Survival of Salmonella spp. and Escherichia coli O157:H7 on fresh and frozen strawberries

For maximum shelf life, fresh strawberries are harvested directly without washing into retail containers. Frozen berries are usually hulled in the field and washed prior to freezing, sometimes with the addition of sucrose. To determine survival of potential bacterial contaminants, cut or intact surfaces of fresh strawberries were spot inoculated with five- or six-strain cocktails of Salmonella or Escherichia coli O157:H7 (log 7.0 CFU/sample). Inoculated strawberries were dried for 1 h at 24 degrees C and were stored in closed containers at 5 or 24 degrees C. Sliced strawberries with or without added 20% sucrose were inoculated with one of two strains of E. coli O157:H7 and frozen at -20 degrees C. An initial population reduction of approximately 0.5-log cycles was observed on intact but not cut berries after the 1-h drying period. During storage at 24 degrees C for up to 48 h, populations of Salmonella and E. coli O157:H7 did not decline further. When strawberries were stored at 5 degrees C for up to 7 days, populations of both pathogens remained constant on cut surfaces but decreased by 1 - to 2-log cycles on intact surfaces. After 30 days of frozen storage, the population of E. coli O157:H7 had declined by 0.7- to 2.2-log cycles (with and without sucrose, respectively). Results of this study indicate that E. coli O157:H7 and Salmonella are capable of survival but not growth on the surface of fresh strawberries throughout the expected shelf life of the fruit and can survive in frozen strawberries for periods of greater than 1 month.     

Survival of Listeria monocytogenes on Processed Poultry

The potential for Listeria monocytogenes to survive and proliferate on processed poultry was investigated. Chicken breasts were inoculated with L. monocytogenes, cooked to one of five different internal temperatures, then either vacuum-packaged or wrapped in an oxygen permeable film and stored for up to four wk at 4^oC and 10^oC. Survivors were encountered at each cooking temperature employed. The magnitude of lethality was directly related to the cooking temperature. No significant increase in microbial populations occurred through 2 wk of storage. Bacterial populations in samples that were overwrapped increased significantly by week 4, except for the 160^oF (71.1^oC) treatment. For samples which were vacuum-packaged, only the bacterial population in the 150^oF (65.6^oC) treatment significantly increased. Differences due to packaging were observed at 4^oC storage but not at 10^oC storage.     

Survival of Listeria monocytogenes in experimental chorizos

Chorizos-Mexican-style raw-meat sausages-are a concern in California because their production in small ethnic food markets is unregulated. Their formulation may cause them to appear cooked to the consumer, who may eat the raw sausage without prior proper cooking. Bacterial pathogens in such products may cause illness or even death. Survivability of Listeria monocytogenes in chorizos was evaluated under different storage conditions selected on the basis of an initial survey of uninspected chorizos in California. Sausages were formulated to five different initial water activity (aw) levels (0.85, 0.90, 0.93, 0.95, 0.97), stored under four conditions (refrigeration, "Ref," 6 to 8 degrees C under convective air circulation; room temperature, "RT," 24 to 26 degrees C under convective air circulation; hood, "Hd," 24 to 26 degrees C under forced air circulation; and incubation, "Inc," 30 to 31 degrees C under convective air circulation), and sampled after 1, 2, 4, and 7 days. The initial pH was 4.8 and remained near 5.0 from day 1 of the sampling period. An inoculated-pack study using a five-strain cocktail of L. monocytogenes was performed twice for each initial aw. Results indicated that the three lowest initial aw levels (0.85, 0.90, 0.93) and the Hd and Inc storage conditions were more effective (P < or = 0.05) at reducing L. monocytogenes levels in chorizos than the two highest initial aw levels (0.95 and 0.97) and the Ref storage condition, irrespective of storage time. These results can provide a scientific basis for guidelines given to uninspected chorizo producers in California and reduce the risk of foodborne illness.     

Survival of Listeria monocytogenes in ground beef

Listeria monocytogenes, due to its association with animals and animal products and its proven pathogenicity, is an organism of potential importance to the meat industry. The survival of L. monocytogenes in ground beef held at 4 degrees C for 2 weeks was investigated. The ground beef was inoculated with L. monocytogenes type 1 or type 4 at a level of 5 x 10(5) to 7 x 10(6) CFU/g and then packaged in either oxygen-permeable or oxygen-impermeable bags. Packages were sampled at random at 0, 2, 3, 5, 7, 11, and 14 days post-inoculation, and assayed for L. monocytogenes counts and pH. The number of L. monocytogenes in ground beef remained constant throughout the sampling period, and survival was not affected by package permeability to oxygen. Listeriae were not isolated from the control ground beef. The pH of the meat increased slightly during storage, but was always in the range of pH 5.6 to 5.9. It appears that L. monocytogenes in ground beef can survive without any substantial increase or decrease in viable cell population during refrigerated storage for 14 days.     

Survival of Listeria monocytogenes in egg washwater

The viability of Listeria monocytogenes strains Scott A, 78-34, and 81-861 in artificial egg washwater at different temperatures and pH values was determined. After a 4-h incubation, less than a 1-log decrease in viability of strains Scott A and 78-34 was found at 33 degrees C with alkaline detergent (pH 8.0-10.5); however, up to a 3-log decrease in viable numbers was found in neutral pH controls lacking detergent. At 42 degrees C, survival was generally poorer; complete loss of viability (greater than 4-log decrease in viable numbers) was found within 2 h at neutral pH. Strain 81-861 was more sensitive to the test conditions than the other two strains. Viability of all strains was markedly lower in synthetic washwater at the lower pH values (pH 7-9) containing whole egg than washwater in which whole egg was omitted. The presence of whole egg appeared to have no effect on survival at pH 10.5. A limited survey of two egg wash facilities in Southeastern Ontario revealed Listeria innocua in environmental samples from both plants, and in washwater from one plant. These results suggest that Listeria spp. can survive normal commercial washwater conditions, and can be found in commercial egg wash plants.     

Antimicrobial effect of electrolyzed oxidizing water against Escherichia coli O157:H7 and Listeria monocytogenes on fresh strawberries (Fragaria x ananassa)

ABSTRACT:  Antibacterial activity of electrolyzed oxidizing (EO) water prepared from 0.05% or 0.10% (w/v) sodium chloride (NaCl) solutions against indigenous bacteria associated with fresh strawberries ( Fragaria × ananassa ) was evaluated. The efficacy of EO water and sodium hypochlorite (NaOCl) solution in eliminating and controlling the growth of Listeria monocytogenes and Escherichia coli O157:H7 inoculated onto strawberries stored at 4 ± 1 °C up to 15 d was investigated at exposure time of 1, 5, or 10 min. Posttreatment neutralization of fruit surfaces was also determined. More than 2 log₁₀ CFU/g reductions of aerobic mesophiles were obtained in fruits washed for 10 or 15 min in EO water prepared from 0.10% (w/v) NaCl solution. Bactericidal activity of the disinfectants against L. monocytogenes and E. coli O157:H7 was not affected by posttreatment neutralization, and increasing exposure time did not significantly increase the antibacterial efficacy against both pathogens. While washing fruit surfaces with distilled water resulted in 1.90 and 1.27 log₁₀ CFU/mL of rinse fluid reduction of L. monocytogenes and E. coli O157:H7, respectively, ≥ 2.60 log₁₀ CFU/mL of rinse fluid reduction of L. monocytogenes and up to 2.35 and 3.12 log₁₀ CFU/mL of rinse fluid reduction of E. coli O157:H7 were observed on fruit surfaces washed with EO water and NaOCl solution, respectively. Listeria monocytogenes and E. coli O157:H7 populations decreased over storage regardless of prior treatment. However, EO water and aqueous NaOCl did not show higher antimicrobial potential than water treatment during refrigeration storage.     

Survival of Listeria monocytogenes in commercial cheese brines.

The survival of Listeria monocytogenes was determined in commercial cheese brines collected from cheese factories in Wisconsin and northern Illinois. Survival of L. monocytogenes inoculated into commercial cheese brines ranged from < 7 d to over 259 d. Survival did not correlate with pH, salt content, nitrogen content, mineral content, or inherent microbial populations but was negatively associated with addition of sodium hypochlorite at the dairy plant. The L. monocytogenes generally survived longer in brines held at 4 degrees C than at 12 degrees C. Sodium hypochlorite or hydrogen peroxide inactivated L. monocytogenes when added to commercial brines in the lab at 10 to 100 ppm or 0.001% to 0.02%, respectively. Addition of 1% potassium sorbate or 1% sodium benzoate also decreased survival of L. monocytogenes. Laboratory filtration of commercial brines had a negative effect on survival in one of three brines tested. The L. monocytogenes did not grow during incubation in any of the commercial brine samples tested.     

Growth modelling of Listeria monocytogenes in packaged fresh green asparagus

Growth of Listeria monocytogenes in packaged fresh green asparagus was investigated at different temperatures. The present study monitored the growth of L. monocytogenes in packaged fresh green asparagus stored at 2, 4, 8, 12 and 20°C. Changes in carbon dioxide, oxygen concentrations and pH were determined. Growth data were fitted to the Baranyi equation. The influence of storage temperature was very apparent, at higher temperatures (8, 12 and 20°C) an increase of two logarithmic units of L. monocytogenes took place, criteria used for the determination of the sanitary risk. The recommended storage temperatures for the optimal and safe conservation of asparagus are the lower ones studied (2 and 4°C), because growth of L. monocytogenes is not allowed, although it can survive, so in cases of a high contamination of this micro-organism it could be a sanitary risk. Predictive modelling appears to be a useful tool in estimating growth rates ofL. monocytogenes in these products.     

Impact of Mechanical Shear on the Survival of Listeria monocytogenes on Surfaces

Abstract: The impact of mechanical surface shear on microbial viability is rarely a subject for exploration in food processing. The objective of this research was to investigate the impact of mechanical shear on the survival of Listeria monocytogenes on surfaces. Mechanical shear created by slicing a model food was explored to investigate the viability of L. monocytogenes. Cell injury/death was readily demonstrated in fluorescence images by confocal microscopy in which the live and dead cells were fluorescently stained green and red, respectively, with a viability dye kit. Images showed that a large percentage of dead cells appeared after slicing, and they were readily transferred from the slicer blade onto the surfaces of sliced agar, indicating that surface shear may cause the lethal effect on L. monocytogenes. Surface transfer results also showed that viable cell counts on agar slices (in a slicing series) followed a consistently decreasing pattern. The cell counts initially at 5 to 6.5 log CFU/slice (slices 1 to 6), decreased to 3 to 4 log CFU/slice (slices 8 to 30), then to 2 to 3 log CFU/slice (slices 31 to 40), and counts would be expected to further decrease if slicing continued. The overall cell recovery (survival) ratio was about 2% to 3% compared to the initial 8.4 log CFU/blade on a 10 cm² edge area. The impact of shear on microbial viability during slicing may contribute 99% of viable cell count reduction. This study provides clear evidence that surface shear can kill foodborne pathogens and reduce cross-contamination. The lethal effects of surface shear may further enhance food safety.     

Bioprotective Leuconostoc strains against Listeria monocytogenes in fresh fruits and vegetables

Ten Leuconostoc mesenteroides and one Ln. citreum strains isolated from fresh fruit and vegetables were tested for their antagonistic capacity against Listeria monocytogenes. Genetic differences among strains were analyzed by Random Amplified Polymorphic DNA (RAPD). All the isolates clustered together and differed from the type strain Ln. mesenteroides ATCC 8293 as well as from Ln. fallax and Ln. citreum. Organic acids, hydrogen peroxide and bacteriocins were detected as main inhibition mechanisms. Characterization of culture supernatants from the bacteriocinogenic strains, CM135 and CM160 revealed a high resistance of antibacterial activity to temperature and pH, and a bactericidal mode of action against L. monocytogenes. Produced bacteriocins belonged to the Class IIa and sequencing of genes showed complete homology with mesentericin Y105. A study of the effect of the relative dose of pathogen and LAB on control of L. monocytogenes in wounds of Golden Delicious apples and Iceberg lettuce leaf cuts was performed. A comparison of the dose of bioprotective strain needed for a ten fold reduction of the viable pathogen concentration (ED(90)) revealed that strain CM160 was the most effective against L. monocytogenes. ED(90) values varied from 1.3.10(4) to 5.0.10(5) cfu.g(-1) or wound, at ranges of pathogen levels from 1.0.10(3) to 5.0.10(4) cfu.g(-1) of lettuce or wound of apple. The efficiency of the strains was also calculated as the ratio of the ED(90) value to the pathogen dose inoculated. The lowest ratio was found for strain CM160 at 5 to 50 cells of LAB per cell of pathogen. The strain offers potential application for prevention of the presence of L. monocytogenes in fresh fruit and vegetables.     

Survival of Listeria monocytogenes, Listeria innocua, and Lactic Acid Bacteria in Chill Brines

ABSTRACT:  Listeria monocytogenes is the pathogen of concern in ready-to-eat (RTE) meat products. Salt brines are used to chill processed meats. L. monocytogenes and lactic acid bacteria (LAB) can grow under saline conditions, and may compete with each other for nutrients. The objective of this study was to determine the effect of lactic acid bacteria ( Enterococcus faecalis , Carnobacterium gallinarum , and Lactobacillus plantarum ) on the survival of L. monocytogenes and Listeria innocua in brines stored under low temperatures for 10 d. Sterile tap water (STW) and 2 brine solutions (7.9% and 13.2% NaCl) were inoculated with 1 of 5 cocktails ( L. monocytogenes , L. innocua , LAB, L. monocytogenes + LAB, or L. innocua + LAB) at initial concentrations of 7 log CFU/mL. Brines were stored for 10 d at 4 or 12 °C. Three replications of each brine concentration/cocktail/temperature combination were completed. No significant reductions of L. monocytogenes occurred in 7.9%[w/v] or 13.2%[w/v] brines when LAB were present; however, there were significant reductions after 10 d of L. monocytogenes in the STW solution when LAB were present (1.43 log CFU/mL at 4 °C and 3.02 log CFU/mL at 12 °C). L. innocua was significantly less resilient to environmental stresses of the brines than L. monocytogenes , both with and without LAB present ( P ≤ 0.05). These strains of lactic acid bacteria are not effective at reducing L. monocytogenes in brines at low temperatures. Furthermore, use of L. innocua as a model for L. monocytogenes is not appropriate under these environmental conditions.     

Effect of gamma irradiation on Listeria monocytogenes in frozen,artificially contaminated sandwiches

Gamma irradiation has been shown to effectively control L monocytogenes in uncooked meats but has not been extensively studied in ready-to-eat foods. The presence of Listeria in ready-to-eat foods is often due to postprocess contamination by organisms in the food-manufacturing environment. Because gamma irradiation is applied after products are packaged, the treated foods are protected from environmental recontamination. Currently, a petition to allow gamma irradiation of ready-to-eat foods is under review by the Food and Drug Administration. This study was conducted to determine if gamma irradiation could be used to control L. monocytogenes in ready-to-eat sandwiches. Ham and cheese sandwiches were contaminated with L. monocytogenes, frozen at -40 degrees C, and exposed to gamma irradiation. Following irradiation, sandwiches were assayed for L. monocytogenes. A triangle test was performed to determine if irradiated and nonirradiated sandwiches differed in sensory quality. We found that the D10-values ranged from 0.71 to 0.81 kGy and that a 5-log reduction would require irradiation with 3.5 to 4.0 kGy. The results of a 39-day storage study of sandwiches inoculated with 10(7) CFU of L monocytogenes per g indicated that counts for nonirradiated sandwiches remained fairly constant. Counts for sandwiches treated with 3.9 kGy decreased by 5 log units initially and then decreased further during storage at 4 degrees C. Sensory panelists could distinguish between irradiated and nonirradiated sandwiches but were divided on whether irradiation adversely affected sandwich quality. Our results suggest that manufacturers of ready-to-eat foods could use gamma irradiation to control L. monocytogenes and improve the safety of their products.     

Survival of Listeria monocytogenes strains in a dry sausage model

The survival of five inoculated Listeria monocytogenes strains (DCS 31, DCS 184, AT3E, HT4E, and HR5E) was studied in dry fermented sausages prepared using two different starter cultures (starter A and B) with or without a protective Lactobacillus plantarum DDEN 2205 strain. L. monocytogenes was detected throughout ripening in every sausage sample in which the L. plantarum DDEN 2205 strain had not been used. The use of either starter A, with a high concentration of protective culture, or starter B, with a low concentration of protective culture, resulted in L. monocytogenes-negative sausages after 17 days of ripening. Differential survival was noted among the L. monocytogenes strains during fermentation. Strains AT3E and DCS 31 survived in sausages with protective cultures more often than did the other strains, whereas HT4E and HR5E were inhibited during ripening by all starter and protective cultures used. Protective cultures such as L. plantarum may be used as part of a hurdle strategy in dry sausage processing, but variations in susceptibility of different L. monocytogenes strains can create problems if other hurdles are not included.     

单核细胞增生李斯特菌在典型食品接触表面的存活建模研究

单核细胞增生李斯特菌在食品接触表面的长期存活,可导致其在食品中暴露可能性升高,增加引发严重食源性疾病的风险。本文研究了无营养冷藏（4 ℃）和室温（25 ℃）条件下,单核细胞增生李斯特菌在7种典型食品接触材质表面的存活情况。同时采用失活模型Bigelow模型进行拟合,便于理解其存活动力学。结果发现,单核细胞增生李斯特菌在木材质表面上的存活时间较短,但在其他材质表面可存活46 h以上。显著性分析表明,除玻璃和ABS塑料材质外,单核细胞增生李斯特菌在4 ℃条件下较25 ℃条件下存活量更高,表明冷藏环境有助于其长期存活。同时,单核细胞增生李斯特菌在玻璃和聚丙烯表面的存活能力相对较强。因此,为了更准确地推断单核细胞增生李斯特菌在食品链中导致危害发生的可能性,应纳入对其在不同材质及不同温度组合环境下存活能力的考量。     

噬菌体对虾仁中单核细胞增生李斯特菌的抑制效果

以虾仁为研究对象,通过人为模拟在虾仁中接种单核细胞增生李斯特菌后,再用噬菌体ListexTMP100在常温(20℃)、冷藏(4℃、0℃)和冷冻(-18℃)环境下进行处理,分析虾仁中该菌的抑制效果,单核细胞增生李斯特菌采用平板计数法进行计数。结果表明:噬菌体浓度大于2×107PFU/mL时,能有效杀灭虾仁中单核细胞增生李斯特菌(P<0.05);该浓度在20℃下作用虾仁1 h,便能降低1.50 Log10CFU/g(死亡率为99%以上)的单核细胞增生李斯特菌;在0℃和4℃,作用24 h下,单核细胞增生李斯特菌的死亡率达到99.9%;在-18℃下贮藏30d,单核细胞增生李斯特菌在第1天下降了1.38Log10CFU/g。这些结果表明,噬菌体ListexTMP100对虾仁中的单核细胞增生李斯特菌具有明显的抑制效果,可作为一种理想的生物杀菌剂用于水产食品中。     

Survival of Listeria monocytogenes in a simulated recirculating brine chiller system

Contamination by Listeria monocytogenes of processed meats after cooking presents a significant food safety risk. The purpose of this study was to determine the survival of L. monocytogenes in a simulated recirculating brine chiller system using pH values of 5, 6, and 7 with free chlorine concentrations of 0, 3, 5, and 10 ppm in 20% salt brine at -12 degrees C. At pH values of 5, 6, and 7 with chlorine concentrations of 2 and 3 ppm, using 10(8) CFU in a test tube system, an immediate drop of 0.28 log CFU/ml with no significance between treatments (P > 0.05), followed by a steady survival phase with a slope close to 0, was observed. In brine at a pH of 5 with 5 and 10 ppm of chlorine, an initial drop of 0.8 log CFU/ml was observed, which was followed by a steady survival phase with a destruction slope close to zero. At an inoculation concentration of 10(2) CFU in a test tube system (pH values of 5 and 7 with 0 and 10 ppm of chlorine), the average initial drop for all treatments was 0.1 log CFU/ml, which was followed by a steady survival phase. In a recirculating system, very few cells were destroyed during the brine chilling process, but only low numbers of L. monocytogenes were recovered from the brine and uninoculated hot dogs. Although little destruction of L. monocytogenes was noted, the dilution effect observed during the study indicates that environmental contamination of a brine chiller system poses little danger of postcooking contamination for processed meats if the system is regularly cleaned and sanitized.     

Survival of Listeria monocytogenes on sliced cooked sausage after treatment with pediocin AcH

A preparation with pediocin AcH bound to its heat-killed producer cells Lactobacillus plantarum WHE 92 (starter culture ALC01, Wisby, Denmark) by adjusting the pH of the preparation to 6.0 was studied for its effects against Listeria monocytogenes ATCC 7644 and (spoilage) lactic acid bacteria on sliced cooked sausage. The pediocin AcH preparation or 0.9% (w/w) NaCl dilution (as a control) were randomly distributed dropwise on the surface of the slices. Treated slices were vacuum-packed and stored at 6 degrees C. Microbiological analysis and determination of pH values were performed after 3, 6, 9, 14 and 21 days of storage. Flavour of the sausages was evaluated after 7 and 11 days of storage. The pediocin preparation had effect (p > 0.05) neither on the growth of lactic acid bacteria, on the pH value nor on the flavour of vacuum-packed sliced sausage during 21 days of storage compared to control. However, during 6 days of storage, the number of L. monocytogenes decreased from the initial level of 2.7 log cfu/g sausage to < 2 log cfu/g, while on the control sausages the number of L. monocytogenes remained at the inoculated level. The numbers of L. monocytogenes remained at those levels to the end of storage period (21 days). However, the treated samples were determined to be Listeria positive, which indicates that the pediocin preparation was not efficient enough to kill all L. monocytogenes.     

Injury and death of frozen Listeria monocytogenes as affected by glycerol and milk components

A cell suspension of Listeria monocytogenes strain Scott A in phosphate buffer solution alone or with added glycerol, milk fat, lactose, or casein was frozen and stored at -18 degrees C. At suitable intervals, samples of cell suspensions were thawed at 35 degrees C and plated on suitable media to distinguish between surviving injured and noninjured cells of L. monocytogenes. Glycerol (2 or 4%) protected L. monocytogenes from death and injury during frozen storage for up to 6 mo; however, when 2% glycerol was present, 30 min of frozen storage had to elapse after completion of freezing before protection against death was evident. During short-term (2 wk or less) frozen storage, lactose, milk fat, and casein, each at 2%, provided better protection to L. monocytogenes than did 2% glycerol. During long-term frozen storage, milk components, each at 2%, protected L. monocytogenes against death and injury, but less than that provided by glycerol. Protection by lactose and milk fat against death during frozen storage was observed during 4 wk and against injury during 5 mo and 4 wk of frozen storage, respectively. Protection by casein against death and injury occurred during frozen storage for up to 6 mo. Salts that simulate milk ultrafiltrate provided almost no protection to L. monocytogenes during freezing and frozen storage. Increasing the concentration of milk fat from 2 to 4% resulted in almost no change in death of L. monocytogenes, but in a decrease in injury only during the first 24 h of frozen storage.(ABSTRACT TRUNCATED AT 250 WORDS)