Gamma irradiation as a means to eliminate Listeria monocytogenes from frozen chicken meat

Cells of Listeria monocytogenes ATCC 35152 were sensitive to gamma irradiation in phosphate buffer, pH 7.00 (D₁₀, dose required for 10% survival—0.15 kGy) at 0–5°C. The cells showed higher radiation survival when irradiated under frozen condition, with a D₁₀ of 0.3 kGy. The protection offered by shrimp/chicken/kheema homogenates (100 g litre^?1) was evidenced by even higher D₁₀ values (0.5 kGy) at both 0–5°C and cryogenic temperature. Boneless chicken meat samples were artificially inoculated with L monocytogenes ATCC 35152 cells at low (5 × 10³) colony-forming unit (cfu) g^?1 and high (5 × 10⁶ cfu g^?1) concentrations and irradiated at 1, 3, 4, 6 kGy doses under cryogenic conditions. The efficacy of the radiation process was evaluated by detecting L monocytogenes during storage at 2–4°C in the irradiated samples. These studies, when repeated with three other serotypes of L monocytogenes, clearly suggested the need for a dose of 3 kGy for elimination of 10³ cfu cells of L monocytogenes g^?1 from air-packed frozen chicken meat.     

Effect of sodium lactate on thermal inactivation of Listeria monocytogenes and Salmonella in ground chicken thigh and leg meat

The effect of sodium lactate on thermal inactivation D- and z-values of Listeria monocytogenes and Salmonella was determined for chicken thigh and leg meat. At 55 to 70 degrees C, the D-value of L. monocytogenes in ground chicken thigh and leg meat with the addition of 4.8% sodium lactate (4.8 g sodium lactate per 100 g of meat) was 53 to 75% higher than that in the meat without sodium lactate. No significant difference was found for the D-values of Salmonella at 55 to 70 degrees C between the meat with and that without sodium lactate (4.8%. wt/wt). The z-values of both L. monocytogenes and Salmonella were not affected by sodium lactate (4.8%). The results from this study are useful for predicting thermal process lethality of L. montocytogenes and Salmonella in formulated chicken thigh and leg meat products.     

Effect of the substrate's microstructure on the growth of Listeria monocytogenes

The effect of the microstructure of the medium on the growth of the food-borne pathogen Listeria monocytogenes was studied. The pathogen's growth kinetics was evaluated using liquid substrates and gels formed from different concentrations of sodium alginate (3.0% w/w) and gelatin (0–30.0% w/w). These results were further verified using a model dairy product with solid concentrations varying from 10.0 to 40.0% w/w. The pathogen's growth was faster in the liquid media than in the gels regardless of the gelling agent employed. The substrate's microstructure, apart from altering the growth pattern from planktonic to colonial, resulted in microbial growth suppression; however, each system affected the microorganism's growth in a different way. The suppressing effect of the substrate's microstructure on microbial growth was also dependent on temperature, while the presence of glucose in the solid medium accelerated microbial growth, thus reducing substantially the difference in growth kinetics between the gels and the liquid media. Any increase in the hydrocolloid concentration, which was also reflected in the rheological properties of the structured samples, resulted in a reduction of growth rate and in an increase of the lag phase of the pathogen. Overall, the gelation of the medium was found to exert a stress on the microorganism since the sol–gel transition, when the pathogen was already at the exponential growth phase, resulted in an additional lag phase or a decrease in the growth rate. The relationship between maximum specific growth rate and loss tangent of the gels (tanδ = G″/G′) was explored, pointing to the possible use of a single structural parameter to describe food matrix effects on microbial growth kinetics.     

Impact of nitrite on detection of Listeria monocytogenes in selected ready-to-eat (RTE) meat and seafood products

ABSTRACT:  The impact of sodium nitrite (NaNO₂) on detection and recovery of Listeria monocytogenes from select ready-to-eat (RTE) foods including smoked salmon, smoked ham, beef frankfurters, and beef bologna was assessed. Nitrite-containing (NC; 100 to 200 ppm NaNO₂) or nitrite-free (NF) foods were inoculated with a 5-strain cocktail of L. monocytogenes by immersion into Butterfield's buffer solution containing 5.4 to 7.4 × 10³ L. monocytogenes per milliliter. Inoculated products were vacuum-packaged and stored at 5 °C. A weekly comparative analysis was performed for presence of L. monocytogenes using 5 detection methods on products held at 5 °C for up to 8 wk. L. monocytogenes initially present at <100 CFU/g during the first 2 wk of storage increased throughout the study, attaining final populations of approximately 1 × 10⁴ to 1 × 10⁵ CFU/g. Lactic acid bacteria predominated throughout the study in all products. Exposure to NaNO₂ (100 to 200 ppm) resulted in 83% to 99% injury to the L. monocytogenes strains tested. The genetic-based BAX^® System (DuPont™ Qualicon, Wilmington, Del., U.S.A.) and modified USDA/FSIS methods detected 98% to 100% of Listeria -positive food samples and were consistently superior to and significantly different ( P < 0.05) from conventional cultural methods in recovering Listeria from NC samples. Data show that nitrite-induced injury adversely affects detection and recovery of L. monocytogenes from NC food, confirming earlier findings that nitrite-induced injury masks L. monocytogenes detection in NC RTE food products. Nitrite-injured Listeria can subsequently repair upon nitrite depletion and grow to high levels over extended refrigerated storage.     

Effect of relative inoculum concentration on Listeria monocytogenes growth in co-culture

Growth suppression of multi-species bacterial populations in batch cultures by a single 'dominant' strain has been referred to in the literature as the 'Jameson Effect'. The effect is often attributed to production of specific inhibitors of growth by one species against another. To explore its basis, we hypothesised that the Jameson Effect is often a non-specific interaction and that growth inhibition of species in co-culture can be controlled by manipulation of inoculum concentration and growth rate so as to enable a selected species to achieve stationary phase first. The hypothesis was assessed by co-culturing pairs of bacterial species under conditions selected to ensure that both strains grew at a similar rate and manipulating the initial concentration of each species. Specifically, the effect of inoculum concentration on the growth kinetics of Listeria monocytogenes when co-cultured in complex laboratory media with Escherichia coli, Pseudomonas fluorescens or a commercial strain of the lactic acid bacterium, Lactobacillus plantarum, was studied. Starting numbers of the second strain were either higher than ( approximately 10(6) cfu mL(-1)), equal to ( approximately 10(4) cfu mL(-1)) or lower than ( approximately 10(3) cfu mL(-1)) L. monocytogenes. In most trials, the initial inoculum concentration governed which species became dominant and suppressed the growth of the other strain. L. monocytogenes was suppressed by all other strains when its inoculum level was lower. Conversely, when L. monocytogenes was initially present at higher concentration than either P. fluorescens or L. plantarum, their growth was suppressed. E. coli, however, was not suppressed by L. monocytogenes even when the E. coli density was initially lower. While simple competition for nutrients could explain most of the observations, in some co-culture experiments pH reduction also seemed to play a role in inhibition of growth of some species. In other cases, available growth substrates were apparently not utilised by both co-cultured species, and the Jameson Effect did not occur. Thus, while it appears that under many conditions the Jameson Effect may be largely due to non-specific inhibition, more complex interactions between co-cultured strains involving species-specific pH limits for growth and differential utilisation of growth substrates may also occur and confound the simple Jameson Effect.     

Antimicrobial activity of Thymus zygis essential oil against Listeria monocytogenes and its application as food preservative

Thymus zygis is an aromatic plant used in folk medicine. This work aimed to evaluate the anti-Listeria monocytogenes activity of T. zygis essential oil (EO), whose thymol is its major compound. Furthermore, the attenuation of this bacterium's virulence, namely by the inhibition of biofilm formation, motility and invasion of human cells, and the possible application of the EO in food were evaluated. The T. zygis EO showed antibacterial activity against L. monocytogenes with a minimum inhibitory concentration (MIC) of 0.05%, while showing a bactericidal effect. The EO significantly reduced the biofilm formation (inhibition from 16.85 to 89.86%) and motility (halos between 6.66 and 10.98 mm, compared to controls 13.12 to 17.22 mm), and not inducing cross-resistance to antibiotics, such as ampicillin, cefotaxime, erythromycin, gentamicin, tetracycline, and vancomycin. L. monocytogenes counts (initial inoculum of ~10⁶ CFU/mL) were lowered by the use of 2× MIC of T. zygis EO in the chicken juice (1.53 log CFU/mL) and lettuce model (to below the detection limit) after two days of storage. The use of EO (0.2% (v/v)) for sanitizing fresh vegetables, reduce L. monocytogenes and natural microbiota for values below the detection limit of the method for iceberg lettuce after an immersion of 5 min. For the spinach, L. monocytogenes was reduced in 4.35 log CFU/mL and the natural microbiota was diminished in a range of 4.47 to 5.94 log CFU/mL, when compared with the washing with water. Overall, the T. zygis EO has demonstrated a promising antimicrobial activity and these findings point to the potential of the EO as a natural food preservative or sanitizer for controlling L. monocytogenes in food products.     

Effect of the inoculum size on Listeria monocytogenes growth in structured media

Obtaining quantitative data concerning the relative impact of various factors that may influence bacterial growth is of great importance for microbial risk assessment and predictive microbiology. The objective of this work was to investigate the effect of the initial Listeria monocytogenes density on all the growth parameters of this pathogen (lag phase duration, growth rate and maximum population density attained) on a sterile solid model system mimicking smoked fishery products, and in real cold-smoked salmon, a product likely to be contaminated with L. monocytogenes. Growth of the pathogen was monitored using a sensitive enumeration method, recently developed, based on membrane filtration followed by the transfer of the filter on a selective media [Gnanou Besse, N., Audinet, N., Beaufort, A., Colin, P., Cornu, M. and Lombard, B., 2004. A contribution to the improvement of Listeria monocytogenes enumeration in smoked salmon. International Journal of Food Microbiology, 91, 119-127.]. Depending on the experimental conditions, we found a significant effect of the inoculum size, both on lag phase duration, and on the maximal population attained. Moreover, the effect of the inoculum size on the growth of L. monocytogenes was dependent on a complex set of interactions. Factors which have appeared to impact on this effect include the cells physiological state, the background microflora, the texture of the media and the packaging system. It is important to understand how these interactions affect the growth of Listeria in order to predict and control its development in food.     

复合保鲜剂对冷鲜肉品质影响

新鲜美味的冷鲜肉受到广大消费者的喜爱。但冷鲜肉由于具有营养丰富、水分活度高等特点,容易滋生微生物。因此,选用安全、有效的保鲜剂对提升冷鲜肉的品质具有十分重要的意义。通过研究使用三种不同配比的复合保鲜剂后,冷鲜肉的菌落总数、挥发性盐基氮（ TVB－N）、pH值、出水损失率和感官指标等理化指标的变化,来分析其对冷鲜肉品质影响。实验结果表明：三种不同配比的复合保鲜剂均能不同程度地延长冷鲜肉的保鲜期,其中配比为0．2％茶多酚＋2％乳酸钠＋0．15％乳酸＋0．03％Nisin的复合保鲜剂对延长冷鲜肉保质期效果最佳。     

散装熟肉制品中单增李斯特菌污染状况分析

目的分析散装熟肉制品中单增李斯特菌污染状况。方法根据GB 4789.30-2016《单核细胞增生李斯特氏菌检验》规定的方法对散装熟肉制品的加工用原料、生产环境、各加工环节中产品以及不同销售环境下产品中的单增李斯特菌进行定性和定量检测。结果原料肉的总体带菌率达到21%;生产环境中第三区(远离食品接触面的区域)检出率为2%,其余区域未检出;各加工环节中产品单增李斯特菌检测结果均小于10CFU/g;不同销售环境下产品中单增李斯特菌检测结果小于10 CFU/g的比例为93%,处于10～50 CFU/g之间的样本占比6%,大于50 CFU/g的占比为1%。结论原料散装熟肉制品中单增李斯特菌污染的主要来源,蒸煮等加工环节能有效地杀灭单增李斯特菌。销售环境也关系到散装熟肉制品单增李斯特菌的污染程度,对于未包装的产品,专卖店优于农产品市场。     

Temperature-assisted pressure inactivation of Listeria monocytogenes in turkey breast meat

Ready-to-eat turkey breast meat samples were surface-inoculated with a five-strain cocktail of Listeria monocytogenes cultures to a final concentration of approximately 10(7) CFU/g. The inoculated meat samples were vacuum-packaged and pressure treated at 300 MPa for 2 min, 400 MPa for 1 min, and 500 MPa for 1 min at initial sample temperatures of 1, 10, 20, 30, 40, 50, and 55 degrees C. L. monocytogenes was most resistant to pressure at temperatures between 10 and 30 degrees C. As temperature decreased below 10 degrees C or increased over 30 degrees C, its pressure sensitivity increased. This enhanced inactivation effect was more pronounced when meat samples were treated at higher temperature than at lower temperature. For example, a 1-min treatment of 500 MPa at 40 degrees C reduced the counts by 3.8 log(10), while at 1 and 20 degrees C the same treatment reduced counts by 1.4 and 0.9 log(10), respectively (P<0.05). The survival curves of L. monocytogenes were obtained at 300 MPa and 55 degrees C, 400 MPa and 50 degrees C, and 500 MPa and 40 degrees C. With increasing treatment time, the three survival curves showed a rapid initial drop in bacteria counts with a diminishing inactivation rate or tailing effect. The survival data were fitted with a linear and a nonlinear, Weibull, models. The Weibull model consistently produced better fit to the survival data than the linear model.     

Influence of different histories of the inoculum on lag phase and growth of Listeria monocytogenes in meat models

The aim of this study was to determine the effect of history of inoculum and preservatives on the lag phase and growth rate of Listeria monocytogenes strains in meat products packaged under modified atmosphere conditions. Inocula with different histories were added to meat models, and growth rate and lag phase of two strains of L. monocytogenes were measured at 5 and 10 degrees C. The meat model stored at 10 degrees C contained sodium lactate, but the model stored at 5 degrees C did not. The five different histories of the inocula included cold propagation, biofilm formation, and starvation. The lag phase ranged from 1 to 10 days and was affected by the history of the inoculum, whereas the growth rate was constant except for one combination of history of inoculum and strain, where growth did not start during the incubation period. In a second series of experiments, the growth rate and lag phase of the two Listeria strains and the effects of two different histories of inoculum were tested in meat models with pH 5.7 or 6.5 and increasing amounts of NaCl. The growth rate depended on salt concentration, bacterial strain, and pH, whereas lag phase duration depended on history of inoculum, salt concentration, and pH. The lag phase duration was highly dependent on the history of the inoculum, and higher amounts of preservative (NaCl) made these effects even more noticeable. The results of this study underline the importance of the effects of the history of the inoculum on lag phase duration and could be used to predict lag phase in industrial meat products.     

体内儿茶素对鸡肉的抗氧化保鲜作用研究

通过在肉鸡饲料中添加一定量的儿茶素,研究了体内儿茶素对鸡肉的抗氧化保鲜作用效果.体内儿茶素明显提高了鸡肉的氧化稳定性能,具有延长其保鲜期的潜力,鸡腿肉的效果比鸡胸肉的效果更加明显,鸡肉的氧化稳定性能随着儿茶素添加量的增多而改善,儿茶素300mg的抗氧化作用效果与维生素E200mg的相近.结果表明,儿茶素可以代替维生素E作为肉鸡饲料中的抗氧化添加剂,以提高鸡肉的氧化稳定性及延长其保鲜期.     

Inhibition of Listeria monocytogenes by Lactobacillus sake strains of meat origin

The ability of two Lactobacillus sake strains of meat origin to inhibit the growth of Listeria monocytogenes at 4, 8, 15, 24 and 32°C in a conventional liquid media was investigated. Growth of L. monocytogenes was affected by Lac. sake strains at all temperatures. The inhibition was higher at 15, 24 and 32°C than at refrigeration temperatures. The inhibitory activity of both lactobacilli was similar perhaps due to the fact that Lac. sake 148 produces a bacteriocin inhibitory to L. monocytogenes, while Lac. sake 23 is a strong lactic acid producer. The antagonism exhibited by the lactobacilli on the L. monocytogenes strains seems to display a bacteriostatic rather than a bacteriocidal effect.     

Effect of high pressure, in combination with antilisterial agents, on the growth of Listeria monocytogenes during extended storage of cooked chicken

A cocktail of Listeria monocytogenes strains was inoculated into cooked chicken (∼2.2 × 10³ CFU g⁻¹) which was then pressure-treated (600 MPa/2 min/20 °C) and stored for up to 105 days at 8 °C. In addition, sodium lactate (2% w/w) or a pressure-resistant Weissella viridescens strain, known to have antilisterial activity, were added to the meat prior to inoculation with the pathogen and pressure treatment, to investigate the effect on Listeria survival.Pressure treatment alone was not sufficient to eliminate all of the Listeria. Numbers of survivors were initially below the level of detection (50 CFU g⁻¹) but increased during storage to reach >10⁸ CFU g⁻¹ by day 21. The presence of W. viridescens significantly extended the lag phase of any Listeria that survived the initial pressure treatment by ∼35 days, but numbers then increased to reach ∼10⁷ CFU g⁻¹ by day 105. The addition of 2% sodium lactate in combination with pressure treatment was most effective at inhibiting the growth of L. monocytogenes and numbers remained below the limit of detection throughout the 105 day storage.The addition of antimicrobial agents, in combination with pressure, could be used to give additional food safety assurance without increasing pressure hold time.     

Use of octanoic acid as a postlethality treatment to reduce Listeria monocytogenes on ready-to-eat meat and poultry products

The antilisterial efficacy and organoleptic impact of an octanoic acid (OA)-based treatment for ready-to-eat (RTE) meat and poultry products were investigated. Whole-muscle and comminuted RTE products were inoculated with a five-strain mixture of Listeria monocytogenes. The OA treatments were applied to the surface of RTE products by dispensing a specific volume of solution directly into the final package prior to vacuum sealing. Once sealed, the vacuum-packaged RTE products containing OA were immersed in water heated to 93.3 degrees C (200 degrees F) for 2 s to effect adequate film shrinkage. Extending the time at which the packaged, treated RTE products were exposed to water heated to 93.3 degrees C was also evaluated with a commercial cascading shrink tunnel fitted with a modified drip pan. Once treated, RTE products were examined for survivor populations of L. monocytogenes after 24 h of storage at 5 degrees C. Sensory evaluation was conducted with a 60-member trained panel on 11 uninoculated, treated RTE products. The OA treatment of RTE products reduced L. monocytogenes numbers to between 0.85 log CFU per sample (oil-browned turkey) and 2.89 log CFU per sample (cured ham) when compared with controls. The antilisterial activity of OA was improved by increasing the duration of the heat shrink exposure. Specifically, reductions of L. monocytogenes ranged from 1.46 log CFU per sample (oil-browned turkey) to 3.34 log CFU per sample (cured ham). Results from the sensory evaluation demonstrated that 10 of the 11 treated RTE products were not perceived as different (P < or = 0.05) from the untreated controls. Panelists detected reduced (P < or = 0.05) smoke flavor intensity with treated mesquite turkey, although the treated product was viewed as acceptable. Results demonstrate the effectiveness of OA as a postlethality treatment meeting U.S. Food Safety and Inspection Service regulatory guidelines for RTE meat and poultry products with minimal impact on sensory quality.     

Quantifying the robustness of a broth-based model for predicting Listeria monocytogenes growth in meat and poultry products

Given the importance of Listeria monocytogenes as a risk factor in meat and poultry products, there is a need to evaluate the relative robustness of predictive growth models applied to meat products. The U.S. Department of Agriculture-Agricultural Research Service Pathogen Modeling Program is a tool widely used by the food industry to estimate pathogen growth, survival, and inactivation in food. However, the robustness of the Pathogen Modeling Program broth-based L. monocytogenes growth model in meat and poultry application has not, to our knowledge, been specifically evaluated. In the present study, this model was evaluated against independent data in terms of predicted microbial counts and covered a range of conditions inside and outside the original model domain. The robustness index was calculated as the ratio of the standard error of prediction (root mean square error of the model against an independent data set not used to create the model) to the standard error of calibration (root mean square error of the model against the data set used to create the model). Inside the calibration domain of the Pathogen Modeling Program, the best robustness index for application to meat products was 0.37; the worst was 3.96. Outside the domain, the best robustness index was 0.40, and the worst was 1.22. Product type influenced the robustness index values (P < 0.01). In general, the results indicated that broth-based predictive models should be validated against independent data in the domain of interest; otherwise, significant predictive errors can occur.     

Influence of the adaptation of Listeria monocytogenes populations to structured or homogeneous habitats on subsequent growth on chilled processed meat

The influence of adaptation to habitat structure on the growth of Listeria monocytogenes on processed meats was investigated. Triplicate populations of each of two L. monocytogenes strains were adapted to growth in homogeneous (Tryptic Soy Broth, TSB) or structured (Tryptic Soy Agar) environments by serial transfer in these media incubated at 25 degrees C for 48 h. Adapted (serial transfer for 56 days) and unadapted control (single 48 h growth period) populations were inoculated onto the surface of bologna or farmers' sausage disks, vacuum packaged and stored at 4 degrees C for 4 weeks. Initially and after 4 weeks, L. monocytogenes numbers were determined on Modified Oxford Agar incubated at 25 degrees C for 48 h. Control populations of both strains grown in the structured habitat reached significantly higher levels (1-1.5 log cfu g(-1), P<0.05) on both products than their counterparts grown in homogeneous habitat. Populations of both strains adapted to homogeneous habitats grew to significantly higher levels (0.4-1 log cfu g(-1), P<0.05), while those adapted to growth in structured habitats grew to significantly lower levels (0.3-1 log cfu g(-1), P<0.05), on both products as compared to their unadapted control populations. Populations of both strains adapted to homogeneous habitats grew to significantly higher levels (0.2 to 0.5 log cfu g(-1), P<0.05) on farmers' sausage, and significantly lower levels (0.2 to 0.4 log cfu g(-1), P<0.05) on bologna, than those adapted to structured habitats. Habitat adaptation affected L. monocytogenes growth on processed meats, but the practical significance and reasons for these findings were not established.     

Effectiveness of trisodium phosphate against Listeria monocytogenes on excised and nonexcised chicken skin

The influence of sample type (i.e., excised versus nonexcised chicken skin) on the efficiency of trisodium phosphate (TSP) solutions in reducing Listeria monocytogenes populations and inhibiting their growth during refrigerated storage was studied. Whole chicken legs and excised chicken leg skin fragments inoculated with 10(8) CFU of L. monocytogenes per ml were dipped for 15 min in sterile tap water (control) or in a solution containing 8, 10, or 12% TSP. L. monocytogenes counts were determined after 0, 1, 3, and 5 days of refrigerated storage (2 degrees C). The decontamination effect of TSP was greater for excised skin than for whole legs. Microbial differences between control and TSP-treated samples were significantly larger for excised skin than for whole legs for 9 (75%) of 12 tested combinations of TSP concentrations and storage times. These differences varied from 1.05 +/- 0.26 log10 cycles (day 1) to 3.30 +/- 0.14 log10 cycles (day 5) for nonexcised-skin samples (whole legs) and from 1.54 +/- 0.48 log10 cycles (day 1) to 4.28 +/- 0.86 log10 cycles (day 5) for excised-skin samples. Significantly larger reductions were observed from the third day of refrigerated storage onward. The TSP concentration was a significant factor in the reduction of L. monocytogenes populations. These results suggest that bacteria are more readily accessible to TSP in excised than in nonexcised chicken skin and that the type of sample used to ascertain the efficacy of antimicrobial surface treatments may influence the findings of this type of study.     

接种量对单增李斯特菌生长期及生长界面的影响

为了初步了解接种量对单增李斯特菌生长状况及生长/非生长界面的影响,本实验对单增李斯特菌在0、4、10、25℃下通过培养菌液在600nm下的吸光光度值对其生长周期曲线分别进行了测定,并分析了不同接种量的单增李斯特菌菌液在25℃下的生长周期状况,探讨了纯培养条件下不同盐度和pH下,接种量对单增李斯特菌生长/非生长状况的影响。结果表明:不同的温度下,单增李斯特菌的生长周期有很大的差别;而在相同温度下,接种量对单增李斯特菌的生长周期有较大的影响,随着接种水平的降低,菌种生长所需的延滞时间越长,接种量为107CFU/mL时,其生长延滞期为0~4h,而当接种量减少为10CFU/mL时,其生长延滞期为0~16h;而对于单增李斯特菌的生长/非生长界面而言,接种量对其也有一定的影响,但其作用机制还有待进一步深入的研究。