Inactivation Of Listeria Monocytogenes Research Articles

Inactivation of Listeria monocytogenes and Escherichia coli O157:H7 inoculated on fresh-cut romaine lettuce by peanut skin extract/benzethonium chloride emulsion washing

The aim of this study was to examine the washing effect of the positively charged emulsion produced by the combination of peanut skin extract (PSE) and benzethonium chloride (BEC) against Listeria monocytogenes and Escherichia coli O157:H7 on fresh-cut romaine lettuce. PSE and BEC showed a good washing effect regardless of pathogenic bacteria type. Specifically, the combination of 5 mg/mL PSE and 0.5 mg/mL BEC (PSE-BEC) resulted in 3.06 and 2.83 log reductions in L. monocytogenes and E. coli O157:H7 populations inoculated on romaine lettuce leaves, respectively, compared to washing with water alone. Further, the PSE-BEC treatment-induced reduction in the levels of these two pathogenic bacteria was greater than that with 0.2 mg/mL sodium hypochlorite. SEM analysis revealed a decrease in the number of bacteria and a change in cell membrane morphology on the lettuce surface following PSE-BEC treatment. Moreover, analysis of contact angle and particle characteristics (i.e. ζ–potential, particle size, and polydispersity index) of the washing solution indicated a correlation between the washing effect and the physicochemical characteristics of the PSE-BEC solution. Furthermore, washing with PSE-BEC did not change the quality of lettuce leaves during storage. These results suggest that PSE-BEC has the potential to be used as an effective washing agent for freshly cut romaine lettuce as an alternative to chlorine-based sanitizers.

Inactivation of Listeria monocytogenes in raw and hot smoked trout fillets by high hydrostatic pressure processing combined with liquid smoke and freezing

High hydrostatic pressure (HHP; 200 MPa for 15 min), liquid smoke (0.50%, v/v) and freezing (−80 °C, overnight) was used to eliminate Listeria monocytogenes in BHI broth, raw and smoked trout. The bactericidal effect of liquid smoke solutions (L9 and G6), HHP and their combinations was evaluated against L.monocytogenes LO28, EGD-e and 10403S and further continued with the most resistant strain (10403S) to the combined treatment. For first time, a synergistic effect of liquid smoke and HHP was observed and was further enhanced by freezing prior to HHP. The effect of HHP and liquid smoke, prior to freezing was highest in BHI compared to raw and smoked trout. A major synergistic effect of HHP, liquid smoke and freezing was observed, reaching a 5.48 or 1.93 log CFU/g reduction when smoked or raw trout was used respectively. Furthermore, high injury levels occurred, among treatments reaching up to 55.98%. Industrial relevanceThis paper illustrates for first time, the possibility of using a very low pressure in combination with liquid smoke and freezing to eliminate L.monocytogenes. It was demonstrated that treatment of trout samples with liquid smoke followed by freezing prior to pressurization at 200 MPa for 15 min reduced the number of L.monocytogenes by more than 5-log CFU/g. Such a remarkable bacterial inactivation at a very low pressure (compared to common industrial practices) is a significant achievement that could allow production of safer and novel products by HHP at an affordable price, as the cost of equipment manufacture as well as the maintenance and running costs could be reduced substantially at lower operation pressures.

Inactivation of Listeria monocytogenes, Escherichia coli O157:H7, and Pre-existing Bacteria on Spinach by Combined Treatment of Cudrania tricuspidata Leaf Extract Washing and Ultraviolet-C Irradiation

Sodium hypochlorite has been used in the food industry to reduce microbial contamination in food; however, the use of sodium hypochlorite is insufficient considering the human health hazards and environmental pollution. To overcome this issue, plant-derived antimicrobial agents have been developed. In our previous study, Cudrania tricuspidata leaf extract (0.4%) was used to wash fresh vegetables for 3 min; however, Cudrania tricuspidata leaf extract was not effective in inhibiting gram-negative pathogens. Therefore, to enhance microbial decontamination by Cudrania tricuspidata leaf extract, ultraviolet-C irradiation (1.8 kJ/m2) was used in combination on spinach as a model system. After the combined treatment, the microbial count of Listeria monocytogenes, Escherichia coli O157:H7, and pre-existing bacteria on spinach were reduced by 1.94, 1.93, and 2.10 log CFU/g, respectively. The log reduction levels of the combined treatment against Listeria monocytogenes, Escherichia coli O157:H7, and pre-existing bacteria on spinach were higher than those of 200 ppm sodium hypochlorite washing (1.46, 1.26, and 1.44 log reduction). Moreover, an increased amount of cellular constituents was released from the pathogens due to the combined treatment compared with the single treatment of Cudrania tricuspidata leaf extract washing or ultraviolet-C irradiation. The distorted and ruptured pathogens after the combined treatment were observed via scanning electron microscopy. Furthermore, combined treatment did not affect the color, texture, and bioactive compounds in spinach. These results suggest that combination of Cudrania tricuspidata leaf extract washing and ultraviolet-C irradiation used in the present study can be an effective method to replace sodium hypochlorite washing.

Multilevel modelling as a tool to include variability and uncertainty in quantitative microbiology and risk assessment. Thermal inactivation of Listeria monocytogenes as proof of concept

Variability is inherent in biology and also substantial for microbial populations. In the context of food safety risk assessment, it refers to differences in the response of different bacterial strains (between-strain variability) and different cells (within-strain variability) to the same condition (e.g. inactivation treatment). However, its quantification based on empirical observations and its incorporation in predictive models is a challenge for both experimental design and (statistical) analysis.In this article we propose the use of multilevel models to quantify (different levels of) variability and uncertainty and include them in the predictions. As proof of concept, we analyse the microbial inactivation of Listeria monocytogenes to thermal treatments including different levels of variability (between-strain and within-strain) and uncertainty. The relationship between the microbial count and time was expressed using a (non-linear) Weibullian model. Moreover, we defined stochastic hypotheses to describe the different types of variation at the level of the kinetic parameters, as well as in the observations (microbial counts). The model parameters (kinetic parameters and variances) are estimated using Bayesian statistics.The multilevel approach was compared against an analogous, single-level model. The multilevel methodology shrinks extreme parameter estimates towards the mean according to uncertainty, thus mitigating overfitting. In addition, this approach enables to easily incorporate different levels of variation (between-strain and/or within-strain variability and/or uncertainty) in the predictions. On the other hand, multilevel (Bayesian) models are more complex to define, implement, analyse and communicate than single-level models. Nevertheless, their ability to incorporate different sources of variability in predictions make them very suitable for Quantitative Microbial Risk Assessment.

Inactivation of Listeria monocytogenes in game meat applying sous vide cooking conditions

This study investigated the effects of sous vide cooking at temperatures between 50 °C and 60 °C on the inactivation kinetics of Listeria (L.) monocytogenes. Nutrient broth and minced game meat (Capreolus capreolus and Sus scrofa) were inoculated with three strains of L. monocytogenes and cooked under sous vide conditions (50, 55 or 60 °C for several hours). Results showed that the decimal reduction values (D-values) were largely dependent on the surrounding matrix. D-values of 125.5, 29.7 and 5.1 min were reached for BHI (brain heart infusion) at 50 °C, 55 °C and 60 °C, respectively. For roe deer, D-values of 49.2, 14.9 and 3.7 min and for wild boar, D-values of 100.2, 23.8 and 4.2 min were reached. It can be concluded that microbiologically safe cooking durations under sous-vide conditions below 60 °C should be considered individually for each meat product due to the dramatic influence of the matrix in comparison to higher temperature conditions.

Ultrasonic decontamination in smoked salmon experimentally contaminated with Listeria monocytogenes: Preliminary results.

The purpose of this work was to evaluate the effects of ultrasound (sonication) and their combination with temperature (thermosonication) on the inactivation of Listeria monocytogenes (LM) in smoked salmon. The trial was conducted on smoked salmon samples experimentally contaminated with a cocktail of 4 strains of Listeria monocytogenes (LM ATCC 19114, LM ATCC 15313, LM ATCC 19111 and LM ATCC 7644) at a final concentration of 8 log cfu/g and kept at 4°C until its use. Thermosonication treatments between 40°C and 50°C for 5, 10 and 15 minutes proved to be more effective without altering the sensory characteristics of the food.

Evaluation of the effectiveness of different preservation techniques on the inactivation of Listeria monocytogenes by using challenge testing protocol in the fresh, soft cheese - paneer

Paneer is a fresh, soft, unripened cheese, falls under the category of ready to eat product. From 2010 to 2015, because of consumption of soft cheese, several multistate outbreaks of Listeria monocytogenes happened in the USA. In the meantime, recalls of soft cheese happened in Australia and Canada due to association of soft cheese with L. monocytogenes. Therefore, this research was undertaken to study the growth potential of L. monocytogenes in this product by challenge testing with three different contamination modes and evaluate the effectiveness of different preservation techniques in inhibiting the growth of L. monocytogenes in this product. Results showed that paneer cheese supports the growth of L. monocytogenes to more than 2 logs (100 cfu/g) during the shelf life. It was found that the none of the preservation techniques evaluated inhibited the growth of L. monocytogenes to less than 1 log cycle during the shelf life. This study provides the scientific basis of assessing the growth potential of L. monocytogenes by challenge testing and mandates the implementation of regulatory guidelines while manufacturing, distribution, storage and importation of this product with the recommendation of use of hurdle technologies to inhibit the growth of L. monocytogenes in this product.

Limonene nanoemulsified with soya lecithin reduces the intensity of non-isothermal treatments for inactivation of Listeria monocytogenes

Consumers’ demands for ready-to-eat, fresh-like products are on the rise during the last years. This type of products have minimal processing conditions that can enable the survival and replication of pathogenic microorganisms. Among them, Listeria monocytogenes is of special concern, due to its relatively high mortality rate and its ability to replicate under refrigeration conditions. Previous research works have shown that nanoemulsified essential oils in combination with thermal treatments are effective for inactivating L. monocytogenes. However, previous research works were limited to isothermal conditions, whereas actual processing conditions in industry are dynamic. Under dynamic conditions, microorganism can respond unexpectedly to the thermal stress (e.g. adaptation, acclimation or increased sensitivity). In this work, we assess the combination of nanoemulsified D-limonene with thermal treatments under isothermal and dynamic conditions. The nanoemulsion was prepared following an innovative methodology using soya lecithin, a natural compound as well as the essential oil. Under isothermal heating conditions, the addition of the antimicrobial enables a reduction of the treatment time by a factor of 25. For time-varying treatments, dynamic effects were relevant. Treatments with a high heating rate (20 °C/min) are more effective than those with a slow heating rate (1 °C/min). This investigation demonstrates that the addition of nanoemulsified D-limonene can greatly reduce the intensity of the thermal treatments currently applied in the food industry. Hence, it can improve the product quality without impacting its safety.

The Complex Effect of Food Matrix Fat Content on Thermal Inactivation of Listeria monocytogenes: Case Study in Emulsion and Gelled Emulsion Model Systems.

Previous studies on the influence of food matrix fat content on thermal inactivation kinetics of food pathogens have shown contradictory results due to the combined influence of fat content and other factors such as composition. Therefore, thermal inactivation of Listeria monocytogenes at 59, 64, and 69°C was systematically investigated in emulsion and gelled emulsion food model systems with various fat content (1, 5, 10, and 20%), such that the effect of fat content was isolated. Thermal conductivity and rheological properties of the model systems were quantified, as well as the effect of these properties on the thermal load of the model systems. Thermal conductivity was complexly related to fat content, the nature of the food matrix (i.e., viscous or gelled), and temperature. For the emulsions, the consistency index K increased with increasing fat content, while the flow behavior index n followed the opposite trend. For the gelled emulsions, the storage modulus G′ was always larger than the loss modulus G″ (i.e., measure of elastic and viscous properties, respectively). The phase angle δ [i.e., arctan (G″/G′)] was proportional with fat content, but this relation became more complex at higher temperatures. The thermal load of the model systems was not largely affected by food matrix fat content. Thermal inactivation of L. monocytogenes was investigated by means of the maximum specific inactivation rate kmax, log reductions, and sublethal injury (SI). Both for emulsions and gelled emulsions, kmax decreased with increasing fat content below approximately 60°C, while a more complex behavior was observed at higher temperatures. In the emulsions, log reductions were considerably lower (i.e., 2–3 log) at 1% fat than in systems with higher fat content. In the gelled emulsions, log reductions generally decreased with increasing fat content. SI decreased with increasing fat content, both in emulsions and gelled emulsions. In conclusion, the inactivation rate (i.e., kmax) of L. monocytogenes was affected by a complex relation between food matrix fat content, thermal conductivity, rheological properties, and inactivation temperature. Due to the small scale of the model systems, differences in kmax did not directly affect the final log reductions in a similar fashion.

Combined ohmic heating and krypton‐chlorine excilamp treatment for the inactivation of Listeria monocytogenes, Salmonella Typhimurium, and Escherichia coli O157:H7 in apple juice

AbstractThe combined effect of the 222‐nm krypton‐chlorine (KrCl) excilamp and ohmic heating for the inactivation of Listeria monocytogenes, Salmonella enterica serovar Typhimurium, and Escherichia coli O157:H7 in apple juice was investigated in this study. When ohmic heating and a KrCl excilamp were applied to apple juice simultaneously, the reduction level of E. coli O157:H7 following 70 s (target temperature of 65.9°C) of combination treatment reaching 4.6 log CFU/ml was significantly higher than that of each treatment alone (2.7 log CFU/ml). The same trend, indicating a synergistic bactericidal effect, was observed for L. monocytogenes and S. Typhimurium. Therefore, the combination treatment of the KrCl excilamp and ohmic heating can be used effectively to control bacterial pathogens in apple juice with a reduced processing time.Practical applicationsDemands for energy‐efficient and environmentally friendly bactericidal apparatuses have been increasing. Although the mercury UV lamp has been widely used to inactivate foodborne pathogens in water or juice products individually or combined with heat treatment, the use of this conventional lamp will be limited continuously in accordance with the Minamata Convention treaty, which restricts the use of mercury. Thus, it is of interest to identify the bactericidal effect of an alternative UV‐C lamp and its combination with heat treatment. The synergistic bactericidal effect of the KrCl excilamp and ohmic heating, which are alternative nonthermal and thermal technologies, respectively, was identified in the present study. The results indicated in this study could be utilized by juice processors to achieve a 5‐log reduction in foodborne pathogens.

Augmenting the Pressure-Based Pasteurization of Listeria monocytogenes by Synergism with Nisin and Mild Heat.

The current study investigated Listeria monocytogenes inactivation using mild heat with elevated hydrostatic pressure and nisin under buffered condition. A four-strain pathogen mixture was exposed to 0 (control) and up to 9 min of (1) 4 °C elevated pressure; (2) 4 °C elevated pressure and nisin; (3) 4 °C nisin; (4) heat at 40 °C; (5) 40 °C elevated pressure; (6) 40 °C elevated pressure and nisin; and (7) 40 °C nisin. Elevated hydrostatic pressure at 400 MPa (Hub880 Explorer, Pressure BioScience Inc., Easton, MA, USA) and nisin concentration of 5000 IU/mL were used in the trials. Analyses of variance were conducted, followed by Dunnett’s- and Tukey-adjusted means separations. Under conditions of these experiments, nisin augmented (p < 0.05) decontamination efficacy of 40 °C heat and elevated hydrostatic pressure treatments, particularly at treatment interval of 3 min. This synergism with nisin faded away (p ≥ 0.05) as the treatment time for thermal, high-pressure, and thermal-assisted pressure processing increased. The results of our study, thus, exhibit that practitioners and stakeholders of pressure-based technologies could benefit from synergism of mild heat and nisin for short-term, high-pressure pasteurization treatments to achieve microbial safety and economic feasibility comparable to traditional heat-treated products.

Effect of gamma radiation and storage at 4°C on the inactivation of Listeria monocytogenes, Escherichia coli and Salmonella enterica Typhimurium in Legon-18 pepper (Capsicum annuum) powder

Abstract Objectives Spices are low moisture foods which have been known to be contaminated with various pathogens and sun-dried Legon-18 pepper powder is not left out. Due to its contamination with various pathogens, a study was conducted to determine the effects of gamma irradiation on the decontamination of Legon-18 pepper powder and on some quality parameters. Methods Samples were obtained from a local farmer from the Eastern Region of Ghana. Sterility tests were carried out. The samples were inoculated with known cfu/ml of Escherichia coli, Listeria monocytogenes and Salmonella enterica Typhimurium. Samples were irradiated at 1, 2, 4, and 5 kilogray (kGy). Zero kilogray served as control (unirradiated). All samples were stored at 4oC for 60 days. Enumeration of the various pathogens was done in appropriate media. Some quality parameters were determined after irradiating unsterile samples at 5 kGy and 0 kGy served as control. Capsaicinoids and carotenoids were quantified using a high performance liquid chromatography. The samples were stored at 4oC for 8 weeks. Results A dose-dependent effect on the inactivation of the pathogens was observed (P &amp;lt; 0.05). Storage time affected the inactivation of the pathogens as well (P &amp;lt; 0.05). Complete inactivation of the pathogens was observed at 5 kGy at day 0. Capsaicin, dihydrocapsaicin and total capsaicinoid content of the samples irradiated at 5 kGy increased at 23.64%, 14.7 % and 20.95% respectively as compared with the contents of the unirradiated samples. A gamma irradiation dose of 5 kGy caused losses of 8.11%, 8.67% and 26.54% in capsanthin, beta carotene and beta cryptoxanthin respectively. Quality parameters measured reduced with storage (P &amp;lt; 0.05). Conclusions Gamma irradiation inactivated pathogens at 5 kGy. Lower doses used during the study could inactivate the pathogens but with time. All quality parameters and carotenoids quantified were affected by gamma irradiation and storage period (P &amp;lt; 0.05).

Efficacy of Commercial Sanitizers Used in Food Processing Facilities for Inactivation of Listeria Monocytogenes, E. Coli O157:H7, and Salmonella Biofilms.

Bacteria entrapped in biofilms are a source of recurring problems in food processing environments. We recently developed a robust, 7-day biofilm microplate protocol for creating biofilms with strongly adherent strains of Listeria monocytogenes, Escherichia coli O157:H7, and Salmonella serovars that could be used to examine the effectiveness of various commercial sanitizers. Listeria monocytogenes 99-38, E.coli O157:H7 F4546, and Salmonella Montevideo FSIS 051 were determined from prior studies to be good biofilm formers and could be recovered and enumerated from biofilms following treatment with trypsin. Extended biofilms were generated by cycles of growth and washing daily, for 7 days, to remove planktonic cells. We examined five different sanitizers (three used at two different concentrations) for efficacy against the three pathogenic biofilms. Quaternary ammonium chloride (QAC) and chlorine-based sanitizers were the least effective, showing partial inhibition of the various biofilms within 2 h (1–2 log reduction). The best performing sanitizer across all three pathogens was a combination of modified QAC, hydrogen peroxide, and diacetin which resulted in ~6–7 log reduction, reaching levels below our limit of detection (LOD) within 1–2.5 min. All treatments were performed in triplicate replication and analyzed by one way repeated measures analysis of variance (RM-ANOVA) to determine significant differences (p < 0.05) in the response to sanitizer treatment over time. Analysis of 7-day biofilms by scanning electron microscopy (SEM) suggests the involvement of extracellular polysaccharides with Salmonella and E. coli, which may make their biofilms more impervious to sanitizers than L. monocytogenes.

Inactivation of Listeria monocytogenes during dry-cured ham processing

The effect of Serrano and Iberian dry-cured ham processing and ripening on Listeria monocytogenes inactivation at the surface of whole hams was investigated. Salted hams were surface inoculated (6.5 log CFU) with a cocktail of 4 L. monocytogenes strains isolated from environment and products of a meat industry. Serrano and Iberian hams were ripened for 16 and 24 months, respectively. A decrease of at least 4.6 log units on the surface of Serrano ham was recorded after 4 months for L. monocytogenes counts, which remained under the detection limit thereafter. L. monocytogenes declined by >5 log units on the surface of Iberian ham during the first 9 months and was not detected afterwards. The higher nitrite content of Serrano ham might have accelerated the decrease of the pathogen. This study validates the inactivation of L. monocytogenes on the surface of whole dry-hams during extended ripening.

Sous-vided Restructured Goat Steaks: Process Optimized by Thermal Inactivation of Listeria monocytogenes and Their Quality Characteristics

The thermal-death times of Listeria monocytogenes were determined in inoculated restructured goat steak at 60°C, 65°C, and 70°C of sous-vide temperatures. D-values of L. monocytogenes in inoculated restructured goat steak ranged from 7.27 min at 60°C to 0.46 min at 70°C. Times need to yield at least a 6 log reduction of L. monocytogenes at their temperatures for this product were 47, 12, and 3 min, respectively. After sous-vide, all microbial counts in non-inoculated samples were not detectable, except the aerobic and anaerobic mesophilie and lactic acid bacteria counts were lower than 2 Log CFU/g. For sous-vided and grilled sous-vided samples, sous-vide loss and surface shrinkage were the lowest in samples sous-vided at 60°C for 47 min (p<0.05). These samples demonstrated the lowest CIE L*, shear force, hardness, gumminess and chewiness and the highest CIE a* and hue angle (p<0.05). Therefore, sous-vide at 60°C for 47 min provided convenient ready-to-cook restructured goat steak for microbiology safety and optimization of physicochemical quality.

Inactivation of Listeria monocytogenes on post-harvest carrot and tomato by gamma radiation, sanitizer, biocontrol treatments and their combinations

Control of foodborne pathogens on post-harvest produce may be improved by combined treatments. Hurdle applications may enhance the lethal effects of non-thermal interventions on pathogenic bacteria. We evaluated the effects of gamma radiation (0.5 kGy), nisin-based sanitizer (100%), and competitive biocontrol microbes (non-pathogenic Pseudomonas fluorescens) on the survival of Listeria monocytogenes on post-harvest tomato and carrot. Produce storage temperatures (5 and 20 °C) and days (0 and 7), with single, two or three treatment combinations of radiation (R), sanitizer (S) and biocontrol (B) were investigated for pathogen survival. Significant differences (P < 0.05) in L. monocytogenes and subsequent pathogen reductions on produce by hurdle treatments were recorded. L. monocytogenes reductions on carrot and tomato varied, as R + S + B and S + R + B hurdle treatments (day 7) reduced Listeria survival by 2.5–5.0 and 2.4–5.0 log CFU/g of produce, respectively, at both temperatures. The sanitizer (S) treatment reduced Listeria on produce by 2.2–4.7 logs (day 7), while R or B treatments also reduced Listeria survival on produce. The treatment efficacies on pathogen survival were sanitizer > radiation > biocontrol. Inactivation of Listeria on post-harvest produce may be improved by treatment combinations as hurdle applications could enhance the safety of post-harvest carrot and tomato.

Listeria monocytogenes inactivation in deboned dry-cured hams by high pressure processing

The effect of high pressure processing (HPP) on Listeria monocytogenes inactivation on the surface and in the interior of deboned dry-cured ham was investigated. Whole deboned hams were pressurized at 450 MPa for 10 min or 600 MPa for 5 min and stored for 60 d at 4 and 12 °C. In control non-pressurized dry-cured hams, L. monocytogenes counts decreased on the surface throughout refrigerated storage, whereas remained unchanged in the interior of the product. Treatments at 600 MPa reduced L. monocytogenes population by 2 logs on the surface and by 3 logs in the interior of hams. Minor changes in the physicochemical characteristics were found. High water activity in the interior of dry-cured ham might enhance the pathogen inhibition by HPP. Treatment at 600 MPa for 5 min in whole dry-cured ham allowed to reach the Food Safety Objective for L. monocytogenes according to the European and the USDA criteria in case of contamination during deboning.

Inactivation of Listeria Monocytogenes at various growth temperatures by ultrasound pretreatment and cold plasma

Effects of ultrasound pretreatments and dielectric barrier discharge (DBD) cold plasma on the inactivation of Listeria monocytogenes (L. monocytogenes) under various growth temperatures were investigated, and membrane fatty acid profiles of L. monocytogenes were also determined. Fatty acids anteiso-C15:0, anteiso-C17:0 and C15:0 were the most abundant fatty acids at any given growth temperature (10, 25, 37 and 42 °C). Principal components analysis (PCA) and redundancy analysis (RDA) displayed that preconditioning growth temperatures induced noticeable modifications in membrane fatty acid profile, which could be separated from each other and classified roughly into four groups. In addition, growth temperature-mediated alterations in fatty acid profile and membrane fluidity of L. monocytogenes were associated with cell viability, with a significant increase in intracellular reactive oxygen species (ROS) levels (P < 0.01) and a significant reduction in the resistance to DBD plasma exposure (P < 0.01). For inactivation kinetics, the sigmoidal-like model (RMSE10 °C = 0.037; RMSE25 °C = 0.055; RMSE37 °C = 0.073; RMSE42 °C = 0.192) with three phases (shoulder region, log-linear phase and tailing phase) was the most appropriate model that could describe the inactivation curves of L. monocytogenes. Survival assays and scanning electron microscopy (SEM) indicated that ultrasound pretreatments caused a weakening effect on the membrane. In general, this study provided new insights on correlating growth temperature-mediated alterations in fatty acid profile with intracellular ROS levels and inactivation efficiency.

Hot water sanitization of a commercial mushroom disk slicer to inactivate Listeria monocytogenes

Industrial food slicing equipment that is difficult to clean and sanitize can become a reservoir of Listeria monocytogenes. Some commercial mushroom slicing operations have attempted to manage cross-contamination risks by periodically immersing detachable slicer heads in heated water maintained at varying temperatures for different times. In this study, the efficacy of hot water disinfection treatments to eliminate L. monocytogenes from commercial mushroom slicers was examined. Fourteen L. monocytogenes strains, including several obtained from mushrooms or mushroom processing environments, were screened for heat tolerance at 60 °C for 6 min. The 6 most heat tolerant strains were used to prepare a microbial cocktail for thermal inactivation studies. Heat tolerance of planktonic cells was not affected by incubation at 30 °C for up to 7 days, or the extent to which they adhered to stainless-steel coupons under the same conditions. Respective L. monocytogenes D50, D60, and D70-values of 11.5 min, 1.90 min, and 1.0 min were not higher than D-values for a L. innocua isolate at the same temperatures. Heat penetration studies, conducted on the slicer head immersed in a heated clean-out-of-place (COP) wash tank, determined that the slowest-to-heat food contact location (cold spot) was the at the interface of one of the blade-spacers with the horizontal slicer drive shaft. A microbial challenge study was conducted by disassembling the slicer head, inoculating the cold spot with 7 log cells L. innocua surrogate, and subjecting the reassembled slicer head to water temperatures of 55 °C, 65 °C, or 75 °C for 93, 16.4, or 6.5 min. Complete elimination of L. innocua cells for each slicer head treatment demonstrated the feasibility of hot water sanitization treatments to minimize L. monocytogenes food safety risks.

Non-thermal approach to Listeria monocytogenes inactivation in milk: The combined effect of high pressure, pediocin PA-1 and bacteriophage P100

Non-thermal food processing and replacement of chemical additives by natural antimicrobials are promising trends in the food industry. The objective of the present work was to evaluate the effect of a process which combines mild high hydrostatic pressure – HHP (200 and 300 MPa, 5 min, 10 °C), phage Listex™ P100 and the bacteriocin pediocin PA-1 as a new non-thermal process for destruction of Listeria monocytogenes (104 CFU mL−1 or 107 CFU mL−1) in milk. For inoculum levels of 104 CFU mL−1, HHP combined with phage P100 eliminated L. monocytogenes immediately after pressurization. When L. monocytogenes was inoculated at levels of 107 CFU mL−1, a synergistic effect between phage P100, pediocin PA-1 and HHP (300 MPa) on the inactivation of L. monocytogenes was observed during storage of milk at 4 °C. For non-pressure treated samples inoculated with phage or pediocin or both, L. monocytogenes counts decreased immediately after biocontrol application, but regrowth was observed in a few samples during storage. Phage particles were stable during refrigerated storage for seven days while pediocin PA-1 remained stable only during three days. Further studies will have to be performed to validate the findings of this work in specific applications (e.g. production of raw milk cheese).