Microbiological quality of maatjes herring stored in air and under modified atmosphere at 4 and 10 degrees C

Microbiological and sensory changes of maatjes herring stored in air (experiment I) and under modified atmosphere (MAP) (experiments II and III) were evaluated during storage at 4 and 10 degrees C. Microbial (total and psychrotrophic viable bacteria, lactic acid bacteria and Enterobacteriaceae) counts and chemical analyses (chloride content, fat content, dry matter, ash and pH) were performed. A Quality Index Method (QIM) scheme developed for maatjes herring was used for sensory evaluation. The main reasons for sensory rejections at both storage temperatures were a strong rancid taste for herring stored in air (Experiment I) and a sour, bitter, rotten taste and an aftertaste like old flower water for MAP herring (Experiments II and III). A soft texture of freshly produced samples (Experiment II) was noticed. The sensory shelf-life of maatjes herring stored in air (Experiment I) was three days at both 4 and 10 degrees C. The MAP herring in Experiments II and III had a shelf-life of 5 and 6 days, respectively, at both storage temperatures. Rancidity due to oxidation of fat was the main spoilage indicator for air-stored maatjes herring. Autolytic enzymes may affect textural deterioration. The characteristic off-odour and off-taste in the MAP herring (Experiments II and III) were may well be attributable to microbial metabolism. On the day of sensory rejection, total viable counts for herring in all three experiments (Experiments I-III) stored at 4 degrees C did not reach 10(6)cfu/g, which is considered the limit of acceptability for maatjes herring given by the Dutch fishery authorities. It appears that total viable counts have minor significance in the sensory assessment of maatjes herring.     

Combined effect of oregano essential oil and modified atmosphere packaging on shelf-life extension of fresh chicken breast meat, stored at 4 degrees C

The combined effect of oregano essential oil (0.1% and 1% w/w) and modified atmosphere packaging (MAP) (30% CO2/70% N2 and 70% CO2/30% N2) on shelf-life extension of fresh chicken meat stored at 4 degrees C was investigated. The parameters that were monitored were: microbiological (TVC, Pseudomonas spp., lactic acid bacteria (LAB), yeasts, Brochothrix thermosphacta and Enterobacteriaceae), physico-chemical (pH, TBA, color) and sensory (odor and taste) attributes. Microbial populations were reduced by 1-5 log cfu/g for a given sampling day, with the more pronounced effect being achieved by the combination of MAP and oregano essential oil. TBA values for all treatments remained lower than 1 mg malondialdehyde (MDA) kg(-1) throughout the 25-day storage period. pH values varied between 6.4 (day 0) and 5.9 (day 25). The values of the color parameters L*, a* and b* were not considerably affected by oregano oil or by MAP. Finally, sensory analysis showed that oregano oil at a concentration of 1% imparted a very strong taste to the product for which reason these lots of samples were not scored. On the basis of sensory evaluation a shelf-life extension of breast chicken meat by ca. 3-4 days for samples containing 0.1% oregano oil, 2-3 days for samples under MAP and 5-6 days for samples under MAP containing 0.1% of oregano oil was attained. Thus oregano oil and MAP exhibited an additive preservation effect.     

Quality attributes of farmed eel (Anguilla anguilla) stored under air, vacuum and modified atmosphere packaging at 0 degrees C

The shelf life of fresh eel in various packaging conditions of atmospheric air, vacuum and modified atmosphere packaging (MAP) (40% CO(2), 30% N(2) and 30% O(2)) at 0 degrees C was investigated. All raw eel samples received acceptable sensory scores during the first 11+/-1 days of storage in atmospheric air, 11+/-1 days of storage in vacuum and finally 18+/-1 days of storage in MAP conditions. Using the microbial quality indicators the shelf life of eel packed in air, vacuum and MAP was estimated to be more than 18, 28 and 34 days, respectively. The main spoilage microorganisms under MAP conditions were lactic acid producing bacteria followed by Shewanella spp., pseudomonads, Enterobacteriaceae and yeasts. Chemical data revealed that pH, ammonia, glucose and lactate examinations might not be useful for monitoring eel quality differences.     

Volatile metabolite production of spoilage micro-organisms on a mixed-lettuce agar during storage at 7 degrees C in air and low oxygen atmosphere

This paper describes the volatile metabolite production of spoilage bacteria (Pantoea agglomerans and Rahnella aquatilis) and spoilage yeasts (Pichia fermentans and Cryptococcus laurentii), previously isolated from mixed lettuce, on a simulation medium of shredded mixed lettuce (mixed-lettuce agar) both under air conditions and modified atmosphere (MA)-conditions at 7 degrees C. These latter conditions simulated the equilibrium modified atmosphere packaging, which is used to extend the shelf-life of shredded mixed lettuce. Besides volatile metabolites, organic acid metabolites and consumption of sugars were measured. Microbiological growth on the mixed-lettuce agar resulted in metabolite production and consumption of sugars. Bacteria and yeasts produced a range of volatile organic compounds both under air conditions and MA-conditions: ethanol, ethyl acetate, 2-methyl-1-propanol, 2-methyl-1-butanol, 3-methyl-1-butanol, 2,3-butanedione, 3-methyl-1-pentanol, 1-butanol and 1-hexanol. Under MA-conditions, 2-methyl-1-butanol, 3-methyl-1-butanol and ethanol were the first compounds that were detected in the headspace as being produced by the inoculated micro-organisms. In the case of the yeast P. fermentans, production of these compounds was detected from a count of 5.0+/-0.1 log cfu/cm(2) with a fast increase when exceeding 6.0-6.5 log cfu/cm(2). Unlike P. fermentans, the yeast C. laurentii showed a slow metabolism under MA-conditions, compared to air conditions. In the case of the bacteria, production of 2-methyl-1-butanol and 3-methyl-1-butanol was detected starting from a count of 6.7+/-0.1 log cfu/cm(2) in the case of R. aquatilis and from a count of 7.1+/-0.4 log cfu/cm(2) in the case of P. agglomerans with a fast increase when exceeding 8 log cfu/cm(2). No production of ethanol by the bacteria under MA-conditions was detected in contradiction to air conditions. It could be concluded that, if these counts are reached on the cut surfaces of shredded mixed lettuce which are simulated by the mixed-lettuce agar, sensorial quality of shredded mixed lettuce could be influenced by the microbiological production of metabolites.     

Antimicrobial activities of spice extracts against pathogenic and spoilage bacteria in modified atmosphere packaged fresh pork and vacuum packaged ham slices stored at 4 °C

The antimicrobial activity of 14 spice extracts against four common meat spoilage and pathogenic bacteria (Listeria monocytogenes, Escherichia coli, Pseudomonas fluorescens and Lactobacillus sake) was screened in cultured media (experiment 1). The results showed that individual extracts of clove, rosemary, cassia bark and liquorice contained strong antimicrobial activity, but the mixture of rosemary and liquorice extracts was the best inhibitor against all four types of microbes. Subsequently, mixed rosemary/liquorice extracts were spray-applied to inoculated fresh pork in modified atmosphere packaging (experiment 2) and to inoculated ham slices in vacuum packaging (experiment 3). The meat samples were stored at 4 °C over a 28-day period and microbial growth was monitored regularly. The L. monocytogenes population on fresh pork by day 28 decreased 2.9, 3.1 and 3.6 logs, the MAB decreased 2.7, 2.9 and 3.1 logs, the Pseudomonas spp. count decreased 1.6, 2.1 and 2.6 logs and the total coliform count decreased 0.6, 0.8 and 1.2 logs, corresponding to 2.5, 5.0 and 10.0 mg/ml of spray, respectively, when compared to control (P < 0.05). The number of L. monocytogenes on ham slices decreased 2.5, 2.6 and 3.0 logs, the MAB plate counts decreased 2.9, 3.0 and 3.2 logs and the LAB counts decreased 2.4, 2.6 and 2.8 logs (P < 0.05), respectively, after 28-days, by the same levels of mixed rosemary/liquorice extract treatments. The results demonstrated strong potential of mixed rosemary and liquorice as a natural preservative in fresh pork and ham products.     

Biogenic amines in vacuum-packaged and carbon dioxide-controlled atmosphere-packaged fresh pork stored at -1.50 degrees C

Biogenic amines are formed in foods as a result of amino acid decarboxylation catalyzed by bacterial enzymes. When consumed in sufficient quantities, these compounds will cause headache, hypertension, fever, and heart failure. Technologies such as vacuum packaging and carbon dioxide-modified atmosphere packaging (CO2-MAP), when combined with low-temperature storage (-1.5 degrees C), allow fresh pork to have a storage life long enough for export to overseas markets. During low-temperature storage of pork in these packaging systems, the lactic acid bacteria (LAB), which possess the enzymes for biogenic amine formation, dominate the microflora. The objectives of this study were to determine the quantities of biogenic amines in packaged fresh pork, to monitor LAB growth, and to determine the storage life by sensory evaluation. Vacuum-packaged and CO2-MAP pork were stored at -1.5+/-0.5 degrees C for 9 and 13 weeks, respectively. Phenylethylamine, putrescine, cadaverine, histamine, tyramine, spermidine, and spermine concentrations were determined weekly by high-performance liquid chromatography and capillary gel electrophoresis. LAB and carnobacteria were enumerated weekly. Samples were evaluated for odor and appearance. The CO2-MAP was successful in delaying bacterial growth and the development of unacceptable off-odors compared with the vacuum packaging. The storage lives of the vacuum-packaged and CO2-MAP pork were 5 and 13 weeks, respectively. High-performance liquid chromatography was the superior method for biogenic amine quantification. Tyramine and phenylethylamine in pork of both packaging treatments approached levels considered to be potentially toxic. Given Canada's increasing role in the export of fresh meat to foreign markets, it is recommended that the formation of biogenic amines in vacuum-packaged and CO2-MAP pork be further investigated.     

The effect of glucose supplementation on the spoilage microflora and chemical composition of minced beef stored aerobically or under a modified atmosphere at 4 °C

Glucose was added to minced meat (pH 6.0) and stored under aerobic or a modified atmosphere (MA) composed of 80% O₂ and 20% CO₂ to assess the effects of carbohydrate on the microbial association and the chemical properties of the meat. The type of packaging affected the size and the final composition of the microflora. The microbial composition of the mince without added glucose in air or MA, given in log₁₀cfu/g respectively, were pseudomonads (9.8 and 7.3), Brochothrix thermosphacta (8.5 and 8.1), lactic acid bacteria (8.8 and 8.7), Enterobacteriaceae (7.2 and 6.1) and yeasts (4.3 and 4.2). In mince supplemented with 0.2% (w/w) glucose, similar composition and numbers were observed. Glucose, glucose 6-phosphate and lactic acid were consumed at slower rates by the flora on meat stored under MA than by the flora on meat stored in air. The addition of glucose enhanced gluconate production by the flora on meat stored in air. D-lactic and acetic acid were produced in all samples stored under the MA.     

The effect of vacuum and modified atmosphere packaging on the shelf-life of lamb primals, stored at different temperatures

Lamb primals (shoulders) were vacuum packaged or packaged in modified atmospheres containing 80% O(2)/20% CO(2), 50% CO(2)/50% N(2) or 100% CO(2), and stored at 5 or 0 °C. They were examined microbiologically at 7 day intervals for total counts obtained under (1) aerobic, (2) CO(2) enriched or (3) anaerobic conditions; B. thermosphacta; pseudomonad and Enterobacteriaceae counts. Off-odour assessments were also carried out at these times. In general, there were no significant differences between the total counts obtained from the different incubation conditions in any of the atmospheres. The only exception was noted in 80% O(2)/20% CO(2) at 5 °C. Significant differences between atmospheres for the total counts were observed at 0 °C only. In the case of B. thermosphacta, the pseudomonads and the Enterobacteriaceae, differences between atmospheres were noted at 5 and 0 °C. In general, vacuum packs and 80% O(2)/20% CO(2), and the two high CO(2) atmospheres fell into distinct groups. Storage temperature had a significant effect on all three counts. The relationship between bacterial counts and time was modelled using regression analysis. Data from total counts gave the equations of best fit. Significant differences between atmospheres in terms of off-odour production were observed at 5 °C only. The effect of temperature on off-odour production was significant in all atmospheres except 100% CO(2). A scheme was devised based on the growth of different groups of organisms which facilitated comparisons between studies on packaged meats. The results of the present work and that of others are discussed in relation to the different growth patterns which developed with packaging treatments and storage temperature.     

A combination of modified atmosphere and antimicrobial packaging to extend the shelf-life of beefsteaks stored at chill temperature

An antimicrobial polyethylene (PE) film was obtained by coating a nisin-based antimicrobial solution. PE sheets were coated on both sides and were used for the packaging of beefsteaks to be stored in air or modified atmosphere packaging (MAP, 60% O(2)-40% CO(2)). Microbial populations, species diversity, headspace volatile organic compounds, colour and sensory properties were monitored after 0, 1, 7 and 12days of storage at 4°C. The viable counts showed that there was an effect of MAP and antimicrobial film on the development of all the spoilage associated microbial populations. Carnobacterium spp., Brochothrix thermosphacta, Pseudomonas fragi and Rhanella aquatilis were found in most of the samples. C. maltaromaticum was identified in MRS bulk cells from samples stored in air as well as MAP. Quantitative data of headspace-SPME-GC/MS analysis showed that during storage the production of volatile organic compounds (VOCs) was affected by the use of the treated film and the MAP storage. Compounds such as phenylethylalcohol, nonanal, decanal and ethylbutanoate were produced only from 7 to 12day of storage and only in the samples stored in air. In agreement with the microbiological and VOCs data, the meat stored in active packaging scored the best rankings in the sensory evaluation. Principal component analysis of microbial, sensory and instrumental data showed that beefsteaks stored with the combination of MAP and active packaging for 12days at 4°C differed from the other samples that were more associated to high microbial loads, VOCs concentration and meat off odour perception. In conclusion, the antimicrobial sheets in combination with MAP storage at 4°C were effective for the storage of beefsteaks by retarding the growth of spoilage bacteria, determining lower concentration of VOCs and keeping acceptable levels of colour and other sensory parameters for more than 10days.     

Effect of packaging atmosphere on the microbial attributes of pearlspot (Etroplus suratensis Bloch) stored at 0-2 degrees C

Effect of packaging atmosphere (air and under different modified atmospheres (MAs), 40% CO2/60% O2, 50%/50% O2, 60% CO2/40% O2, 70% CO2/30% O2 and 40% CO2/30% O2/30% N2) on the microbial and biochemical attributes of fresh pearlspot (Etroplus suratensis Bloch) stored at 0-2 degrees C was investigated. Trimethylamine nitrogen (TMA-N) and thiobarbituric acid (TBA) values remained lower than the proposed acceptability limits throughout the storage period. Results demonstrated that storage of pearlspot under air and MA 40% CO2/30% O(2)/30% N(2) resulted in growth of Enterobacteriaceae, Aeromonas and H(2)S-producing bacteria including Shewanella putrefaciens, while all other packaging atmospheres did not allow multiplication of Enterobacteriaceae and Aeromonas within 3 weeks. Aeromonas spp. identified were Aeromonas hydrophila, Aeromonas veronii biovar sobria and A. veronii biovar veronii. Significant reduction (p<0.01) was noticed in Aeromonas population of pearlspot stored under MA 60% CO2/40% O2 and 70% CO2/30% O2. A delay of growth of Pseudomonas below 5.0log(10)cfug(-1) was observed during the 15th day of storage at 0-2 degrees C under modified atmosphere packaging (MAP) conditions. Growth of faecal streptococci was significantly inhibited in all the packaging atmospheres at 0-2 degrees C during the entire storage period. Survival of coagulase positive Staphylococci (<50cfug(-1)) in low numbers was noticed during storage in all the packaging atmospheres. Clostridium botulinum toxin was not detected. All the packaging atmospheres did not allow multiplication of sulphite-reducing clostridia at 0-2 degrees C during the entire storage period. Packaging in MA 60% CO2/40% O2 resulted in the inhibition of growth of Aeromonas and Enterobacteriaceae, and the slowest growth of psychrotrophic bacteria, H(2)S-producing bacteria, including Shewanella putrefaciens and Pseudomonas and extended microbiological shelf life to 9-10 days. This study confirms the survival of potentially pathogenic A. hydrophila, A. veronii biovar sobria and A. veronii biovar veronii capable of growth at low temperature in pearlspot stored under MA.     

Effect of irradiation and modified atmosphere packaging on the microbiological and sensory quality of pork stored at refrigeration temperatures

The effect of combining low-dose irradiation (1.75 kGy) with modified atmosphere packaging (MAP) on the microbiological and sensory quality of pork chops stored at refrigeration temperatures was studied. The microflora of irradiated MAP pork was almost exclusively composed of lactic acid bacteria, predominantly Lactobacillus spp. Modified atmospheres containing either 25 or 50% CO₂, balance N₂, resulted in the best microbial control in irradiated pork held at 4°C, compared to an unirradiated MAP control, and these atmospheres were subsequently used in sensory studies. The atmosphere containing 25% CO₂ 75% N₂ maintained the uncooked colour and odour of irradiated pork chops more effectively than 50% CO₂ 50% N₂. Therefore packaging in a modified atmosphere containing 25% CO₂, balance N₂, followed by irradiation to a dose of 1.75 kGy is recommended to improve the microbiological and sensory quality of pork chops.     

Effect of irradiation and modified atmosphere packaging on the microbiological safety of minced pork stored under temperature abuse conditions

The safety of irradiated pork packed in 25% CO₂:75% N₂ and stored at abuse temperature (10 or 15°C) was assessed by inoculation studies involving Salmonella typhimurium, Listeria monocytogenes, Escherichia coli, Yersinia enterocolitica and Clostridium perfringens . Irradiation to a dose of 1.75 kGy reduced pathogen numbers to below the detection limit of 10² cells g^-1. When higher inoculum levels were used (10⁶ cells g^-1) irradiation at 1.75 kGy reduced pathogen numbers by 1 –>5 log₁₀ cycles depending on strain. Clostridium perfringens was the most resistant, and Y. enterocolitica the most sensitive of the pathogens studied.
In all cases when high numbers (10⁶ to 10⁷g^-1) of spoilage and/or pathogenic bacteria were present initially on the pork the meat appeared spoiled, and although irradiation reduced the number of microorganisms, the meat was still unacceptable from a sensory viewpoint after treatment.
It was concluded that the microbiological safety of irradiated, modified atmosphere packaged (MAP) pork is better than that of unirradiated MAP pork.     

Quality of fresh retail pork cuts stored in modified atmosphere under temperature conditions simulating export to distant markets

The effect of storage temperature on microbial and sensory quality of retail cuts of pork was determined on samples stored under temperature regimens designed to simulate conditions that could be encountered in accessing distant markets with retail-ready product. Samples were packaged in modified atmosphere with 100% CO(2) and <200 ppm O(2) in plastic film with extremely low gas transmission rates. All samples were stored at -1·5°C for three weeks. Reference samples were held at -1·5°C for the duration of the study; experimental samples were transferred to 4°C (-1·5 4° C ) or 7°C (-1· 517° C ) and analyzed for microbial content and sebsory attributes including appearance, confinement and meat odours. Storage life of reference samples at -1·5°C was seven weeks before rejection for loss of acceptable appearance. With transfer of samples to 4 and 7°C after three weeks at -1·5°C, samples remained acceptable for retail sale for two weeks and one week, restpectively. The microbial flora was dominated by lactic acid bacteria under all three storage conditions. Appearance of the cuts was the principal criterion limiting storage life. Discoloration of the meat was not a problem in this study, but purge and odour, including sour and sulphur notes, became a problem with time. The study indicated that export of retail-ready pork cuts to distant markets with a three-week time for delivery to market at -1·5°C can be achieved with one to two weeks of marketing time at retail market at 4 to 7°C.     

Relation of biogenic amines to microbial and sensory changes of precooked chicken meat stored aerobically and under modified atmosphere packaging at 4 °C

The formation of biogenic amines and their correlation to microflora and sensory characteristics of a precooked chicken meat product stored aerobically and under modified atmosphere packaging (MAP) (30% CO₂, 70% N₂) was studied. Putrescine was the main amine formed both in aerobically and MA-packaged chicken samples. For the rest of the biogenic amines, including tyramine, histamine, and cadaverine, a stepwise increase was recorded throughout the 23-day storage period under the above packaging conditions. Spermidine was found in higher amounts, as compared to spermine in both aerobically and MA-packaged chicken samples at 4 °C. Formation of these amines in precooked chicken stored either aerobically or under a 30% CO₂, 70% N₂ atmosphere followed an inconsistent trend during the entire storage period at 4 °C. Agmatine, β-phenyl-ethylamine, and tryptamine were not detected in precooked chicken. Of the bacterial groups monitored, lactic acid bacteria (LAB) became the dominant bacteria after day 8 of storage under MAP while LAB were the dominant population of natural microflora of precooked chicken stored both aerobically or under MAP, reaching 7.5 and 8.0 log cfu/g, respectively, on day 23 of refrigerated storage. Enterobacteriaceae populations in chicken meat were below the detection limit (<1 log cfu/g) by pour plating throughout the 23-day storage period, irrespective of packaging conditions. Based on sensory data, after ca. 8 days for the precooked chicken meat stored aerobically and after 12 days under MAP (time to reach initial decomposition stage, score of 2) the putrescine and tyramine content of chicken samples were ca. 14–19 and 1.4 mg/kg, values that may be proposed as the limit for spoilage initiation of precooked chicken meat (respective TVC for both aerobically and MA-packaged chicken meat were ca. 6.5 log cfu/g).     

Survival of Salmonella enteritidis phage type 30 on inoculated almonds stored at -20, 4, 23, and 35 degrees C

To evaluate the survival of Salmonella on raw almond surfaces, whole almond kernels were inoculated with Salmonella Enteritidis phage type (PT) 30 collected from a 24-h broth culture or by scraping cells from an agar lawn. Kernels inoculated with lawn-collected cells to 8, 5, 3, and 1 log CFU per almond after a 24-h drying period were stored for 161 days at 23 +/- 3 degrees C. Calculated rates of reduction were similar for the four inoculum levels (0.22, 0.28, 0.29, and 0.22 log CFU/month, respectively). Kernels inoculated to 7.1 or 8.0 log CFU per almond after drying were stored for 171 or 550 days, respectively, at selected temperatures, including -20 +/- 2 degrees C, 4 +/- 2 degrees C, 23 +/- 3 degrees C, and 35 +/- 2 degrees C. No significant reductions of Salmonella were observed during storage at -20 and 4 degrees C over 550 days. At 35 degrees C, a biphasic survival curve was observed, with calculated reductions of 1.1 log CFU/month from days 0 to 59 and no significant reduction from days 59 to 171. At 23 degrees C, reductions of 0.18 and 0.30 log CFU/month were calculated for 171 and 550 days of storage, respectively. When combined with data from the study of inoculum levels, an overall average calculated reduction at 23 degrees C was 0.25 +/- 0.05 log CFU/month. Significantly greater reductions were observed during the 24-h drying period when broth-collected cells were used as the inoculum, suggesting that cells collected from agar lawns were more resistant to drying. However, after initial drying, the rates of reduction at 23 degrees C did not differ significantly between the inoculum preparation methods. Salmonella Enteritidis PT 30 survives for long periods on almond kernels under a variety of commonstorage conditions.     

The growth of Aeromonas hydrophila K144 in ground pork at 5 degrees C

The influence of NaCl, pH, atmosphere, and background microflora on the growth and/or survival of Aeromonas hydrophila K144 was studied in ground pork held at 5 degrees C. In ground pork, A hydrophila was sensitive to pH values below 6.0 in the form of either a low starting pH in the pork itself or induced by lactic acid bacteria action on added glucose. Growth of the organism is inhibited by NaCl levels of 3% (w/w) (approx. 4% brine content). A hydrophila grew in vacuum-packaged ground pork; its growth was diminished by the presence of the naturally occurring meat microflora. Except for pH values below 6.0, conditions which inhibited growth permitted survival of the organisms for extended periods. Data indicate that the growth of A. hydrophila in ground pork can be controlled by factors such as NaCl, pH, and background microflora. In general, measures designed to control other foodborne pathogens appear adequate to limit A. hydrophila.     

Inhibition of Listeria monocytogenes using nisin with grape seed extract on turkey frankfurters stored at 4 and 10 degrees C

Recontamination of cooked ready-to-eat (RTE) chicken and beef products with Listeria monocytogenes has been a major safety concern. Natural antimicrobials in combinations can be an alternative approach for controlling L. monocytogenes. Therefore, the objectives of this study were to evaluate the inhibitory activities against L. monocytogenes of nisin (6,400 IU/ ml), grape seed extract (GSE; 1%), and the combination of nisin and GSE both in tryptic soy broth with 0.6% yeast extract (TSBYE) and on the surface of full-fat turkey frankfurters. TSBYE was incubated at 37 degrees C for 72 h and turkey frankfurters at 4 or 10'C for 28 days. Inocula were 6.7 or 5 log CFU per ml or g for TSBYE or frankfurters, respectively. After 72 h in TSBYE, nisin alone did not show any inhibitory activity against L. monocytogenes. The combination of nisin and GSE gave the greatest inhibitory activity in both TSBYE and on turkey frankfurters with reductions of L. monocytogenes populations to undetectable levels after 15 h and 21 days, respectively. This combination of two natural antimicrobials has the potential to control the growth and recontamination of L. monocytogenes on RTE meat products.     

Changes in the proteome of Escherichia coli during growth at 15 degrees C after incubation at 2, 6 or 8 degrees C for 4 days

For better understanding of the complex behaviour of Escherichia coli at chiller temperatures, log phase E. coli grown at 15 degrees C were incubated at 8, 6, or 2 degrees C for 4 days, and were then incubated at 15 degrees C for 12 h. Cultures were sampled after incubation at the lower temperatures, and during subsequent incubation at 15 degrees C. Proteins extracted from the samples were separated by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE). Spots of 45 previously identified proteins that were differentially expressed at 15 or < or =8 degrees C were quantified by image analysis. After incubation at 8 or 6 degrees C for 4 days cells were growing with or without formation of elongated cells (filaments), respectively, but growth did not occur at 2 degrees C. In cells incubated at 8 or 6 degrees C proteins associated with the stress response and energy generation were upregulated and proteins associated with protein synthesis were downregulated, while protein levels in cells incubated at 2 degrees C were little changed. When cells were then incubated at 15 degrees C, the levels of differentially expressed proteins in cells that had been incubated at 8 or 6 degrees C decreased or increased towards the levels found in cells growing at 15 degrees C, but some proteins were still under or over expressed after 12 h. In cells incubated at 15 degrees C after incubation at 2 degrees C, the levels of many of the proteins declined but the levels of proteins associated with protein synthesis increased. The findings indicate that the physiological states of log phase E. coli incubated at < or =2 degrees C or at higher chiller temperature are different, but that for both states incubation at an above chiller temperature for >3 generations is required before protein levels adjusted to those usual for the higher temperature. Cells in these different physiological states may respond differently to other stresses encountered during warming of chilled foods.     

Reduction of normal flora by irradiation and its effect on the ability of Listeria monocytogenes to multiply on ground turkey stored at 7 degrees C when packaged under a modified atmosphere

Listeria monocytogenes did not multiply faster during storage at 7 degrees C on irradiated than on nonirradiated raw ground turkey, and there was a concentration-dependent inhibition of its multiplication by CO2. Ground turkey was gamma irradiated at 5 degrees C to 0, 1.5, and 2.5 kGy and inoculated (approximately 100 CFU/g) after irradiation with a cocktail of L. monocytogenes ATCC 7644, 15313, 49594, and 43256. The meat was then packaged in air-permeable pouches or under atmospheres containing 30 or 53% CO2, 19% O2, and 51 or 24% N2 and stored at 7 degrees C for up to 28 days. A dose of 2.5 kGy extended the time for the total plate count (TPC) to reach 10(7) CFU/g from 4 to 19 days compared to that for nonirradiated turkey in air-permeable pouches. Following a dose of 2.5 kGy at the end of the 28-day study, the TPCs were 10(6.42) and 10(4.98) under 25% and 50% CO2 atmospheres, respectively. Under air, 30% CO2, and 53% CO2 atmospheres, the populations of L. monocytogenes after 19 days incubation were 10(4.89), 10(3.60), and 10(2.67) CFU/g. The populations of lactic acid bacteria and anaerobic or facultative bacteria were also reduced by irradiation. Irradiating ground turkey did not decrease its safety when it was contaminated following processing with L. monocytogenes.     

The shelf-life of beef steaks treated with dl-lactic acid and antioxidants and stored under modified atmospheres

Beef steaks were treated with 1.5% lactic acid alone or supplemented with antioxidants (0.1% rosemary extract and 0.05% ascorbic acid). The steaks were stored under modified atmospheres containing either 60% O₂/40% CO₂ or 70% O₂/20% CO₂/10% N₂. Both the 40% CO₂ atmosphere and the lactic acid treatment significantly (P<0.05) inhibited growth of lactic acid bacteria, Brochothrix thermosphacta and Pseudomonas spp. Neither CO₂ in the pack atmosphere, treatment with lactic acid, nor a combination of both, affected formation of thiobarbituric acid reactive substances, myoglobin oxidation, or CIE a^* values. However, treatment with antioxidants significantly (P<0.05) delayed oxidation of both myoglobin and lipids, and so extended the storage-life.