (MAP3). Pseudomonas, Enterobacteriaceae, Brochothrix thermosphacta, and lactic acid bacteria were monitored by viable counts and PCR-denaturing gradient gel electrophoresis (DGGE) analysis during 14 days of storage. Moreover, headspace gas composition, weight loss, and beef color change were also determined at each sampling time. Overall, MAP2 was shown to have the best protective effect, keeping the microbial loads and color change to acceptable levels in the first 7 days of refrigerated storage. The microbial colonies from the plate counts of each microbial group were identified by PCR-DGGE of the variable V6-V8 region of the 16S rRNA gene. Thirteen different genera and at least 17 different species were identified after sequencing of DGGE fragments that showed a wide diversity of spoilage-related bacteria taking turns during beef storage in the function of the packaging conditions. The countable species for each spoilage-related microbial group were different according to packaging conditions and times of storage. In fact, the DGGE profiles displayed significant changes during time and depending on the initial atmosphere used. The spoilage occurred between 7 and 14 days of storage, and the microbial species found in the spoiled meat varied according to the packaging conditions. Rahnella aquatilis, Rahnella spp., Pseudomonas spp., and Carnobacterium divergens were identified as acting during beef storage in air (MAP1). Pseudomonas spp. and Lactobacillus sakei were found in beef stored under MAP conditions with high oxygen content (MAP2), while Rahnella spp. and L. sakei were the main species found during storage using MAP3. The identification of the spoilage-related microbiota by molecular methods can help in the effective establishment of storage conditions for fresh meat.

Shelf life of fresh sausages stored under modified atmospheres

The aim of this study was to investigate differences in modified atmosphere packaging (MAP) for the improvement of the shelf life of fresh meat products. Three different conditions for preserving fresh sausages were tested: MAP1 (20% CO2, 70% O2, and 10% N2), MAP2 (40% CO2 and 60% O2), and MAP3 (40% CO2, 30% O2, and 30% N2). Samples from the MAP2 group had fewer spoilage bacteria, stable red color (no change of a* value), and good physical attributes (high water-holding capacity, little loss from cooking, and low shear force needed for cutting) compared with samples from other treatment groups. Thus, high concentrations of CO2 (40%) and O2 (60%) resulted in a longer shelf life for fresh sausages.

Effect of prestorage treatmlents and storage conditions on the survival of Salmonella enteritidis PT4 and Listeria monocytogenes on fresh marine and freshwater aquaaculture fish

The effect of prestorage treatments, such as immersion in a sorbate solution (5%, wt/vol), heating (60 degrees C, 1 min), and a combination of the two treatments, and the subsequent storage in air or under modified atmosphere packaging (MAP; 40% CO2, 30% O2, and 30% N2) at chill temperatures (0 +/- 1 degrees C), on Listeria monocytogenes and Salmonella Enteritidis PT4 was studied. The prestorage treatments affected the pathogenic bacteria, and in all cases, there was a decrease in their population, with the sorbate and combination (hot water and sorbate) treatment being most effective. The beneficial effect of the prestorage treatments, which was more pronounced in storage under MAP conditions, suggests an interaction of the treatments with the CO2 of MAP against injured bacterial cells.

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.

Preservation of fresh meat with active and modified atmosphere packaging conditions

The sensory, microbiological and physicochemical attributes of fresh meat stored at 5 and 15 degrees C were affected by the combined effect of volatile compounds of oregano essential oil and modified atmosphere packaging conditions (40% CO2/30% N2/30% O2, 100% CO2, 80% CO2/20% air, vacuum pack and air). It was found that the extension of shelf life of meat samples depended on the packaging conditions and augmented in the order: air < vacuum pack < 40% CO2/30% N2/30% O2 < 80% CO2/ 20% air < 100% CO2. Longer shelf life was observed in samples supplemented with the volatile compounds of oregano essential oil and stored under the same packaging conditions mentioned above. The extension of shelf life may be due to the synergistic effect of volatile compounds of oregano essential oil and the modified atmosphere packaging used on the microbiological and physicochemical characteristics of meat. Indeed, both these hurdles can prolong and delay microbial growth or suppress the final counts of the spoilage microorganisms in comparison with the 'control' samples. The effect of essential oil volatile compounds was even more pronounced on the physicochemical changes of meat samples caused by microbial association. Oregano essential oil delayed glucose and lactate consumption, both indicators of meat spoilage aerobically as well as under 40% CO2/30% N2/30% O2, and 100% CO2. Finally, changes in other metabolites such as formic acid were also observed.

Behaviour of Listeria monocytogenes and autochthonous flora on meat stored under aerobic, vacuum and modified atmosphere packaging conditions with or without the presence of oregano essential oil at 5 degrees C

The effect of aerobic, modified atmosphere packaging (MAP; 40% CO2/30% O2/30% N2) and vacuum packaging (VP) on the growth/survival of Listeria monocytogenes on sterile and naturally contaminated beef meat fillets was studied in relation to film permeability and oregano essential oil. The dominant micro-organism(s) and the effect of the endogenous flora on the growth/survival of L. monocytogenes were dependent on the type of packaging film. The fact that L. monocytogenes increased whenever pseudomonads dominated, i.e. aerobic storage and MAP/VP in high-permeability film, and even earlier than on sterile tissue, suggests that this spoilage group enhanced growth of the pathogen. Brochothrix thermosphacta constituted the major proportion of the total microflora in MAP/VP within the low-permeability film, where no growth of L. monocytogenes was detected either on naturally contaminated or sterile meat fillets. The addition of 0.8% (v/w) oregano essential oil resulted in: (i) an initial reduction of 2-3 log10 of the majority of the bacterial population, with lactic acid bacteria and L. monocytogenes indicating the most apparent decrease in all gaseous environments, and (ii) limited growth aerobically and survival/death of L. monocytogenes in MAP/VP, regardless of film permeability.

Comparison between the growth of Yersinia enterocolitica, listeria monocytogenes, escherichia coli O157:H7 and salmonella spp. in ground beef packed by three commercially used packaging techniques

Growth of the pathogens Yersinia enterocolitica, Listeria monocytogenes, Escherichia coli O157:H7 and strains of Salmonella were compared in ground beef packed in modified atmospheres of 60% CO2/40% N2/0.4% CO (high CO2/low CO mixture), 70% O2/30% CO2 (high O2 mixture) and in chub packs (stuffed in plastic casings). The ground beef was inoculated with rifampicin-resistant or nalidixic acid/streptomycin-resistant strains of the pathogens (final concentration 10(2) - 10(3) bacteria/g) and stored at 4 and 10 degrees C for up to 14 days. At 4 degrees C the shelf life, based on colour stability and background flora development, was prolonged for the high CO2/low CO mixture compared to the two other packaging methods, but at 10 degrees C the shelf life was < 8 days for all the packaging methods. Growth of Y. enterocolitica was nearly totally inhibited both at 4 and 10 degrees C in the high CO2/low CO mixture, while the bacterial numbers in the samples packed in the high O2 mixture increased from about 5 x 10(2) bacteria/g at day 0 to about 10(4) at day 5 at 4 degrees C and to 10(5) at 10 degrees C. Growth in the chub packs was even higher. L. monocytogenes showed very little growth at 4 degrees C in all treatments. At 10 degrees C there was slow growth from about 5 x 10(3) bacteria/g to about 10(4) at day 5 in the high CO2/low CO mixture, while the numbers in the high O2 mixture and the chub packs were about 10 times higher. Growth of E. coli O157:H7 at 10 degrees C in the ground beef was nearly totally inhibited in both the high CO2/low CO mixture and the high O2 mixture. Growth in the chub packs was higher, as the number of bacteria increased 3 log in 5 days. The Salmonella strains (S. typhimurium, S. dublin, S. enteritidis and S. enterica 61:k:1,5,(7)) in the ground beef stored at 10 degrees C for 5 and 7 days grew to a higher number in the high CO2/low CO mixture than in the high O2 mixture. This study shows that the growth of Y. enterocolitica and L. mononcytogenes in ground beef stored in the high CO2 /low CO mixture was not increased as a result of prolonging the shelf life. However, the observed growth of strains of Salmonella at 10 degrees C in this mixture and in chub packs does emphasise the importance of temperature control during storage.

Predictive model for the growth of Yersinia enterocolitica under modified atmospheres

A quadratic response surface model is presented to describe the maximum specific growth rate of Yersinia enterocolitica, at refrigeration temperatures, under modified atmospheres. The presence of CO2 affected mainly the lag phase of the organism. The length of the lag phase increased with higher levels of CO2 in the atmosphere, and this effect was more noticeable at low temperatures. The effect of oxygen was similar but less pronounced. The observed growth was slower with higher CO2. Oxygen also decreased the growth rate, but its effect was significant only when its proportion in the atmosphere was greater than about 40%. Model predictions were compared with growth rates obtained in sea food inoculated with Y. enterocolitica and packaged under modified atmospheres. Predictions were also checked to determine whether they were inside the strict interpolation region of the model.

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.

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.

Growth/survival of Salmonella enteritidis on fresh poultry and fish stored under vacuum or modified atmosphere

The effect of vacuum and modified atmosphere packaging on the growth/survival of Salmonella enteritidis on fresh poultry and fish (Boops boops) is described. Salmonella enteritidis survived but did not grow significantly in all samples (poultry or fish) at 3 degrees C. At 10 degrees C the numbers of Salm. enteritidis increases rapidly in vacuum-packed samples and in samples flushed with 100% N2, 20% CO2/80% O2 of both types of proteinaceous food. Growth was also evident in fish and poultry flushed with 100% CO2; however the rate of growth was greater in fish samples rather than in poultry.

Model for the combined effects of temperature, pH, and sodium lactate on growth rates of Listeria innocua in broth and Bologna-type sausages

A modified Monod equation was successfully applied to describe the maximum specific growth rate of Listeria innocua in a broth model in the presence of various concentrations of sodium lactate or NaCl. The combined effects of temperature and pH were assessed by translating the parameters of the modified Monod equation mu(m), alpha, and p') as functions of pH and/or temperature. As a result, the area in which the growth rate could be predicted was extended to include as a variable not only the salt concentration but also pH and temperature. The number of parameters needed to describe the experimental data was thereby reduced from 48 to 4 (NaCl) and from 42 to 5 (sodium lactate). The decline in the goodness of fit that accompanied the reduction in the number of parameters was within statistically acceptable ranges. The resulting model was compared with a polynomial fit, and it was proposed that the former was more suitable for the purpose of this study. The broth model for sodium lactate was evaluated with Bologna-type sausages. Because of the "worst-case" design of the broth model, it was necessary to reestimate one or all parameters to obtain a good description of the growth rate of L. innocua in the meat product. However, the simplicity of the model and the practical usefulness of its parameters offer considerable prospects for its use in predictive microbiology.