The influence of mayonnaise pH and storage temperature on the growth of Listeria monocytogenes in seafood salad

Seafood salad has been identified as a ready-to-eat food with a relatively high incidence of contamination by Listeria monocytogenes; however, little is known about the behavior of this pathogen in seafood salad as a function of product pH and storage temperature. To produce data towards the development of a predictive growth model, a 6-strain cocktail of L. monocytogenes was inoculated onto the surface of a shrimp-crabmeat product, mixed with mayonnaise that was previously adjusted with NaOH to pH 3.7, 4.0, 4.4, 4.7 or 5.1, and then stored at 4 degrees , 8 degrees or 12 degrees C under both aerobic and vacuum conditions. At each storage temperature, L. monocytogenes was able to grow in the seafood salad under both aerobic and vacuum conditions. The slowest growth of L. monocytogenes was observed in seafood salad with a mayonnaise pH of 3.7 and a storage temperature of 4 degrees C under vacuum condition. In salad with the same mayonnaise pH, the growth rate (GR, log10 cfu/h) of L. monocytogenes increased as a function of storage temperature. At the same storage temperature, the lag phase duration (LPD, h) of L. monocytogenes decreased as mayonnaise pH increased. At the same mayonnaise pH and temperature, LPD of L. monocytogenes was greater under aerobic than under vacuum conditions. Regression analyses indicated that mayonnaise pH is the main effector on the LPD of L. monocytogenes in seafood salad, and storage temperature was the main effector on the GR. Secondary models that describe LPD and GR of L. monocytogenes in seafood salad as a function of mayonnaise pH and storage temperature were produced.     

Growth of Listeria monocytogenes in egg salad and pasta salad formulated with mayonnaise of various pH and stored at refrigerated and abuse temperatures

This study investigated and modeled the behavior of Listeria monocytogenes in egg salad and pasta salad as affected by mayonnaise pH (3.8, 4.2, 4.6, and 5.0) and storage temperature (4, 8, and 12 degrees C). At each storage temperature, L. monocytogenes was able to grow in both salads regardless of the mayonnaise pH. The lag-phase durations (LPD) of L. monocytogenes in egg salad ranged from 33 to 85, 15 to 50, and 0 to 19 h, and the growth rates (GR) ranged from 0.0187 to 0.0318, 0.0387 to 0.0512, and 0.0694 to 0.1003 log(10)cfu/h at 4, 8, and 12 degrees C, respectively. The LPD of L. monocytogenes in pasta salad ranged from 210 to 430, 49 to 131, and 21 to 103 h, and GR ranged from 0.0118 to 0.0350, 0.0153 to 0.0418, and 0.0453 to 0.0718 log(10)cfu/h at 4, 8, and 12 degrees C, respectively. The growth of L. monocytogenes was more rapid in egg salad than in pasta salad, indicating that a better growth environment for L. monocytogenes existed in egg salad. In both salads, the LPD decreased and the GR increased as the storage temperature increased. Mathematical models and response surface plots describing the LPD and GR of L. monocytogenes in both salads as affected by the mayonnaise pH and storage temperature were developed. The models confirmed that the growth of L. monocytogenes in egg salad and pasta salad was primarily promoted by higher storage temperatures and, secondarily, by higher mayonnaise pH. The conditions under which the models may be applied to estimate the growth of L. monocytogenes in both salads were identified.     

Effect of temperature and pH on survival of Listeria monocytogenes in coleslaw

The survival and growth of Listeria monocytogenes in fresh coleslaw, pH 3.9, and in coleslaw adjusted to pH 4.0, 5.0, 6.0 or 7.0 before inoculation was studied at three temperatures (4, 15 and 25 degrees C). L. monocytogenes was not detectable after 5 days incubation in fresh coleslaw nor in coleslaw adjusted to pH 4.0. Coleslaw at pH 5.0 was also inhibitory to L. monocytogenes at all three temperatures studied. A decline in viable numbers of L. monocytogenes in coleslaw at pH 6.0 occurred at 4 degrees C and at 15 degrees C, whereas at 25 degrees C the viable count of L. monocytogenes increased initially and remained high after incubation for 25 days. L. monocytogenes grew rapidly in coleslaw at pH 7.0 at all three temperatures studied, followed by an equally rapid decline in viable count.     

Effect of packaging and storage temperature on the survival of Listeria monocytogenes inoculated postprocessing on sliced salami

The survival of postprocess Listeria monocytogenes contamination on sliced salami, stored under the temperatures associated with retail and domestic storage, was investigated. Sliced salami was inoculated with low and high concentrations of L. monocytogenes before being packaged under vacuum or air. Survival of L. monocytogenes was determined after storage of sausages for 45 or 90 days for low or high sample inocula, respectively, at 5, 15, and 25 degrees C. All survival curves of L. monocytogenes were characterized by an initial rapid inactivation within the first days of storage, followed by a second, slower inactivation phase or "tailing." Greater reduction of L. monocytogenes was observed at the high storage temperature (25 degrees C), followed by ambient (15 degrees C) and chill (5 degrees C) storage conditions. Moreover, vacuum packaging resulted in a slower destruction of L. monocytogenes than air packaging, and this effect increased as storage temperature decreased. Although L. monocytogenes numbers decreased to undetectable levels by the end of the storage period, the time (in days) needed for this reduction and for the total elimination of the pathogen decreased with high temperature, aerobic storage, and high inoculum. Results of this study clearly indicated that the kinetics of L. monocytogenes were highly dependent on the interaction of factors such as storage temperature, packaging conditions, and initial level of contamination (inoculum). These results may contribute to the exposure assessment of quantitative microbial risk assessment and to the establishment of storage-packaging recommendations of fermented sausages.     

Use of antimicrobial biodegradable packaging to control Listeria monocytogenes during storage of cooked ham

The antimicrobial effect against L. monocytogenes of biodegradable films (alginate, zein and polyvinyl alcohol) containing enterocins was investigated. Survival of the pathogen was studied by means of challenge tests performed at 6 degrees C during 8 and 29 days, for air-packed and vacuum-packed sliced cooked ham, respectively. Air packaging was tested with two concentrations of enterocins (200 and 2000 AU/cm2). Control air-packed cooked ham showed an increase of L. monocytogenes from 10(4) to 10(7) CFU/g after 8 days. By contrast, packaging with antimicrobial films effectively slowed down the pathogen's growth, leading to final counts lower than in control lots. Air-packaging with alginate films containing 2000 AU/cm2 of enterocins effectively controlled L. monocytogenes for 8 days. An increase of only 1 log unit was observed in zein and polyvinyl alcohol lots at the same enterocin concentration. Vacuum packaging with films containing enterocins (2000 AU/cm2) also delayed the growth of the pathogen. No increase from inoculated levels was observed during 15 days in antimicrobial alginate films. After 29 days of storage, the lowest counts were obtained in samples packed with zein and alginate films containing enterocins, as well as with zein control films. The most effective treatment for controlling L. monocytogenes during 6 degrees C storage was vacuum-packaging of sliced cooked ham with alginate films containing 2000 AU/cm2 of enterocins. From the results obtained it can concluded that antimicrobial packaging can improve the safety of sliced cooked ham by delaying and reducing the growth of L. monocytogenes.     

High-pressure processing and antimicrobial biodegradable packaging to control Listeria monocytogenes during storage of cooked ham

The efficiency of combining high-pressure processing (HPP) and active packaging technologies to control Listeria monocytogenes growth during the shelf life of artificially inoculated cooked ham was assessed. Three lots of cooked ham were prepared: control, packaging with alginate films, and packaging with antimicrobial alginate films containing enterocins. After packaging, half of the samples were pressurized. Sliced cooked ham stored at 6 degrees C experienced a quick growth of L. monocytogenes. Both antimicrobial packaging and pressurization delayed the growth of the pathogen. However, at 6 degrees C the combination of antimicrobial packaging and HPP was necessary to achieve a reduction of inoculated levels without recovery during 60 days of storage. Further storage at 6 degrees C of pressurized antimicrobial packed cooked ham resulted in L. monocytogenes levels below the detection limit (day 90). On the other hand, storage at 1 degrees C controlled the growth of the pathogen until day 39 in non-pressurized ham, while antimicrobial packaging and storage at 1 degrees C exerted a bacteriostatic effect for 60 days. All HPP lots stored at 1 degrees C led to counts <100CFU/g at day 60. Similar results were observed when combining both technologies. After a cold chain break no growth of L. monocytogenes was observed in pressurized ham packed with antimicrobial films, showing the efficiency of combining both technologies.     

Effect of inoculum size on the combined temperature, pH and aw limits for growth of Listeria monocytogenes

The growth/no growth responses of Listeria monocytogenes inoculated at four levels (0.90, 2.58, 4.20 and 6.81 log cfu/well; 300 microl medium/well) into tryptic soy broth (TSB) were monitored at different combinations of temperature (4 to 30 degrees C), pH (3.76 to 6.44) and aw (0.888 to 0.997) for 60 days. The study was conducted in 96-well microtiter plates and growth was monitored visually and by recording the turbidity of the medium with an automated microplate reader. The growth limits of the pathogen and hence the position of the growth boundary were found to be affected by the size of the inoculum. For example, at 25 degrees C and aw 0.997 the minimum pH values that allowed growth were 4.45 and 3.94 for inoculum levels of 0.90 and 6.81 log cfu/well, respectively. Similarly, at 25 degrees C and pH 6.44 the minimum aw values where growth was observed ranged from 0.900 to 0.928 depending on the inoculum level. The results showed that at temperatures from 10 to 30 degrees C the difference in the growth limits between the tested inoculum levels was higher in environments where a single factor (pH or aw) was inhibitory than in environments where pH and aw together were inhibitory. The data were used to develop a growth/no growth interface model for each inoculum level tested, using logistic polynomial regression. The concordance indices of the models ranged from 99.8% to 99.9% and showed a good fit to the observed data in all models. This study indicates the importance of inoculum size for microbial growth initiation and provides quantitative data that show how the combinations of hurdles which prevent growth vary with inoculum size.     

Effect of inoculum preparation procedure and storage time and temperature on the fate of Listeria monocytogenes on inoculated salami

Although dry/semidry fermented sausages are characterized as being of low-to-moderate risk for human listeriosis on a per-serving and per-annum basis, data are lacking relative to the fate of postprocessing Listeria monocytogenes contamination during storage of such products. This study evaluated the effect of inoculum preparation and storage conditions on the fate of L. monocytogenes on vacuum-packaged salami. Commercially produced salami was sliced and inoculated (4 +/- 1.3 log CFU/ cm2) with one of four types of inocula. All inocula consisted of the same 10-strain L. monocytogenes composite, cultivated as individual strains prior to mixing for inoculation. Active cultures of individual strains were prepared (30 degrees C, 24 h) in either tryptic soy broth (containing 0.25% glucose) plus 0.6% yeast extract (TSBYE), tryptic soy broth without glucose plus 0.6% yeast extract (TSBYE-G), TSBYE-G plus 1% glucose (TSBYE+G), or in TSBYE, and then habituated (7 degrees C, 72 h) in sterile salami homogenate (10% [wt/wt] with distilled water). Inoculated salami slices were vacuum packaged, stored at 4, 12, or 25 degrees C, and analyzed (three samples per treatment in each of two replicates) periodically for surviving bacterial counts. In general, pathogen levels decreased during storage and reached levels below the detection limit (-0.4 log CFU/cm2) between 27 and 90 days of storage, depending on temperature of storage and inoculum type. Death rates (log CFU/cm2/day) were found to increase as storage temperature increased, with the exception of the acid-adapted (TSBYE+G) cells, which decreased more rapidly at 4 degrees C than at 12 or 25 degrees C. The habituated inoculum was inactivated at a faster rate than other inocula at 12 and 25 degrees C, but performed similarly to nonadapted (TSBYE-G) and partially acid-adapted (TSBYE) inocula at 4 degrees C. These data may be used to supplement existing information for use in future risk assessments.     

Inhibition of Listeria monocytogenes using natural antimicrobials in no-nitrate-or-nitrite-added ham

Consumer demand for foods manufactured without the direct addition of chemical preservatives, such as sodium nitrite and organic acid salts, has resulted in a unique class of "naturally" cured meat products. Formulation with a natural nitrate source and nitrate-reducing bacteria results in naturally cured processed meats that possess traits similar to conventionally cured meats. However, previous research has shown that the naturally cured products are more susceptible to pathogen growth. This study evaluated Listeria monocytogenes growth on ham manufactured with natural curing methods and with commercially available clean-label antimicrobials (cultured sugar and vinegar blend; lemon, cherry, and vinegar powder blend) and assessed impacts on physicochemical characteristics of the product. Hams made with either of the antimicrobials supported L. monocytogenes growth similar to that in the traditionally cured control (P > 0.05). Hams made with prefermented celery juice powder had the lowest residual nitrite concentrations (P < 0.05), and when no antimicrobial was added, L. monocytogenes growth was similar to that of the uncured control (P > 0.05). Aside from residual nitrite and nitrate concentrations, few physicochemical differences were identified. These findings show that ham can be produced with natural curing methods and antimicrobials to provide similar L. monocytogenes inhibition and physicochemical traits as in traditionally cured ham.     

Effect of salt, smoke compound, and storage temperature on the growth of Listeria monocytogenes in simulated smoked salmon

Smoked salmon can be contaminated with Listeria monocytogenes. It is important to identify the factors that are capable of controlling the growth of L. monocytogenes in smoked salmon so that control measures can be developed. The objective of this study was to determine the effect of salt, a smoke compound, storage temperature, and their interactions on L. monocytogenes in simulated smoked salmon. A six-strain mixture of L. monocytogenes (10(2) to 10(3) CFU/g) was inoculated into minced, cooked salmon containing 0 to 10% NaCl and 0 to 0.4% liquid smoke (0 to 34 ppm of phenol), and the samples were stored at temperatures from 0 to 25 degrees C. Lag-phase duration (LPD; hour), growth rate (GR; log CFU per hour), and maximum population density (MPD; log CFU per gram) of L. monocytogenes in salmon, as affected by the concentrations of salt and phenol, storage temperature, and their interactions, were analyzed. Results showed that L. monocytogenes was able to grow in salmon containing the concentrations of salt and phenol commonly found in smoked salmon at the prevailing storage temperatures. The growth of L. monocytogenes was affected significantly (P < 0.05) by salt, phenol, storage temperature, and their interactions. As expected, higher concentrations of salt or lower storage temperatures extended the LPD and reduced the GR. Higher concentrations of phenol extended the LPD of L. monocytogenes, particularly at lower storage temperatures. However, its effect on reducing the GR of L. monocytogenes was observed only at higher salt concentrations (>6%) at refrigerated and mild abuse temperatures (< 10 degrees C). The MPD, which generally reached 7 to 8 log CFU/g in salmon that supported L. monocytogenes growth, was not affected by the salt, phenol, and storage temperature. Two models were developed to describe the LPD and GR of L. monocytogenes in salmon containing 0 to 8% salt, 0 to 34 ppm of phenol, and storage temperatures of 4 to 25 degrees C. The data and models obtained from this study would be useful for estimating the behavior of L. monocytogenes in smoked salmon.     

Control of Listeria monocytogenes in boned dry-cured ham by E-beam treatment

Boning of dry-cured hams involves additional handling which increases the risk of contamination with Listeria monocytogenes. This work studies the ability of E-beam to eliminate this pathogen from boned dry-cured ham (BDH). The destruction kinetics of four L. monocytogenes strains (S2, S4-2, S12-1, and S7-2) and L. innocua NCTC 11288 as a surrogate were determined at doses up to 3 kGy. L. innocua and L. monocytogenes S4-2 were the most radioresistant (D-value = 0.55 kGy). E-beam penetration was studied in blocks extracted from BDH and irradiated with 2–6 kGy. The bilateral treatment of 2 kGy allows the required dose (1.32 kGy) to be absorbed in the innermost part of the block to achieve the food safety objective of “zero tolerance” for this microorganism. This listericidal treatment was also confirmed in whole BDH intentionally contaminated with L. innocua. The sensory analysis of irradiated BDH showed off-odors that were imperceptible after 15 days of storage at 4°C.     

Effect of preincubation temperature and pH on the individual cell lag phase of Listeria monocytogenes, cultured at refrigeration temperatures

The impact of precultural temperature and pH on the distribution of the lag phase of individual Listeria monocytogenes cells was assessed during preincubation at 7 degrees C, using a dilution protocol to obtain single cells, and optical density measurements to estimate the individual lag phase. Firstly, the pure temperature effect (37, 15, 10, 7, 4 and 2 degrees C) was investigated on a subsequent growth at 7 degrees C and pH 7.4. Secondly, low precultural temperatures (10, 7 and 4 degrees C) were combined with a controlled pH at 7.4 and 5.7 with a subsequent growth at 7 degrees C and at different pH values (7.4, 6.0 and 5.5). For all temperature-pH combinations, the individual cell lag phase was determined using a three-phase linear growth model. It was observed that at low precultural temperatures (2, 4 and 7 degrees C), a high proportion of L. monocytogenes cells were able to grow at 7 degrees C with almost no lag phase, consequently, the resulting distributions were positively skewed. Beside this, the variability observed was lower than at higher precultural temperatures. Regarding the precultural pH effect, at pH 7.4 the mean values of the lag phases were shorter at lower preincubation temperatures; while at pH 5.7 small pH transitions produced shorter individual lag phases at all precultural temperatures. The quantification of the effect of precultural conditions on the individual cell lag phase duration would improve the accuracy of the existing growth models, especially when a series of processing and storage steps are linked together in a process model or exposure assessment. Distributions will be fitted to the data for every set of conditions, generating useful tools for further risk assessment purposes.     

Production of mortadella: behavior of Listeria monocytogenes during processing and storage conditions

The presence of Listeria monocytogenes in ready-to-eat meat products is cause of concern to the food industry as well as to health authorities. Studies were conducted to evaluate the presence of L. monocytogenes in mortadellas acquired at retail stores and to evaluate the fate of two levels of a L. monocytogenes pool spiked in two different formulations of the product, cooked under commercial conditions and stored at refrigeration (7°C) and room temperature (25°C). Among the samples collected at different retail stores, 26.7% harboured L. monocytogenes. Regarding to the fate of L. monocytogenes in mortadella, periodically, samples were taken and the surviving L. monocytogenes cells in the spiked products were counted by the MPN procedure. Populations of <0.35 MPN/g of L. monocytogenes were found in these samples. It could be concluded that the heat treatment was effective to reduce 3-log of L. monocytogenes independent of formulation or storage conditions.     

温度及乳酸链球菌素对单增李斯特菌的抑制作用

通过Matlab软件拟合Gompertz生长模型,研究了温度对单增李斯特菌(Listeria.monocytogenes,LM)的影响;同时,研究了不同浓度(5、10、50、100、150μg/m L)、p H(1、3、5、7、9)、温度(45、75、95、115、121℃)条件下的乳酸链球菌素对LM杀菌活性的影响,旨在为LM的监控技术发展提供理论依据。结果表明:低温的抑制效果明显,最大细胞密度减少了4.3455 lg cfu/m L;乳酸链球菌素浓度低于5μg/m L时能促进LM的生长,高于10μg/m L时,对LM有一定的杀菌作用,高于150μg/m L时,48 h之内几乎可以杀死营养肉汤中的所有LM;乳酸链球菌素对酸性有协同效应;并有较好的热稳定性。      

超高压协同温度处理对烟熏火腿中单增李斯特菌生长预测模型的建立

为了建立超高压协同温度处理烟熏火腿中单增李斯特菌生长预测模型。本研究分别以20、30、40、50℃结合600MPa对接种106cfu/g单增李斯特菌烟熏切片火腿进行5min的超高压处理,4℃贮藏条件下,每10d测定处理后样品中单增李斯特菌的数量。结果表明:当超高压协同处理的温度高于40℃时显著延长了单增李斯特菌在烟熏火腿修复生长的延滞期,所得生长曲线用修正的Gompertz模型进行了拟合,在35、45d进行验证,建立了烟熏火腿中单增李斯特菌在超高压协同温度处理后的生长预测模型,经验证其精确因子(Af)、偏差因子(Bf)、根平均方差(RMSE)和决定系数(R2)均在较好范围内,能较好的模拟单增李斯特菌的生长情况,为低温肉制品中单增李斯特菌的控制提供理论参考。     

Combined effect of packaging atmosphere and storage temperature on growth of Listeria monocytogenes on ready-to-eat shrimp

Cooked, peeled, and deveined shrimp were inoculated with a 5 strain mixture of Listeria monocytogenes and packaged in air, vacuum, and a 100% carbon dioxide modified atmosphere. The packaged shrimp were then stored at 3, 7, and 12 degrees C for 15 days to monitor the growth of L. monocytogenes and psychrotrophic bacteria. Uninoculated shrimp were also subjected to sensory evaluation by a trained panel to measure odor and appearance over the storage period. Results demonstrated that shrimp packaged in CO(2) and stored at 3 degrees C did not permit growth of L. monocytogenes during the 15-day storage period, while all other packaging/temperature combinations allowed for multiplication of the bacterium. Carbon dioxide packaging also resulted in the slowest growth of psychrotrophic bacteria and resulted in shrimp having acceptable sensory odor and appearance scores at the end of storage. When strict temperature control is difficult, such as during processing, transportation, retail display, or home use, additional antimicrobial hurdles may be necessary to ensure safety.     

Simultaneous detection of Listeria monocytogenes and Salmonella by multiplex PCR in cooked ham

A multiplex PCR (m-PCR) assay with an internal amplification control (IAC) was developed for the simultaneous detection of Salmonella spp. and Listeria monocytogenes through invA and prfA genes, respectively. To ensure the detection of the pathogens in cooked ham, samples were enriched in both buffered peptone-water and Half Fraser broth. Subsequently, equal volumes of enrichment broths were mixed and DNA purification was performed prior to m-PCR reaction, saving considerable time and effort. The m-PCR also proved to be very useful as a simple and ready-to-go method for simultaneous confirmation of presumptive L. monocytogenes and Salmonella spp. colonies directly from agar plates without any DNA extraction steps.     

Contributor factors for the occurrence of salmonellosis during preparation,storage and consumption of homemade mayonnaise salad

This study aimed to analyze the impact of practices adopted for preparation, storage and consumption of homemade mayonnaise salad (HMS) as contributor factors for the occurrence of salmonellosis. A total of 493 individuals answered a questionnaire composed of demographic and socioeconomic questions and, preparing, storage and consumption practices of HMS. The level of good hygiene practices (GHP) of respondents was calculated using Good Practice and Outbreak Prevention Indexes. The consumer behavior and the correlation between practices were performed using Multiple Correspondence Analysis (MCA). 75% of respondents consume HMS, being 43% of interviewers doing this at least once per month. HMS was prepared from 30 min to 1 h before being served by 46% of individuals, it remaining for 30 min to 1 h at room temperature during the meal by 63% of people. Furthermore, 51% of respondents left the HMS container open or improperly closed, which may lead to cross-contamination. Most respondents (66%) stated reuse the HMS for less than one day (44%), at once (76%). 77% of participants declared that they prepare HMS, being only 7% preparing HMS without eggs. The majority (51%) used a cooked and a raw egg yolk. In addition, 75% of the cases showed raw eggs in recipe. The Weighted Good Practice Index, Weighted Outbreak Prevention Index and Weighted Harmonic Outbreak Prevention Index were 63%, 62% and 27% of participants, respectively. The MCA gathered the participants into two groups one commits various errors and other commits few errors on GHP. Thus, a consumer would probably perform multiple either good or bad practices simultaneously.     

Elimination of Listeria monocytogenes from vacuum-packed dry-cured ham by E-beam radiation

The inactivation kinetics for Listeria monocytogenes Scott A (CIP 103575, serotype 4b) and Listeria innocua (NTC 11288) after E-beam radiation were studied in vacuum-packed ready-to-eat dry-cured ham to optimize the sanitation treatment of this product. A treatment of 1.12 kGy was calculated to reach the food safety objective according to the U.S. Department of Agriculture criterion. No irradiation treatment is necessary to meet the European Union microbiological criterion for this bacterium. No changes (at doses < or =4 kGy) in the 2-thiobarbituric acid reactive substances values and texture were observed. Dry-cured hams treated with 1 and 2 kGy had negligible sensory modifications (appearance, odor, and flavor). However, the application of 3 and 4 kGy resulted in an increase in the intensity of off-odors and off-flavors. Despite these effects, all irradiated vacuum-packed dry-cured hams treated at < or =4 kGy were deemed acceptable for trading.