Reduction and survival of Listeria monocytogenes in ready-to-eat meats after irradiation

A five-strain Listeria monocytogenes culture was inoculated onto six different types of ready-to-eat (RTE) meats (frankfurters, ham, roast beef, bologna, smoked turkey with lactate, and smoked turkey without lactate). The meats were vacuum packed and stored at 4 degrees C for 24 h prior to irradiation. Populations of L. monocytogenes were recovered by surface plating on nonselective and selective media. The margins of safety studied include 3-log (3D) and 5-log (5D) reduction of pathogenic bacteria to achieve an optimal level of reduction while retaining organoleptic qualities of the meats. A 3-log reduction of L. monocytogenes was obtained at 1.5 kGy when nonselective plating medium was used. The dosages for 3-log reduction were 1.5 kGy for bologna, roast beef, and both types of turkey and 2.0 kGy for frankfurters and ham on the basis of use of selective medium. The D10-values ranged from 0.42 to 0.44 kGy. A 5-log reduction of L. monocytogenes was obtained at 2.5 kGy with nonselective medium. With selective medium, the dosages were 2.5 kGy for bologna, roast beef, and both types of turkey and 3.0 kGy for frankfurters and ham. Survival of L. monocytogenes in the same RTE meat types after irradiation was also studied. Meats were inoculated with 5 log L. monocytogenes per g and irradiated at doses of 2.0 and 4.0 kGy. Recovery of the surviving organisms was observed during storage at temperatures of 4 and 10 degrees C for 12 weeks. Preliminary results showed no growth in meats irradiated at 4.0 kGy. Survivors were observed for irradiated meats at 2.0 kGy stored at 10 degrees C after the second week. No growth was observed in samples irradiated at 2.0 kGy stored at 4 degrees C until the fifth week.     

Prepackage surface pasteurization of ready-to-eat meats with a radiant heat oven for reduction of Listeria monocytogenes

In this paper, a thermal process for the surface pasteurization of ready-to-eat (RTE) meat products for the reduction of Listeria monocytogenes on such products (turkey bologna, roast beef, corned beef, and ham) is described. The process involves the passage of products through a "tunnel" of heated coils on a stainless steel conveyor belt at various treatment times relevant to the manufacture of processed meat for the surface pasteurization of RTE meat products. Two inoculation procedures, dip and contact inoculation, were examined with the use of a four-strain cocktail of L. monocytogenes prior to heat processing. With the use of radiant heat prepackage surface pasteurization, 1.25 to 3.5-log reductions of L. monocytogenes were achieved with treatment times of 60 to 120 s and air temperatures of 475 to 750 degrees F (246 to 399 degrees C) for these various RTE meats. Reduction levels differed depending on the type of inoculation method used, the type of product used, the treatment temperature, and the treatment time. Prepackage pasteurization (60 s) was also combined with postpackage submerged water pasteurization for formed ham (60 or 90 s), turkey bologna (45 or 60 s), and roast beef (60 or 90 s), resulting in reductions of 3.2 to 3.9. 2.7 to 4.3, and 2.0 to 3.75 log cycles, respectively. These findings demonstrate that prepackage pasteurization, either alone or in combination with postpackage pasteurization, is an effective tool for controlling L. monocytogenes surface contamination that may result from in-house handling.     

Survival and recovery of Listeria monocytogenes on ready-to-eat meats inoculated with a desiccated and nutritionally depleted dustlike vector

Dust from construction was theorized to serve as a vector for L. monocytogenes transmission to ready-to-eat (RTE) meats after heat processing but before packaging. A five-strain Listeria monocytogenes culture including serotype 4b was continually stressed on a sand vector under four sets of nutritionally depleted and dry conditions to simulate postprocessing contamination by dustlike particulates. The stresses included that associated with sand stored at different temperatures (10 and 22 degrees C) and levels of humidity (40% relative humidity [RH], 88% RH, or complete desiccation). Irradiated RTE meats, including frankfurters, bologna, chopped ham, and deli-style roast beef, were inoculated with the L. monocytogenes-contaminated sand every 2 to 3 days over a period of 1 1/2 months. After inoculation, the RTE meats were vacuum packed and stored at 4 degrees C for 24 h. Populations of L. monocytogenes were enumerated by surface plating on nonselective and selective media to recover cells on the basis of the different stresses presented (osmotic or antibiotic). L. monocytogenes was demonstrated to be capable of surviving on the sand vector for > 151 days at 10 degrees C and 88% RH, 136 days at 10 degrees C and 0% RH, 73 days at 22 degrees C and 40% RH, and 82 days at 22 degrees C and 0% RH. These results show that under the most conservative scenario, the 73-day-old L. monocytogenes-contaminated sand was able to attach to and be recovered from the RTE meats. This study illustrated that dust contaminated with L. monocytogenes, once in contact with meat surfaces, can survive and grow, posing a health hazard to consumers.     

Gamma irradiation of fine-emulsion sausage containing sodium diacetate

Listeria monocytogenes, a psychrotrophic foodborne pathogen, is a frequent postprocess contaminant of ready-to-eat (RTE) meat products, including frankfurters and bologna. Ionizing radiation can eliminate L. monocytogenes from RTE meats. Sodium diacetate (SDA) incorporated into fine-emulsion sausages inhibits the growth of L. monocytogenes. Irradiation of L. monocytogenes suspended in SDA solutions resulted in synergistic reductions of the microorganism. L. monocytogenes populations were reduced by > 9 log10 units at a radiation dose of 1.5 kGy when suspended in 0.125% SDA solution. In contrast, the D10-values (the ionizing radiation doses required to reduce the population by 90%) were 0.58, 0.59, 0.57, and 0.53 kGy for L. monocytogenes populations suspended in emulsions containing 0, 0.125, 0.25, and 0.5% SDA, respectively. The D10-values for L. monocytogenes surface inoculated onto frankfurters dipped in 0, 0.125, 0.25, and 0.5% SDA solutions were 0.58, 0.53, 0.54, and 0.52 kGy, respectively. Postirradiation growth of L. monocytogenes suspended in beef bologna emulsion at 9 degrees C was dependent on SDA concentration and ionizing radiation dose. Very small, but statistically significant, changes in bologna redness, lipid oxidation, and shear force were observed for the beef bologna emulsion with the highest SDA concentration (0.5%) and irradiation dose (3.0 kGy). SDA can inhibit the proliferation of L. monocytogenes surviving the irradiation process with minimal impact on fine-emulsion sausage color, lipid oxidation, and firmness when used within regulatory limits.     

Postpackage pasteurization of ready-to-eat deli meats by submersion heating for reduction of Listeria monocytogenes

A mixed cocktail of four strains of Listeria monocytogenes was resuspended in product purge and added to a variety of ready-to-eat (RTE) meat products, including turkey, ham, and roast beef. All products were vacuum sealed in shrink-wrap packaging bags, massaged to ensure inoculum distribution, and processed by submersion heating in a precision-controlled steam-injected water bath. Products were run in pairs at various time-temperature combinations in either duplicate or triplicate replications. On various L. monocytogenes-inoculated RTE deli meats, we were able to achieve 2- to 4-log cycle reductions when processed at 195 degrees F (90.6 degrees C), 200 degrees F (93.3 degrees C), or 205 degrees F (96.1 degrees C) when heated from 2 to 10 min. High-level inoculation with L. monocytogenes (approximately 10(7) CFU/ml) ensured that cells infiltrated the least processed surface areas, such as surface cuts, folds, grooves, and skin. D- and z-value determinations were made for the Listeria cocktail resuspended in product purge of each of the three meat categories. However, reduction of L. monocytogenes in product challenge studies showed much less reduction than was observed during the decimal reduction assays and was attributed to a combination of surface phenomena, including surface imperfections, that may shield bacteria from the heat and the migration of chilled purge to the product surface. The current data indicate that minimal heating regimens of 2 min at 195 to 205 degrees F can readily provide 2-log reductions in most RTE deli meats we processed and suggest that this process may be an effective microbial intervention against L. monocytogenes on RTE deli-style meats.     

Radiation (gamma) resistance and postirradiation growth of Listeria monocytogenes suspended in beef bologna containing sodium diacetate and potassium lactate

Listeria monocytogenes, a psychrotrophic foodborne pathogen, is a frequent postprocessing contaminant of ready-to-eat (RTE) meat products, including frankfurters and bologna. Ionizing radiation can eliminate L. monocytogenes from RTE meats. When they are incorporated into fine-emulsion sausages, sodium diacetate (SDA) and potassium lactate (PL) mixtures inhibit the growth of L. monocytogenes. The radiation resistance of L. monocytogenes, and its ability to proliferate during long-term refrigerated storage (9 degrees C), when inoculated into beef bologna that contained 0% SDA-0% PL, 0.07% SDA-1% PL, and 0.15% SDA-2% PL, were determined. The radiation doses required to eliminate 90% of the viable L. monocytogenes cells were 0.56 kGy for bologna containing 0% SDA-0% PL, 0.53 kGy for bologna containing 0.07% SDA-1% PL, and 0.46 kGy for bologna containing 0.15% SDA-2% PL. L. monocytogenes was able to proliferate on bologna containing 0% SDA-0% PL during refrigerated storage, but the onset of proliferation was delayed by the addition of the SDA-PL mixtures. An ionizing radiation dose of 3.0 kGy prevented the proliferation of L. monocytogenes and background microflora in bologna containing 0.07% SDA-1% PL and in bologna containing 0.15% SDA-2% PL over 8 weeks of storage at 9 degrees C. Little effect on lipid oxidation and color of the control bologna, or bologna containing SDA-PL mixtures, was observed upon irradiation at either 1.5 or 3.0 kGy.     

Competitive inhibition of Listeria monocytogenes in ready-to-eat meat products by lactic acid bacteria

Forty-nine strains of lactic acid bacteria (LAB), isolated from commercially available ready-to-eat (RTE) meat products, were screened for their ability to inhibit the growth of Listeria monocytogenes at refrigeration (5 degrees C) temperatures on agar spot tests. The three most inhibitory strains were identified as Pediococcus acidilactici, Lactobacillus casei, and Lactobacillus paracasei by 16S rDNA sequence analysis. Their antilisterial activity was quantified in associative cultures in deMan Rogosa Sharpe (MRS) broth at 5 degrees C for 28 days, resulting in a pathogen reduction of 3.5 log10 cycles compared to its initial level. A combined culture of these strains was added to frankfurters and cooked ham coinoculated with L. monocytogenes, vacuum packaged, and stored at 5 degrees C for 28 days. Bacteriostatic activity was observed in cooked ham, whereas bactericidal activity was observed in frankfurters. Numbers of L. monocytogenes were 4.2 to 4.7 log10 and 2.6 log10 cycles lower than controls in frankfurters and cooked ham, respectively, after the 28-day refrigerated storage. In all cases, numbers of LAB increased by only 1 log10 cycle. The strain identified as P. acidilactici was possibly a bacteriocin producer, whereas the antilisterial activity of the other two strains was due to the production of organic acids. There was no significant difference (P > 0.05) in the antilisterial activity detected in frankfurters whether the LAB strains were used individually or as combined cultures. Further studies over a 56-day period indicated no impact on the quality of the product. This method represents a potential antilisterial intervention in RTE meats, because it inhibited the growth of the pathogen at refrigeration temperatures without causing sensory changes.     

Application of electrolyzed oxidizing water to reduce Listeria monocytogenes on ready-to-eat meats

Experiments were conducted to determine the effectiveness of acidic (EOA) or basic electrolyzed oxidizing (EOB) water, alone or in combination, on ready-to-eat (RTE) meats to reduce Listeria monocytogenes (LM). Frankfurters or ham surfaces were experimentally inoculated with LM and subjected to dipping or spraying treatments (25 or 4°C for up to 30 min) with EOA, EOB, and other food grade compounds. LM was reduced the greatest when frankfurters were treated with EOA and dipped at 25°C for 15 min. A combination spray application of EOB/EOA also resulted in a slight reduction of LM on frankfurters and ham. However, reductions greater than 1log CFU/g were not observed for the duration of the study. Even with a prolonged contact time, treatments with EOA or EOB were not enough to meet regulatory requirements for control of LM on RTE meats. As such, additional studies to identify food grade antimicrobials to control the pathogen on RTE meats are warranted.     

Radiation Resistance of Listeria monocytogenes in the Presence or Absence of Sodium Erythorbate

ABSTRACT: Listeria monocytogenes is a common contaminant of ready-to-eat meat products that can be eliminated by low dose ionizing radiation. Sodium erythorbate (SE), an antioxidant, is commonly included in cured meat emulsions or applied to the surfaces of cured meats as a solution prior to packaging. The radiation resistance (D_γ) of L. monocytogenes increased when suspended in SE solutions of 0.1% and greater. However, no differences in D_γ, which ranged from 0.67 kGy to 0.70 kGy, were observed when L. monocytogenes was inoculated onto cooked cured meat products (frankfurters or bologna slices) which contained no SE, 0.05% SE in the emulsion, or a 10% SE solution applied to the product surface. Surface antioxidant power of the cured meats did not reach a level sufficient to protect L. monocytogenes against the lethal effects of ionizing radiation. Therefore, the industrial practice of applying SE to the surfaces of cured meat products would not compromise the efficacy of irradiation as an antimicrobial process.     

Determination of Ascorbic Acid, Erythorbic Acid, and Uric Acid in Cured Meats by High Performance Liquid Chromatography

A paired-ion reversed-phase high-performance liquid chromatographic procedure with amperometric detection was applied to the determination of ascorbic acid (AA), erythorbic acid (EA), and uric acid (UA) in 5% meta-phosphoric acid extracts of beef bologna, beef frankfurters, bacon, and ham cured with AA or EA. AA concentrations (mg/ g meat) ranged from 0.008 in EA-cured beef frankfurters to 0.584 in AA-cured bacon. EA concentrations ranged from 0.149 in EA-cured beef frankfurters to 0.545 in EA-cured bacon. UA was quantified only in EA-cured bologna and frankfurters with 0.013 and 0.010 mg/g, respectively. Correlations of total ascorbate (AA + EA) by HPLC with fluorometric and calorimetric procedures were 0.992 and 0.988, respectively.     

Growth of Listeria monocytogenes in different retail delicatessen meats during simulated home storage

Delicatessen meats are reported to be the leading vehicle of foodborne listeriosis in the United States. Listeria monocytogenes can reach high numbers in these products during storage, and the growth rate is largely dictated by product formulation and storage temperature. To assess the impact of product age on Listeria growth, five commercial brands each of cured and uncured turkey breast, ham, and roast beef (three lots per brand) were sliced (approximately 25 g per slice) at the beginning of the shelf life, the midpoint, and the last allowable day of sale, surface inoculated with an eight-strain cocktail of L. monocytogenes (approximately 40 CFU/g), and then quantitatively examined for Listeria, lactic acid bacteria, and mesophilic aerobic bacteria during aerobic storage at 4, 7, or 10°C. As expected, L. monocytogenes grew faster in deli meats without rather than with Listeria inhibitors (lactate and/or diacetate) and at the highest storage temperature (10°C). Lag-phase durations for L. monocytogenes in deli meats with and without Listeria inhibitors were 9.21, 6.96, and 5.00 and 6.35, 3.30, and 2.19 days at 4, 7, and 10°C, respectively. Generation times for L. monocytogenes in deli meats with and without Listeria inhibitors were 1.59, 1.53, and 0.85 and 0.94, 0.50, and 0.36 at 4, 7, and 10°C, respectively. Maximum population densities for L. monocytogenes in deli meats with and without Listeria inhibitors were 5.26, 5.92, and 5.97 and 8.47, 8.96 and 9.34 log CFU/g at 4, 7, and 10°C, respectively. Although lactate and diacetate suppressed L. monocytogenes growth, the extent of inhibition differed, ranging from total inhibition in roast beef to only partial inhibition in ham and cured turkey. Listeria growth was also impacted by lot-to-lot variation in the concentrations of Listeria inhibitors, product pH, and background microflora. These data will be useful for developing recommendations for "best consumed by" dating for deli meats using a risk-based approach.     

Inhibition of Listeria monocytogenes in turkey and pork-beef bologna by combinations of sorbate, benzoate, and propionate

The control of Listeria monocytogenes was evaluated with ready-to-eat uncured turkey and cured pork-beef bologna with combinations of benzoate, propionate, and sorbate. Three treatments of each product type were formulated to include control with no antimycotic agents; a combination of 0.05% sodium benzoate and 0.05% sodium propionate; and a combination of 0.05% sodium benzoate and 0.05% potassium sorbate. Ingredients were mixed, stuffed into fibrous, moisture-impermeable casings, cooked to an internal temperature of 73.9 degrees C, chilled, and sliced. The final product was surface inoculated with L. monocytogenes (4 log CFU per package), vacuum packaged, and stored at 4 degrees C for 13 weeks. The antimycotic addition to the second and third uncured turkey treatments initially slowed the pathogen growth rate compared with the control, but populations of L. monocytogenes increased 5 log or more by 6 weeks. In contrast, the addition of antimycotic combinations in the cured bologna prevented growth of L. monocytogenes during the 13-week storage period at 4 degrees C, compared with a more than 3.5-log increase in listerial populations in the control bologna, to which no antimicrobial agents had been added. These data suggest that low concentrations of antimycotic agents can prevent L. monocytogenes growth in certain ready-to-eat meats. Additional research is needed to define the levels needed to prevent growth of L. monocytogenes in high-moisture cured and uncured ready-to-eat meat and poultry and for gaining governmental approval for their use in such formulations.     

Combining organic acid treatment with steam pasteurization to eliminate Listeria monocytogenes on fully cooked frankfurters

An organic acid solution of 2% acetic, 1% lactic, 0.1% propionic, and 0.1% benzoic acids was combined with steam surface pasteurization to treat frankfurters during vacuum packaging to eliminate potential postcook contamination with Listeria monocytogenes. The thermal lethality of L. monocytogenes from steam was evaluated at an inoculation concentration of 1 to 6 log CFU/cm2. About 3-log reductions of L. monocytogenes were achieved when frankfurters were treated by steam for 1.5 s. Combining organic acid treatment with steam pasteurization further inhibited the growth of surviving L. monocytogenes cells for 19 and 14 weeks when the packaged frankfurters were stored at 4 and 7 degrees C, respectively. The results from this study provide meat processors with useful information for controlling L. monocytogenes on ready-to-eat meats.     

Antioxidant power,lipid oxidation,color, and viability of Listeria monocytogenes in beef bologna treated with gamma radiation and containing various levels of glucose

Ionizing radiation can be used to pasteurize ready-to-eat (RTE) meat products. Thermal processing of RTE meats that contain dextrose results in the production of antioxidants that may interfere with ionizing radiation pasteurization of RTE meat products. Beef bologna was manufactured with dextrose concentrations of 0, 2, 4, 6, and 8%. Antioxidant activity, as measured by the Ferric Reducing Antioxidant Power assay, increased with dextrose concentration but was unaffected by ionizing radiation. Lipid oxidation increased significantly in irradiated bologna (4 kGy) that contained dextrose. Hunter color analysis indicated that the addition of dextrose reduced the ionizing radiation-induced loss of redness (a-value) but promoted the loss of brightness (L-value). The radiation resistance, D10-value, of Listeria monocytogenes that was surface-inoculated onto bologna slices was not affected by dextrose concentration. L. monocytogenes strains isolated from RTE meats after listeriosis outbreaks were utilized. Increased antioxidant activity generated by thermal processing of dextrose in fine emulsion sausages does not present a barrier to radiation pasteurization of RTE meats. However, a high dextrose concentration in combination with gamma irradiation increases lipid oxidation significantly.     

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.     

COMPUTER‐CONTROLLED MICROWAVE HEATING TO IN‐PACKAGE PASTEURIZE BEEF FRANKFURTERS FOR ELIMINATION OF LISTERIA MONOCYTOGENES†

The objective of this study was to develop an in‐package pasteurization technology to kill Listeria monocytogenes in ready‐to‐eat meats using microwave heating. This technology utilized an infrared sensor to monitor the surface temperature of beef frankfurters during microwave heating. The aim was to increase the surface temperature of frankfurters to a set point lethal to L. monocytogenes. A feedback control mechanism was used to control the power to the microwave oven. Results indicated that the simple on‐off control mechanism was able to maintain the surface temperature of beef frankfurters near the respective set points of 75, 80 or 85C used in this study. This pasteurization process was able to achieve a 7‐log reduction of L. monocytogenes in inoculated beef frankfurters using a 600‐W nominally rated microwave oven within 12–15 min. If optimized, this system may provide the food industry with a terminal, postlethality pasteurization technology to kill L. monocytogenes in ready‐to‐eat meats within the final packages.     

Edible chitosan films on ready-to-eat roast beef for the control of Listeria monocytogenes

The use of chitosan as an edible film was evaluated for its antimicrobial activity against Listeria monocytogenes (LM) on the surface of ready-to-eat (RTE) roast beef. L. monocytogenes, decimally diluted to give an initial inoculation of >6.50logCFU/g, was inoculated onto the surface of RTE roast beef cubes, and air-dried. The samples were dipped into chitosan (high or low molecular weights) solutions dissolved with acetic or lactic acid at 0.5% (w/v) or 1% (w/v) then bagged and refrigerated at 4 degrees C. The bacterial counts were determined on days 0, 7, 14, 21, and 28. The samples were spread plated onto modified Oxford agar plates and incubated at 37 degrees C for 48h. An initial 6.50logCFU/g of L. monocytogenes inoculated onto the surface of the non-coated RTE roast beef increased too >10logCFU/g by day 28. On day 14, L. monocytogenes counts were significantly different for all the chitosan-coated samples from the control counts by 2-3logCFU/g and remained significantly different on day 28. Our results have shown that the acetic acid chitosan coating were more effective in reducing L. monocytogenes counts than the lactic acid chitosan coating. Our study indicated that chitosan coatings could be used to control L. monocytogenes on the surface of RTE roast beef.     

FoodNet survey of food use and practices in long-term care facilities

Foodborne illness is an important problem among the elderly. One risk factor for foodborne illness and diarrhea-associated mortality among the elderly is residence in a long-term care facility (LTCF); thus, these facilities must implement measures to ensure safe food. To assess safe food practices, knowledge, and policies, we used a mailed, self-administered questionnaire to survey food service directors at LTCFs that were certified to receive Medicare or Medicaid at eight Foodborne Diseases Active Surveillance Network (FoodNet) sites. Surveys were distributed to 1,630 LTCFs; 55% (865 of 1,568) of eligible facilities returned a completed questionnaire. Only three LTCFs completely followed national recommendations for prevention of Listeria monocytogenes contamination. Nine percent of LTCFs reported serving soft cheeses made from unpasteurized milk. Most LTCFs reported routinely serving ready-to-eat deli meats; however, few reported always heating deli meats until steaming hot before serving (only 19% of the LTCFs that served roast beef, 13% of those that served turkey, and 11% of those that served ham). Most LTCFs (92%) used pasteurized liquid egg products, but only 36% used pasteurized whole shell eggs. Regular whole shell eggs were used by 62% of facilities. Few LTCFs used irradiated ground beef (7%) or irradiated poultry products (6%). The results of this survey allowed us to identify several opportunities for prevention of foodborne illnesses in LTCFs. Some safety measures, such as the use of pasteurized and irradiated foods, were underutilized, and many facilities were not adhering to national recommendations on the avoidance of certain foods considered high risk for elderly persons. Enhanced educational efforts focusing on food safety practices and aimed at LTCFs are needed.     

Application of a multiplex PCR for the simultaneous detection of Escherichia coli O157:H7, Salmonella and Shigella in raw and ready-to-eat meat products

Escherichia coli O157:H7, Salmonella and Shigella might contaminate similar types of meat products and cause deadly diseases in humans. Traditional microbiological analyses to detect these pathogens are labor-intensive and time-consuming. The objective of this study was to apply a multiplex PCR for simultaneous detection of the pathogenic bacteria in certain raw and ready-to-eat meat matrices. The tested samples had aerobic plate counts ranging from non-detectable, in chicken nuggets and salami, to 8.36log(10)CFU/g in ground pork. The pH of homogenates spanned from 6.86, in ground beef, to 7.17 in salami. Following a 24-h enrichment, the multiplex PCR assay could concurrently detect the three pathogens at 0.2log(10)CFU/g in ground beef, roast beef, beef frankfurters, chicken nuggets, salami and turkey ham, and 1.2log(10)CFU/g in ground pork. This multiplex PCR offers an efficient microbiological tool for presumptive detection of E. coli O157:H7, Salmonella and Shigella in meat.     

Acidified sodium chlorite treatment for inhibition of Listeria monocytogenes growth on the surface of cooked roast beef

The effects of acidified sodium chlorite (ASC) against Listeria monocytogenes on the surface of cooked roast beef were investigated. L. monocytogenes, strain V7, serotype 1/2a, was inoculated at numbers of 6.0 log CFU/g onto 5-g cubes of cooked regular or spicy roast beef. The samples were allowed to air dry for 1 h. The cooked roast beef samples were dipped into ASC or sprayed with ASC solutions of 250, 500, 750, or 1,000 ppm, then placed in bags with or without a vacuum and refrigerated at 4 degrees C. L. monocytogenes counts were determined after 0, 7, 14, 21, and 28 days of storage by spread plating roast beef samples onto Oxford agar plates that were incubated at 37 degrees C for 48 h. At day 28, the number of L. monocytogenes on the > or = 500 ppm ASC-treated spicy roast beef samples had count reductions that were >4.0 log CFU/g, whereas the same concentrations of ASC-treated regular roast beef samples had approximately a 2.5 log CFU/g reduction in L. monocytogenes counts when compared with the untreated samples. No significant differences (P > 0.05) were observed in L. monocytogenes counts between the vacuum- or nonvacuum-packaged ASC-treated cooked roast beef samples. Sensory evaluation showed no significant differences (P > 0.05) between ASC-treated and untreated roast beef. ASC can be used as a processing aid in the form of a dip or spray treatment to control L. monocytogenes on the surface of cooked roast beef.