Ready-to-eat Meat Products Research Articles

Synergistic effect of gamma (γ)-irradiation and microencapsulated antimicrobials against Listeria monocytogenes on ready-to-eat (RTE) meat

Oregano essential oil (Origanum compactum; 250 μg/ml), cinnamon essential oil (Cinnamomum cassia; 250 μg/ml) and nisin (16 μg/ml) were used alone or in combination to evaluate their efficiency to inhibit the growth of Listeria monocytogenes on RTE ham. Microencapsulation of the antimicrobial formulations was done to verify the potential effect of the polymer to protect the antimicrobial efficiency during storage. Combined treatments of antimicrobial formulation with γ-irradiation were done to verify the synergistic effect against L. monocytogenes. Microencapsulation of essential oils-nisin and γ-irradiation treatment in combination showed synergistic antimicrobial effect during storage on RTE meat products. Microencapsulated cinnamon and nisin in combination with γ-irradiation (at 1.5 kGy) showed 0.03 ln CFU/g/day growth rate of L. monocytogenes whereas the growth rate of non-microencapsulated cinnamon and nisin in combination with γ-irradiation was 0.17 ln CFU/g/day. Microencapsulation significantly (P ≤ 0.05) improved the radiosensitivity of L. monocytogenes. Microencapsulated oregano and cinnamon essential oil in combination with nisin showed the highest bacterial radiosensitization 2.89 and 5, respectively, compared to the control.

Safety and quality assessment of ready-to-eat pork products in the cold chain

It is of crucial importance for Ready-To-Eat (RTE) foodstuffs producers to guarantee the quality and safety of their products under the cold chain variations related to different time–temperature profiles. Experimental designs were used to investigate and model the effects of temperature on safety and quality attributes of selected RTE meat products. Three types of RTE sliced pork products (cooked ham, cooked paté and smoked ham) were stored at different temperatures (5, 8, 12 and 15°C) up to 6weeks. Microbiological and physico-chemical attributes were followed. Growth parameters of Listeria monocytogenes were investigated by challenge testing for the three RTE products at the four temperatures. Two lactic acid bacteria (Lactobacillus sakei and Leuconostoc mesenteroïdes) were also investigated by challenge testing but only for cooked ham and cooked paté at 8°C. Changes in quality indicators including colour, texture and water content, water activity and water dripping were evaluated over storage time for the three RTE products. Spoilage experiments were conducted (at 2, 8, 12, 15°C for 48days) on cooked ham and the production of ethanol, as a representative volatile deriving from bacterial metabolism, was correlated to bacterial outgrowth. Growth parameters of the three strains for the given food were mathematically modelled and validation tests were performed for L. monocytogenes in cooked ham and cooked paté. Physico-chemical attributes were not significantly affected by time–temperature storage. The production of ethanol on spoiled cooked ham was related to growth of lactic acid bacteria, especially Leuconostoc. A threshold value of ethanol concentration was defined in relation with a threshold count numbers of LAB under the conditions studied.

Enhanced Inactivation of Food-Borne Pathogens in Ready-To-Eat Sliced Ham by Near-Infrared Heating Combined with UV-C Irradiation and Mechanism of the Synergistic Bactericidal Action

The objective of the study described in this article was, first, to investigate the effect of the simultaneous application of near-infrared (NIR) heating and UV irradiation on inactivation of Escherichia coli O157:H7, Salmonella enterica serovar Typhimurium, and Listeria monocytogenes in ready-to-eat (RTE) sliced ham and as well as its effect on product quality and, second, to elucidate the underlying mechanisms of the synergistic bactericidal action of NIR heating and UV irradiation. With the inoculation amounts used, simultaneous NIR-UV combined treatment for 70 s achieved 3.62, 4.17, and 3.43 log CFU reductions of E. coli O157:H7, S. Typhimurium, and L. monocytogenes, respectively. For all three pathogens, the simultaneous application of both technologies resulted in an additional log unit reduction as a result of their synergism compared to the sum of the reductions obtained after the individual treatments. To investigate the mechanisms of NIR-UV synergistic injury for a particular microorganism in a food base, we evaluated the effect of four types of metabolic inhibitors using the overlay method and confirmed that damage to cellular membranes and the inability of cells to repair these structures due to ribosomal damage were the primary factors related to the synergistic lethal effect. Additionally, NIR-UV combined treatment for a maximum of 70 s did not alter the color values or texture parameters of ham slices significantly (P > 0.05). These results suggest that a NIR-UV combined process could be an innovative antimicrobial intervention for RTE meat products.

Effect of E-beam treatment on the bioaccessibility of folic acid incorporated to ready to eat meat products

Meat products are a potential matrix to fortify with folic acid (FA). In previous studies the effect of E-beam treatment on the stability of FA in ready to eat (RTE) meat products has been described. However, the FA bioaccessibility in both traditional and treated (3 kGy) RTE meat products has not yet been studied. We investigated the availability for intestinal absorption (bioaccessibility) of FA using a dynamic gastrointestinal model (TIM system). Three different types of meat products, hamburgers, cooked sausages and dry fermented sausages, initially fortified with 24 μg/g FA were tested as freshly prepared products and as RTE products. The bioaccessibility of FA during the passage through the TIM system was high for all three products, irrespective the E-beam treatment. From hamburgers the bioaccessibility was 82% and 100%, from cooked sausages, 61 and 79% and from dry fermented sausages 81 and 82%, with or without e-beam treatment, respectively.

Antimicrobial resistance of Listeria monocytogenes and Listeria innocua from meat products and meat-processing environment

A total of 336 Listeria isolates from ready-to-eat (RTE) meat products and meat-processing environments, consisting of 206 Listeria monocytogenes, and 130 Listeria innocua isolates, were characterized by disc diffusion assay and minimum inhibitory concentration (MIC) values for antimicrobial susceptibility against twenty antimicrobials. Resistance to one or two antimicrobials was observed in 71 L. monocytogenes isolates (34.5%), and 56 L. innocua isolates (43.1%). Multidrug resistance was identified in 24 Listeria isolates, 18 belonging to L. innocua (13.9%) and 6 to L. monocytogenes (2.9%). Oxacillin resistance was the most common resistance phenotype and was identified in 100% Listeria isolates. A medium prevalence of resistance to clindamycin (39.3% isolates) and low incidence of resistance to tetracycline (3.9% isolates) were also detected. Listeria isolates from RTE meat products displayed higher overall antimicrobial resistance (31.3%) than those from the environment (13.4%). All the strains assayed were sensitive to the preferred antibiotics used to treat listeriosis. Results showed that although antimicrobial resistance in L. monocytogenes still occurs at a low prevalence, L. innocua can form a reservoir of resistance genes which may transfer between bacterial species, including transference to organisms capable of causing disease in humans.

Antimicrobial films and coatings for inactivation of Listeria innocua on ready-to-eat deli turkey meat

Edible antimicrobial coating solutions incorporating chitosan, lauric arginate ester (LAE) and nisin were developed to reduce foodborne pathogen contamination on ready-to-eat (RTE) meats. RTE deli meat samples were directly coated with the solutions, or treated with solution-coated polylactic acid (PLA) films. The antimicrobial efficacy of the coatings and films against Listeria innocua inoculated onto the surface of RTE meat samples was investigated. Antimicrobial coatings with 1.94 mg/cm2 of chitosan and 0.388 mg/cm2 of LAE reduced L. innocua by ca. 4.5 log CFU/cm2. Nisin (486 IU/cm2) showed less effectiveness than LAE (0.388 mg/cm2) and addition of nisin to the antimicrobial coatings or films containing LAE (0.388 mg/cm2) did not enhance the total antimicrobial effectiveness. Combining antimicrobial coatings or films with flash pasteurization (FP), which uses short burst of steam under pressure, further reduced L. innocua, achieving over a 5 log reduction. There was no significant difference in the effectiveness of antimicrobial films versus the coatings (p > 0.05). These data show the potential use of antimicrobial packaging alone, or in combination with FP, in preventing foodborne illness due to post-processing contamination of RTE meat products.

Quantification of viable Escherichia coli O157:H7 in meat products by duplex real-time PCR assays

Rapid and specific detection of viable Escherichia coli O157:H7 cells in ready-to-eat (RTE) meat products, by duplex quantitative PCR (qPCR) procedures with mRNA and SYBR Green and TaqMan methodologies were developed. Specific primers and probes were designed based on the serotype of E. coli O157:H7, fliCh7 and rfbE genes. No cross-reactivity with other microorganisms was observed. The detection limit of the assays was 101 or 102CFU/g for artificially contaminated meat products, and after a 4h enrichment period at 37°C, the detection limit decreased to about 1CFU/g. Time-to completion of the assay was approximately 8h. Thus, these qPCR methods offer a useful, rapid and efficient tool for screening viable E. coli O157:H7 in RTE meat products. This tool could also be proposed for monitoring these foodborne pathogens in HACCP programs.

A simplified modelling approach established to determine the Listeria monocytogenes behaviour during processing and storage of a traditional (Italian) ready-to-eat food in accordance with the European Commission Regulation N° 2073/2005

This paper considers the applications of the European Commission (EC) Regulation N° 2073/2005 concerning Listeria monocytogenes in ready-to-eat (RTE) food products. A simplified modelling approach (SMA), which is intended for a practical evaluation of the behaviour of the pathogen during processing and storage of an RTE meat product (Pitina) from traditional producers, was presented. This approach included a growth/no-growth model, which was developed by using the limits adopted as safety criteria by the EU, a model based on the gamma concept (GM) and a kinetic three-phase model (TPL). Based on the findings of the present study, Pitina was incapable of supporting the growth of L. monocytogenes and the no-growth conditions assessed through the shelf life by the model were confirmed by challenge testing. When the simplified approach was used to estimate the total pathogen growth relative to the Pitina multistage process, taking into account the effects of various hurdles, it provided predictions of L. monocytogenes growth corresponding to the observed data in the inoculation studies. Therefore, this simplified approach is expected to enable the food producers to identify appropriate processing conditions. The successfully validated SMA was found sufficiently complex to predict growth responses of L. monocytogenes in RTE foods, but at the same time easy-to-use for practical processing situations.

The Effects of Sodium Chloride on the Physiological Characteristics of Listeria monocytogenes

Sodium chloride is used to improve various properties of processed meat products, e.g., taste, preservation, water binding capacity, texture, meat batter viscosity, safety, and flavor; however, many studies have shown that sodium chloride increases the resistance of many foodborne pathogens to heat and acid. Listeria monocytogenes has been isolated from various readyto- eat (RTE) meat and dairy products formulated with sodium chloride; therefore, the objective of this paper was to review the effects of sodium chloride on the physiological characteristics of L. monocytogenes. The exposure of L. monocytogenes to sodium chloride may increase biofilm formation on foods or food contact surfaces, virulence gene transcription, invasion of Caco-2 cells, and bacteriocin production, depending on L. monocytogenes strain and serotype as well as sodium chloride concentration. When L. monocytogenes cells were exposed to sodium chloride, their resistance to UV-C irradiation and freezing temperatures increased, but sodium chloride had no effect on their resistance to gamma irradiation. The morphological properties of L. monocytogenes, especially cell elongation and filament formation, also change in response to sodium chloride. These findings indicate that sodium chloride affects various physiological responses of L. monocytogenes and thus, the effect of sodium chloride on L. monocytogenes in RTE meat and dairy products needs to be considered with respect to food safety. Moreover, further studies of microbial risk assessment should be conducted to suggest an appropriate sodium chloride concentration in animal origin foods.

Efficacy of Fir and Qysoom essential oils, alone and in combination, in controlling Listeria monocytogenes in vitro and in RTE meat products model

Listeria monocytogenes, frequently associated with ready-to-eat meat products (RTE-MP), is the causal agent of listeriosis, the virulent foodborne disease. Accordingly, this work aimed to study the effectiveness of essential oils (EOs) of different plants to control growth of L. monocytogenes in RTE-MP model. EOs antilisteric activities were screened by disk diffusion method. Then, efficacy of EOs (1% v/w) with strong inhibition activities were further examined in meat luncheon model, against 2 levels of L. monocytogenes strains cocktail (3 and 6 log CFU/g) coupled with storage at 4 °C for 14 days. The EOs of Fir and Qysoom showed to have the highest significant (p < 0.05) antilisteric activity. In the food model, L. monocytogenes populations in control samples increased by 4 log cycles after 14 days of storage at 4 °C. At the end of storage, for samples with low contamination; Fir, Qysoom, and EOs mixture had approximately 6.37, 6.04, and 5.53 log CFU/g of L. monocytogenes respectively, compared to 6.90 log of control. Whereas in the samples with high contamination level, populations reached to 8.43, 8.88 and 6.75 log CFU/g for Fir, Qysoom, and EOs mixture respectively, compared to 9.90 log of the control. The application of 1% EOs (v/w) to RTE-MP surfaces significantly showed to reduce (p < 0.05) the L. monocytogenes populations growth rate as compared to control in the 2 levels treatments after 14 days of storage at 4 °C. Accordingly, our results suggest that these EOs could be used as natural bio-preservatives in many food products produced in Jordan and worldwide, particularly in RTE-MP.

Evaluation of a novel Listeria enrichment broth combined with a real‐time PCR diagnostics assay for the specific detection of Listeria monocytogenes in RTE pork products

Foodborne outbreaks of listeriosis, human infection with Listeria monocytogenes, are relatively uncommon with approximately 0.1–1.1 cases /100 000 population globally, but high mortality rates of 20–30% are associated with infection (Siegman-Igra et al., 2002; Anonymous, 2012). Those at particular risk of infection include the immunocomprised, the elderly, neonates and pregnant women (Anonymous, 2011; Rocourt et al., 2003). Since the 1990s, over 40% of foodborne listeriosis outbreaks have been associated with ready to eat (RTE) meat products (Warriner & Namvar, 2009). The most significant outbreak of recent times took place in Canada in 2008, with a total of 57 listeriosis cases associated with RTE deli meats which resulted in 23 deaths (Anonymous, 2010; Gilmour et al., 2010). Other significant outbreaks were related with meat frankfurters [USA, 108 cases and 14 deaths in 1998–99] (Anonymous, 1998, 1999; Mead et al., 2006); pât e [Australia, 11 cases and 6 deaths in 1990] (Kittson, 1992; Watson & Ott, 1990); turkey deli meats [USA, 54 cases and 8 deaths in 2002] (Anonymous, 2002; Gottlieb et al., 2006); ‘Quargel’, a brand of acid ripened curd cheese [Austria, Germany and Czech Republic, 34 cases and 8 fatalities in 2009/2010] (Fretz et al., 2010a,b). Listeriosis outbreaks such as these highlight the importance of surveillance in RTE foods. In spite of high salt content and low water activity (Aw), cured and ripened RTE meat products such as those examined in this study can also harbour L. monocytogenes. The organism is capable of growing in 10% NaCl with a water activity (Aw) of 0.90 which is typical of production conditions (AFSSA, 2000; Ryser & Donnelly, 2001). Traditional methods for the specific detection of L. monocytogenes in food are both time consuming and laborious. Recently, a new culture-based method for the identification of L. monocytogenes from foods has been developed, namely the Listeria Precis method. This method uses a single enrichment in Oxoid Novel Enrichment Broth Listeria (ONE Broth-Listeria) followed by selective plating on chromogenic agar [ALOA (Agar Listeria selon Ottaviani & Agosti) One Day] and can provide presumptive results 48 h earlier than ISO 11290-1 (Anonymous, 2004a), a current standard method (Oxoid, 2010). This initial study was performed to determine the feasibility of reducing turnaround time of the Listeria Precis method (considered the reference method used in this study) by incorporating a rapid molecular test after culture enrichment in the ONE Broth-Listeria. A previously described qPCR assay for the specific detection of L. monocytogenes in food (O’Grady et al., 2008, 2009) was combined with the ONE Broth-Listeria culture enrichment step (24 h) of the Listeria Precis method (considered the alternative method used in this study). *Correspondent: Fax: +35391494598; e-mail: thomas.barry@nuigalway.ie Present address: Centre for Clinical Microbiology, University College London (Royal Free Hospital campus), London NW3 2PF

Use of active packaging structures to control the microbial quality of a ready-to-eat meat product

Antimicrobial packaging appears to be a promising application of active food packaging technology. This project was undertaken to evaluate the effectiveness of selected antimicrobial packaging structures in improving the microbial quality of a ready-to-eat (RTE) meat product. Cooked ham samples (25 g) inoculated with a five-strain mixture of Listeria monocytogenes (2 or 4 log CFU g−1) were packaged into three different antimicrobial packaging structures, respectively, and samples in the non-antimicrobial packaging structure served as controls. The samples were stored at 4, 10, or 22 °C, and populations of total aerobic bacteria, Enterobacteriaceae, and inoculated L. monocytogenes were determined twice a week over a 4 w storage period. Packaging structures with an O2 scavenger or a CO2 generator were more effective than the structure with an allyl isothiocyanate (AIT) generator. Listeria populations in the packaging structure with the O2 scavenger were 1.80–2.65, 3.69–4.76, and 4.62–4.67 log CFU g−1 lower than the controls at 4, 10, or 22 °C. Samples in the packaging structure with the CO2 generator were 1.11–1.63, 4.30–4.45, and 4.01–4.44 log CFU g−1 lower than the controls at 4, 10, or 22 °C. Significant inhibitions on total aerobic bacteria and Enterobacteriaceae counts were also observed in packaging structures with the CO2 generator and O2 scavenger. However, packaging structures with the AIT generator only significantly inhibited L. monocytogenes, total aerobic bacteria, and Enterobacteriaceae in some samples, mainly those stored at 22 °C. These results suggest that some of the evaluated packaging structures can effectively control bacterial populations, particularly Listeria populations on RTE meat products.

IMPROVING FOOD THERMAL PROCESSING: A DEATH-TIME STUDY ON PROCESSED MEAT PRODUCTS

ABSTRACT Ready-to-eat (RTE) meat products, consumed directly or after warming, are a source for the transmission of Listeria monocytogenes. With the growing demand and the increasing variety of RTE meat products, the food industries face serious challenges with regard to their safety. The appropriate application of heat treatments becomes an essential issue for food processors. However, quantitative data on thermal inactivation of pathogenic and spoilage microorganisms in different meat products cannot be extrapolated directly because of the particularities of the formulations, the variability of heat resistance among bacterial strains, and the heating procedures. Therefore, the heat resistance parameters (D and z values) of L. monocytogenes, Lactobacillus delbrueckii and accompanying microflora in meat slurry were established in order to provide more accurate F values and improve operation conditions. Results showed overprocessing. L. delbrueckii had the largest z value (9.3C) and adjusted F values of 2.4 and 3.8 min would render hot dogs and cured coarse sausages safe. PRACTICAL APPLICATIONS Thermal processing is still the dominant technology to assure food safety. However, consumer demands for more convenient, easy-to-prepare food with whole nutritional content drive to reevaluate and improve commercial processes. In this study, the heat resistance of target microorganisms in a meat product was calculated and modeling was used to assess and document process lethality. This study may assist the meat industry in the quantification of bacterial lethalities and the proper design of thermal processes.

Transcriptome analysis of Listeria monocytogenes grown on a ready-to-eat meat matrix.

The contamination of ready-to-eat (RTE) meat products with Listeria monocytogenes is a major concern for the food industry. For a better understanding of the adaptation and survival ability of L. monocytogenes grown on turkey deli meat, the transcriptome of L. monocytogenes strain F2365 was determined with a microarray. Microarray data were validated with a quantitative real-time reverse transcription PCR assay. Based on the microarray data, 39 and 45 genes from L. monocytogenes were transcriptionally upregulated and down-regulated, respectively. The genes regulated at the transcriptional level were mainly involved in energy metabolism, fatty acid and phospholipid metabolism, biosynthesis of proteins, transport and binding proteins, DNA metabolism, cellular processes, and regulatory functions. No significant change was noted for the expression of genes encoding known virulence factors such as sigB, prfA, inlA, inlB, plcA, plcB, and hly. These results suggest that L. monocytogenes grown on RTE deli meat changes its transcription of proteins involved in its metabolic pathways to obtain an energy source or to adapt to environmental change without increasing the expression of virulence factors. The global transcriptome profiles provide a better understanding of the growth or adaptation of L. monocytogenes in RTE meat products.

Development of PCR assays for detection of Escherichia coli O157:H7 in meat products

A multiplex polymerase chain reaction (PCR) procedure based on fliC h7 and rfbE genes was developed for the detection of Escherichia coli O157:H7 in raw pork meat and ready-to-eat (RTE) meat products. Two different DNA extraction procedures were evaluated for application on meat products. MasterPure™ DNA Purification kit in combination with immunomagnetic separation was found to be the best method in a meat system. The optimized PCR included an enrichment step in brilliant green bile 2% broth at 37 °C. This method was applied to artificially inoculated meat and RTE meat products with different concentrations of E. coli O157:H7. The results indicate that the PCR assay developed could sensitively and specifically detect E. coli O157:H7 in raw pork meat and RTE meat products in approximately 10 h, including a 6 h enrichment step. Thus, this method could be proposed for screening E. coli O157:H7 in raw pork and RTE meat products.

Heterocyclic amine content in commercial ready to eat meat products

Heterocyclic amines (HCAs) are produced in meats cooked at high temperature, which are potent mutagens and a risk factor for human cancers. The aim of this study was to estimate the amount of HCAs in some commonly consumed ready-to-eat (RTE) meat products. The RTE products were purchased from a local grocery store, and HCA were analyzed using an analytical method based on solid-phase extraction followed by HPLC. The primary HCAs in these samples were PhIP (2-amino-1-methyl-6-phenylimidazo [4,5- b]pyridine) (not detected—7.9 ng/g) and MeIQx (2-amino-3,8-dimethylimidazo [4,5- f]quinoxaline) (not detected—3.6 ng/g). Products ranked in order of increasing total HCA content: pepperoni (0.05 ng/g) < hot dogs and deli meat products (0.5 ng/g) < fully cooked bacon (1.1 ng/g) < rotisserie chicken meat (1.9 ng/g) < rotisserie chicken skin (16.3 ng/g). We believed that cooking conditions and ingredients influenced the level of HCAs in these RTE products and concluded that consumption of RTE meat products contributes very little to HCA intake. Results from this study can be used in risk assessment study to estimate human exposure to HCAs due to food consumption.

Irradiation is useful for manufacturing ready-to-eat cooked meat products enriched with folic acid

Cooked sausages enriched with folic acid (0.6, 1.2 and 2.4 mg/100 g) were manufactured as ready-to-eat (RTE) products using E-beam radiation (2–4 kGy) as a non-thermal technology. The effects of this treatment on the folic acid content, colour, texture and sensory properties of the final products were studied. The characteristics of sausages were not affected by the presence of folic acid, independently of the amount added, and their overall acceptability was good. Doses of 4 kGy caused losses of folic acid close to 20–30% and significantly decreased the sensory quality ( P < 0.05). Despite this, the final content of folic acid in all products was sufficient so that 50 g of product gave 100% of the recommended daily allowance (RDA). This new RTE meat product can be considered as a source of folic acid that can help assure adequate levels of this vitamin in the general population.

Molecular and phenotypic characterization of Listeria monocytogenes from U.S. Department of Agriculture Food Safety and Inspection Service surveillance of ready-to-eat foods and processing facilities.

A panel of 501 Listeria monocytogenes isolates obtained from the U.S. Department of Agriculture Food Safety and Inspection Service monitoring programs for ready-to-eat (RTE) foods were subtyped by multilocus genotyping (MLGT) and by sequencing the virulence gene inlA, which codes for internalin. MLGT analyses confirmed that clonal lineages associated with previous epidemic outbreaks were rare (7.6%) contaminants of RTE meat and poultry products and their production environments. Conversely, sequence analyses revealed mutations leading to 11 different premature stop codons (PMSCs) in inlA, including three novel PMSC mutations, and revealed that the frequency of these virulence-attenuating mutations among RTE isolates (48.5%) was substantially higher than previously appreciated. Significant differences (P < 0.001) in the frequency of inlA PMSCs were observed between lineages and between major serogroups, which could partially explain differences in association of these subtypes with human listeriosis. Interrogation of single-nucleotide polymorphisms responsible for PMSCs in inlA improved strain resolution among isolates with the 10 most common pulsed-field gel electrophoresis (PFGE) patterns, 8 of which included isolates with a PMSC in inlA. The presence or absence of PMSCs in inlA accounted for significant differences (P < 0.05) in Caco-2 invasion efficiencies among isolates with identical PFGE patterns, and the proportion of PulseNet entries from clinical sources was significantly higher (P < 0.001) for PFGE patterns exclusively from isolates with full-length inlA. These results indicated that integration of PFGE and DNA sequence-based subtyping provides an improved framework for prediction of relative risk associated with L. monocytogenes strains from RTE foods.

Predicting Growth–No Growth of Listeria monocytogenes on Vacuum-Packaged Ready-to-Eat Meats

Compliance with U.S. Department of Agriculture (USDA) composition-based labeling standards often has been regarded as evidence of the shelf stability of ready-to-eat (RTE) meats. However, the USDA now requires further proof of shelf stability. Our previous work included development of equations for predicting the probability of Staphylococcus aureus growth based on the pH and a(w) of an RTE product. In the present study, we evaluated the growth-no-growth during 21 degrees C storage of Listeria monocytogenes on 39 vacuum-packaged commercial RTE meat products with a wide range of pH (4.6 to 6.5), a(w) (0.47 to 0.98), and percent water-phase salt (%WPS; 2.9 to 34.0). Pieces of each product were inoculated with a five-strain cocktail of L. monocytogenes and vacuum packaged, and L. monocytogenes levels were determined immediately after inoculation and after storage at 21 degrees C for up to 5 weeks. L. monocytogenes grew on 13 of 14 products labeled "keep refrigerated" but not on any of the 25 products sold as shelf stable. Using bias reduction logistic regression data analysis, the probability of L. monocytogenes growth (Pr) could be predicted as a function of pH and a(w): Pr = exp[-59.58 + (4.67 x pH) + (35.05 x a(w))]/{1 + exp[-59.58 + (4.67 x pH) + (35.05 x a(w))]}. Pr also could be predicted as a function of pH and %WPS: Pr = exp[-20.52 + (4.10 x pH) - (0.51 x %WPS)]/{1 + exp[-20.52 + (4.10 x pH) - (0.51 x %WPS)]}. The equations accurately predicted L. monocytogenes growth (Pr values of 0.68 to 0.99) or no growth (Pr values of <0.01 to 0.26) and with our equations for predicting S. aureus growth will be useful for evaluating RTE meat shelf stability.

Incidence and Contamination Level of Listeria monocytogenes and Other Listeria spp. in Ready-to-Eat Meat Products in Jordan

The objective of the present study was to investigate the incidence and contamination levels of different Listeria monocytogenes serovars in ready-to-eat meat products (RTE-MP) collected from different outlets and processing plants in Jordan in order (i) to provide information to Jordanian health authorities on the incidence of L. monocytogenes in RTE-MP sold and consumed in Jordan and (ii) to ascertain the risks of these products for consumers. Two hundred forty RTE-MP samples, 120 beef and 120 poultry, were analyzed. European International Organization for Standardization (EN ISO) 11290-1 and -2 standard protocols were used for detection and enumeration of L. monocytogenes. The identity of suspected L. monocytogenes was confirmed using PCR. Three Listeria spp., L. monocytogenes, L. innocua, and L. welshimeri, were isolated. L. innocua and L. welshimeri were the most and least frequently isolated with 56 and 36 samples, respectively. L. monocytogenes was isolated from 41 samples (17.1%): 23 from beef and 18 from poultry samples. The contamination levels of L. monocytogenes were <or=100 CFU/g in 97.5% (40 samples) of the positive samples. Only one beef sample with a count of >100 CFU/g was found. The L. monocytogenes strains isolated fell into two serotypes (1 and 4) and four different serovars (1/2a, 1/2b, 1/2c, and 4b).