Occurrence and characterization of Listeria spp. in ready-to-eat retail foods from Vancouver, British Columbia

The occurrence of Listeria spp. and Listeria monocytogenes in retail RTE meat and fish products in Vancouver, British Columbia (B.C.) was investigated. To assess potential consumer health risk, recovered L. monocytogenes isolates were subjected to genotypic and phenotypic characterization. Conventional methods were used to recover Listeria spp. from deli meat (n = 40) and fish (n = 40) samples collected from 17 stores. Listeria spp. were recovered only from fish samples (20%); 5% harboured Listeria innocua, 5% had L. monocytogenes and 10% contained Listeria welshimeri. L. monocytogenes isolates serotyped as 1/2a and 1/2b, possessed dissimilar PFGE patterns, and had full-length InlA. Three 1/2a clonal isolates encoded the 50 kb genomic island, LGI1. Antimicrobial resistance (AMR) profiling showed all Listeria spp. possessed resistance to cefoxitin and nalidixic acid. L. monocytogenes were resistant to clindamycin, two were resistant to streptomycin, and one to amikacin. Reduced susceptibility to ciprofloxacin was seen in all L. monocytogenes, L. innocua and three L. welshimeri isolates. Reduced susceptibility to amikacin and chloramphenicol was also observed in one L. monocytogenes and three L. welshimeri isolates, respectively. Recovery of L. monocytogenes in fish samples possessing AMR, full-length InlA, LGI1, and serotypes frequently associated with listeriosis suggest B.C. consumers are exposed to high-risk strains.     

Prevalence and level of Listeria monocytogenes and other Listeria species in retail pre-packaged mixed vegetable salads in the UK

As part of the European Commission (EC) co-ordinated programme for 2005, a study of pre-packaged ready-to-eat (RTE) mixed salads containing meat or seafood ingredients from retail premises was undertaken in the UK to determine the frequency and level of Listeria monocytogenes in these products. Almost all (99.8%; 2682/2686) samples were of satisfactory/acceptable microbiological quality. Two (0.1%) samples exceeded EC legal food safety criteria due to the presence of L. monocytogenes in excess of 100 cfu g(-1) (1.7 x 10(2), 9.9 x 10(2)cfu g(-1)) while another two (0.1%) were unsatisfactory due to L. welshimeri levels over 100 cfu g(-1) (1.2 x 10(3), 6.0 x 10(3) cfu g(-1)). Overall contamination of Listeria spp. and L. monocytogenes found in samples of mixed salads in the UK was 10.8% and 4.8%, respectively. Almost twice as many salad samples with meat ingredients were contaminated with Listeria spp. and L. monocytogenes (14.7% and 6.0%, respectively) compared to samples with seafood ingredients (7.4% and 3.8%, respectively). Pre-packaged mixed salads were contaminated with Listeria spp. and L. monocytogenes more frequently when: collected from sandwich shops; not packaged on the premises; stored or displayed above 8 degrees C. This study demonstrates that the control of L. monocytogenes in food manufacturing and at retail sale is essential in order to minimize the potential for this bacterium to be present in mixed salads at the point of consumption at levels hazardous to health.     

Reduction of counts of Listeria monocytogenes in cheese by means of high hydrostatic pressure

Inactivation of Listeria monocytogenes (strains NCTC 11994 and Scott A) was evaluated in model cheeses submitted to 10 min HHP treatments of 300, 400 or 500 MPa at 5 or 20 degrees C. Counts were measured immediately after high hydrostatic pressure (HHP) treatment (day 1) and after 2, 15 and 30 days of storage at 8 degrees C. Both strains behaved significantly different after 400 and 500 MPa, being NCTC 11994 more sensitive. Scarce differences were found among final values at both HHP treatment temperatures. Initial reductions (log cfu/g) for 400 MPa at 20 degrees C were 2.9 +/- 0.2 for strain NCTC 11994 and 1.5 +/- 0.2 for Scott A. They reached after 30-day storage 5.3 +/- 0.2 and 4.6 +/- 0.4 log cfu/g for NCTC 11994 and Scott A, respectively. For 500 MPa treatments, day-1 reductions of both strains were around 5-log cfu/g, and counts fell below quantification limit after 30 days. Injured cells (around 0.8-log cfu/g) were mostly observed in 400 MPa treated samples on days 1 and 2. Starter cells suffered higher inactivation and injury. For 20 degrees C treatments, its final counts (log cfu/g) at 300, 400 and 500 MPa were: 8.5 +/- 0.2, 5.4 +/- 0.3 and 2.5 +/- 0.1, respectively. These figures evidence the HHP potential to improve safety of cheese products.     

Control of meatborne Listeria monocytogenes and Brochothrix thermosphacta by a bacteriocinogenic Brochothrix campestris ATCC 43754

The effect of a bacteriocinogenic Brochothrix campestris ATCC 43754 upon the growth of Brochothrix thermosphacta and a 4 strain mixture of Listeria monocytogenes was determined in All Purpose Tween (APT) broth and on pork adipose tissue discs at 4 degrees C. Inocula were prepared to give initial numbers of B. campestris of 6-7 log cfu/ml or cm(2) and 3-4 log cfu/ml or cm(2) of B. thermosphacta and L. monocytogenes. Adipose tissue discs were evaluated by a sensory panel to determine the intensity and acceptability of any off-odours produced during the growth of B. campestris. During co-culture in APT broth with B. campestris the growth of B. thermosphacta or L. monocytogenes was 4 log cycles less than growth in its absence. B. campestris showed limited growth on inoculated pork adipose tissue, increasing from initial numbers of about 6 log cfu/cm(2) to a maximum of 7 log cfu/cm(2) within 7d. B. campestris at numbers of 7 log cfu/cm(2) produced slight off-odours but these were not perceived by the panel as unacceptable. When co-inoculated on adipose tissue discs with B. campestris the numbers of B. thermosphacta or L. monocytogenes was limited to about 2-3 log units less than the numbers attained in its absence. B. campestris ATCC 43754 may be useful for meat preservation because it can inhibit B. thermosphacta and L. monocytogenes in situ while producing little change in the sensory properties of the product.     

A note on the incidences of Salmonella spp., Listeria spp. and Escherichia coli O157:H7 serotypes in Turkish sausage (Soudjouck)

The incidence of Salmonella spp., Listeria monocytogenes and Escherichia coli O157:H7 was determined in 100 Turkish sausage (soudjouck) samples collected from shops and markets in the Afyon province, Turkey. Salmonella spp. were detected in 7% of the samples. All of the isolates were S. enterica Paratyphi B. In addition, Listeria spp. were detected in 9% of the samples. Its distribution was 7% L. monocytogenes and 1% each of L. ivanovii and L. innocua. Serological study of the seven L. monocytogenes isolates showed that three of these were 1/2 ab, three were 5/6 ab and one was 1 ab. E. coli O157:H7 was not detected in any of the samples. The pH values of the samples ranged from 4.8 to 6.5. In conclusion, increasing number of listeriosis and salmonellosis cases in Turkey and the contamination levels found indicate that risk assessment and improved preventive measures are required for these sausages.     

Post-processing application of chemical solutions for control of Listeria monocytogenes, cultured under different conditions, on commercial smoked sausage formulated with and without potassium lactate-sodium diacetate

This study evaluated post-processing chemical solutions for their antilisterial effects on commercial smoked sausage formulated with or without 1.5% potassium lactate plus 0.05% sodium diacetate, and contaminated (approximately 3-4 log cfu/cm(2)) with 10-strain composite Listeria monocytogenes inocula prepared under various conditions. Inoculated samples were left untreated, or were immersed (2 min, 25 +/- 2 degrees C) in solutions of acetic acid (2.5%), lactic acid (2.5%), potassium benzoate (5%) or Nisaplin (0.5%, equivalent to 5000 IU/ml of nisin) alone, and in sequence (Nisaplin followed by acetic acid, lactic acid or potassium benzoate), before vacuum packaging and storage at 10 degrees C (48 days). Acetic acid, lactic acid or potassium benzoate applied alone reduced initial L. monocytogenes populations by 0.4-1.5 log cfu/cm(2), while treatments including Nisaplin caused reductions of 2.1-3.3 log cfu/cm(2). L. monocytogenes on untreated sausage formulated with antimicrobials had a lag phase duration of 10.2 days and maximum specific growth rate (mu(max)) of 0.089 per day, compared to no lag phase and mu(max) of 0.300 per day for L. monocytogenes on untreated product that did not contain antimicrobials in the formulation. The immersion treatments inhibited growth of the pathogen for 4.9-14.8 days on sausage formulated without potassium lactate-sodium diacetate; however, in all cases significant (P < 0.05) growth occurred by the end of storage. The antilisterial activity of chemical solutions was greatly enhanced when applied to product formulated with antimicrobials; growth was completely inhibited on sausage treated with acetic or lactic acid alone, and in sequence with Nisaplin. In general, habituation (15 degrees C, 7 days) of L. monocytogenes cells, planktonically or as attached cells to stainless-steel coupons in sausage homogenate prior to contamination of product, resulted in shorter lag phase durations compared with cells cultivated planktonically in a broth medium. Furthermore, when present, high levels of spoilage flora were found to suppress growth of the pathogen. Findings of this study could be useful to US meat processors in their efforts to select required regulatory alternatives for control of post-processing contamination in meat products.     

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

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

Survival of Listeria monocytogenes, Listeria innocua, and Lactic Acid Bacteria in Chill Brines

ABSTRACT:  Listeria monocytogenes is the pathogen of concern in ready-to-eat (RTE) meat products. Salt brines are used to chill processed meats. L. monocytogenes and lactic acid bacteria (LAB) can grow under saline conditions, and may compete with each other for nutrients. The objective of this study was to determine the effect of lactic acid bacteria ( Enterococcus faecalis , Carnobacterium gallinarum , and Lactobacillus plantarum ) on the survival of L. monocytogenes and Listeria innocua in brines stored under low temperatures for 10 d. Sterile tap water (STW) and 2 brine solutions (7.9% and 13.2% NaCl) were inoculated with 1 of 5 cocktails ( L. monocytogenes , L. innocua , LAB, L. monocytogenes + LAB, or L. innocua + LAB) at initial concentrations of 7 log CFU/mL. Brines were stored for 10 d at 4 or 12 °C. Three replications of each brine concentration/cocktail/temperature combination were completed. No significant reductions of L. monocytogenes occurred in 7.9%[w/v] or 13.2%[w/v] brines when LAB were present; however, there were significant reductions after 10 d of L. monocytogenes in the STW solution when LAB were present (1.43 log CFU/mL at 4 °C and 3.02 log CFU/mL at 12 °C). L. innocua was significantly less resilient to environmental stresses of the brines than L. monocytogenes , both with and without LAB present ( P ≤ 0.05). These strains of lactic acid bacteria are not effective at reducing L. monocytogenes in brines at low temperatures. Furthermore, use of L. innocua as a model for L. monocytogenes is not appropriate under these environmental conditions.     

A 2-step cooking method of searing and hot water pasteurization to maximize the safety of refrigerated, vacuum packaged, chicken breast meat

ABSTRACT:  Americans consume almost 40 kg per capita of chicken each year. Increasing consumption of chicken surpassed pork in 1982 and beef in 1992. The objectives of this study were to examine the effectiveness of a novel, 2-step cooking method of grilling, slicing, vacuum packaging, and hot water pasteurization to inhibit the growth of Listeria monocytogenes in chicken breast meat. Because this study required the use of pilot plant scale pasteurization equipment, Listeria innocua M1, a nonpathogen with slightly greater heat resistance than L. monocytogenes , was used as a surrogate. We first examined the lethal effects of grilling on a boneless skinless chicken breast to mimic cross-contaminated, surface-inoculated Listeria . Searing produced a mean reduction of 2.5 log CFU/g of Listeria and a moisture loss of only 7% (w/w). A 2nd experiment studied the lethal effect of pasteurization of the sliced seared chicken breast. L. innocua M1 inoculated between the slices mimiced contamination in deep muscle. Pasteurization in a 71 °C bath (final internal temperature of 66 °C) gave an additional 2.3 log CFU/g reduction. L. innocua M1 did not show significant regrowth during a wk of refrigerated storage. The combined 2-step cooking method of searing and pasteurization gave a combined 4.8 log reduction in LI M1. In parallel tests a non- Listeria indicator, Corynebacterium glutamicum , inoculated between sliced, seared chicken, showed a 3 log reduction after pasteurization for 10 min in a 71 °C bath compared to 2.3 log reduction of Listeria . Corynebacterium regrowth occurred much faster than did L. innocua M1.     

Microbiological quality of retail cheeses made from raw, thermized or pasteurized milk in the UK

Two studies of retail fresh, ripened and semi-hard cheeses made from raw, thermized or pasteurized milk were undertaken in the UK during 2004 and 2005 to determine the microbiological quality of these products. Using microbiological criteria in European Commission Recommendations 2004/24/EC and 2005/175/EC, 2% of both raw, thermized (37/1819 samples) and pasteurized (51/2618 samples) milk cheeses were of unsatisfactory quality. Raw or thermized milk cheeses were of unsatisfactory quality due to levels of Staphylococcus aureus at 10(4)cfu g(-1), Escherichia coli at 10(5)cfu g(-1), and/or Listeria monocytogenes at 10(2)cfu g(-1), whereas pasteurized milk cheeses were of unsatisfactory quality due to S. aureus at 10(3)cfu g(-1) and/or E. coli at 10(3)cfu g(-1). Salmonella was not detected in any samples. Cheeses were of unsatisfactory quality more frequently when sampled from premises rated as having little or no confidence in management and control systems, and stored/displayed at above 8 degrees C. Raw or thermized milk cheeses were also more likely to be of unsatisfactory quality when they were unripened types, and pasteurized milk cheeses when they were: semi-hard types; from specialist cheese shops or delicatessens; cut to order. These results emphasize the need for applying and maintaining good hygiene practices throughout the food chain to prevent contamination and/or bacterial growth. Labelling of cheeses with clear information on whether the cheese was prepared from raw milk also requires improvement.     

Inhibition of Escherichia coli O157:H7, Listeria monocytogenes and Staphylococcus aureus on sliced roast beef by cetylpyridinium chloride and acidified sodium chlorite

The effects of 0.5% cetylpyridinium chloride (CPC), 0.12% acidified sodium chlorite (ASC) and a mix of equal volume of the two (0.25% CPC-0.06% ASC) on Escherichia coli O157:H7, Listeria monocytogenes and Staphylococcus aureus were evaluated on inoculated sliced roast beef. The antimicrobial agents were, respectively, sprayed on the beef surfaces and tray absorbent pads, and samples were stored at 4 degrees C for 10 days (d). At 0 d, L. monocytogenes and S. aureus were reduced to undetectable levels in 2 h after spraying with CPC. CPC-ASC treatment reduced E. coli O157:H7, L. monocytogenes and S. aureus by 4.07, 6.37 and 4.32 log cfu/cm2, respectively, at 0 d. ASC treatment reduced the population of E. coli O157:H7 by 6.09 log cfu/cm2 at 10 d. CPC treatment caused a slight discoloration and ASC-treated beef surfaces demonstrated the lowest redness and highest lightness. The grey colour and off-odour were significant in the ASC-treated beef samples, while CPC-treated samples demonstrated less off-odor and brown colour from 0 to 4 d. Based on our results, it appears that the application of CPC on sliced roast beef can extend the shelf-life of the product without impairing its quality.     

Synergistic inhibition of Listeria monocytogenes on cold-smoked rainbow trout by nisin and sodium lactate

The inhibition of Listeria monocytogenes and mesophilic aerobic bacteria in cold-smoked rainbow trout by nisin, sodium lactate or their combination was studied. Nisin (4000-6000 IU/ml), sodium lactate (60%) or their combination (1:1) were injected into rainbow trout at an industrial scale before the smoking process, or injected into the finished smoked product. Both types of fish samples were smoked, sliced and vacuum-packed according to normal practice in the plant. Packages were opened and L. monocytogenes was inoculated (10(3)-10(4) log colony forming units (cfu)/g) onto the fish samples, which were then vacuum packed again. Samples were stored at 8 degrees C for 17 days or at 3 degrees C for 29 days. Listeria and mesophilic aerobic bacteria counts were measured once a week. The effects of treatments on sensory characteristics and storage stability were also analyzed. Both nisin and lactate inhibited the growth of L. monocytogenes in smoked fish, but the combination of the two compounds was even more effective. The combination of nisin and sodium lactate injected into smoked fish decreased the count of L. monocytogenes from 3.26 to 1.8 log cfu/g over 16 days of storage at 8 degrees C. The level of L. monocytogenes remained almost constant (4.66-4.92 log cfu/g) for 29 days at 3 degrees C in the samples injected before smoking and which contained both nisin and sodium lactate. The treatments did not affect the sensory characteristics of cold-smoked rainbow trout. Based on a triangle test, the sensory quality of all test samples remained unchanged for 23 days of storage at 3 degrees C, whereas the control fish prepared without additives or additional salt remained unchanged only for 16 days.     

The occurrence in the U.K. of Listeria species in raw chickens and soft cheeses

Retail samples of 100 raw chickens and 222 U.K. and imported soft cheeses were examined for the presence of Listeria species. 60% of raw chickens (fresh and frozen) were contaminated with L. monocytogenes and 28% with other Listeria spp. including L. welshimeri, L. seeligeri and L. innocua. Six serotypes of L. monocytogenes were represented (1/2, 3a, 3b, 3c, 4b, 4d) of which more than one were isolated from some samples. 10% of the soft cheeses examined were found to contain L. monocytogenes at levels from less than 10(2) cfu/g to 10(5) cfu/g. The incidences in cheeses from various countries were Italy (16%), France (14%), Cyprus (10%) and the U.K. (4%). Only 2 serotypes (1/2 and 4b) were isolated, some samples containing both. L. innocua was the only other Listeria sp. found. There was no correlation between either the contamination with E. coli or the processing of the original milk used to make the cheeses (raw or pasteurized) and the presence of L. monocytogenes or other Listeria spp. The contribution of contaminated food to the epidemiology of listeriosis in the U.K. is discussed.     

Growth characteristics of Listeria monocytogenes, Listeria welshimeri and Listeria innocua strains in broth cultures and a sliced bologna-type product at 4 and 7 degrees C

The growth characteristics of meat processing plant-derived field strains of Listeria monocytogenes, L. welshimeri and L. innocua were analyzed. The strains were inoculated in BHI broth cultures and incubated at 4 and 7 degrees C. Growth curves were determined by colony counting for 28 days. Significant variations were detected in the growth properties of these field-derived strains. In particular some of the L. monocytogenes strains displayed better cold stress tolerance. These discrepancies in growth behavior were more apparent in the cultures at 4 degrees C compared to 7 degrees C. Similar growth characteristics were observed for selected L. monocytogenes strains also in food challenge tests based on a sliced bologna-type product. The results stress the need for more evaluation of field strain growth characteristics and incorporation of such information in relevant predictive microbial growth models for L. monocytogenes risk assessment in naturally contaminated food products.     

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.     

Detection of Listeria monocytogenes in pork and beef using the VIDAS® LMO2 automated enzyme linked immunoassay method

Listeria (L.) monocytogenes, a foodborne pathogen, is known to be a possible contaminant of foods during production and processing. Samples (n=985) of raw meat and by-products obtained from beef and pork were first screened by the VIDAS system for the presence of Listeria spp., followed by testing for the presence of L. monocytogenes. Positive L. monocytogenes results were confirmed by plating on selective agars: 14% of the samples were positive for Listeria and 4% tested positive for L. monocytogenes, of which 3% were confirmed on selective agars. In by-products (17%) the contamination with listeriae was higher than in meat cuts (10%). Only samples strongly positive for Listeria spp. by VIDAS were positive for L. monocytogenes. Overall, the prevalence of L. monocytogenes in beef and pork samples was rather low in comparison to most previous studies. The VIDAS system was shown to be a suitable method for screening out Listeria-negative samples; the main advantage being a markedly reduced assay time.     

Occurrence and Antimicrobial Susceptibility of Listeria monocytogenes Isolated from Brined White Cheese in Jordan

Listeria monocytogenes is a serious foodborne pathogen that has been isolated from different dairy food products. Several foodborne outbreaks of listeriosis have been associated with consumption of cheese. The aims of this study were to determine the occurrence of L. monocytogenes and Listeria spp. in brined white cheese (BWC) sold in Jordan, and to determine the susceptibility of isolated L. monocytogenes to antimicrobials. Three hundred and fifty samples of 5 different types of BWC (akkawi, boiled, halloumi, pasteurized, and shellal) were collected from a local market in Jordan. The ISO (11290-1) procedure was followed for isolation and identification of Listeria spp. from cheese samples and a polymerase chain reaction (PCR) technique was used for confirmation of L. monocytogenes isolates. The VITEK2 automated system was used for testing antimicrobial susceptibility of L. monocytogenes isolates. The overall prevalence of Listeria spp. in cheese sample was 27.1%. L. monocytogenes was isolated from 39 (11.1%) samples. Other isolated species were L. grayi (6.9%), L. innocua (2%), L. ivanovii (4%), L. seeligeri (2%), and L. welshimeri (0.3%). The pH values and salt concentrations of L. monocytogenes positive cheese samples ranged from 5.10 to 6.32 and 5.64 to 13.16, respectively. L. monocytogenes isolates were sensitive or intermediate susceptible to imipenem, gentamicin, linezolid, teicoplanin, vancomycin, fusidic acid, trimethoprim/sulfamethoxazole, benzylpenicillin, ciprofloxacin, erythromycin, tetracycline, and rifampicin, but resistant to fosfomycin, oxacillin, and clindamycin.     

Use of natural antimicrobials to improve the control of Listeria monocytogenes in a cured cooked meat model system

Concern about nitrite in processed meats has increased consumer demand for natural products manufactured without nitrite or nitrate. Studies on commercial meat products labeled as "Uncured" and "No-Nitrite-or-Nitrate-Added" have shown less control of nitrite in these products and greater potential growth of bacterial pathogens. To improve the safety of the "naturally cured" meats, several natural ingredients were studied in a cured cooked meat model system (80:20 pork, 10% water, 2% salt, and 150 or 50 ppm ingoing sodium nitrite) that closely resembled commercial frankfurters to determine their inhibitory effect on Listeria monocytogenes. Results showed that cranberry powder at 1%, 2% and 3% resulted in 2-4 log cfu/g less growth of L. monocytogenes compared to the control with nitrite alone (P<0.05). Other natural compounds, such as cherry powder, lime powder and grape seed extract, also provided measureable inhibition to L. monocytogenes when combined with cranberry powder (P<0.05).     

Inactivation of Listeria innocua on frankfurters that contain potassium lactate and sodium diacetate by flash pasteurization

ABSTRACT:  Listeria monocytogenes , a psychrotrophic foodborne pathogen, is a recurring postprocess contaminant on ready-to-eat meat (RTE) products, including frankfurters. Potassium lactate (PL) and sodium diacetate (SDA) are FDA-approved antimicrobials that inhibit the growth of L. monocytogenes when incorporated into the formulation of fine emulsion sausage. Flash (steam) pasteurization (FP) has been shown to reduce levels of L. monocytogenes , and its surrogate L. innocua , on frankfurter surfaces. The ability of FP to inactivate and prevent the growth of the L. monocytogenes surrogate L. innocua in a pilot plant setting was investigated. FP treatment (1.5 s, 121 °C) of single layers of frankfurters that were surface-inoculated with either 5, 4, or 3 log CFU/g of L. innocua immediately before FP (1.5 s, 121 °C) resulted in log reductions of 1.97 (± 0.11), 2.03 (± 0.10), or 2.07 (± 0.14), respectively. Inoculum level had no effect on the inactivation of L. innocua . Following 8 wk of refrigerated storage (4 °C), L. innocua levels decreased by 0.5 log in non-FP-treated frankfurter packs, while the 2 log reduction of L. innocua was maintained for FP-treated frankfurters. FP (1.5 s, 121 °C) had no effect on frankfurter color or texture. Because the numbers of L. monocytogenes associated with contaminations of ready-to-eat meats are typically very low, the use of FP in combination with PL and SDA has the potential to reduce the number of frankfurter recalls and foodborne illness outbreaks.     

Enhancing the antilisterial effect of Lactobacillus curvatus CWBI-B28 in pork meat and cocultures by limiting bacteriocin degradation

This work focused on Listeria monocytogenes growth inhibition and growth rebound in raw and cooked pork meat inoculated with Lactobacillus curvatus strains. During storage of raw meat homogenates in the presence of the bacteriocin-producing strain Lactobacillus curvatus CWBI-B28wt, the Listeria monocytogenes cfu count was initially reduced to an undetectable level, but a growth rebound occurred after two weeks, coinciding with loss of 70% of the bacteriocin activity present at the end of week 2. The Listeria growth rebound was suppressed when proteolysis of bacteriocin was countered by the absence of proteases (bacteriocin addition to cooked meat) or the presence of 1% soy flour (added to provide competing substrates). Further experiments confirmed that bacteriocin is sensitive to the action of proteolytic enzymes isolated from both Lactobacillus curvatus CWBI-B28wt and the meat matrix. Bacteriocin proteolysis thus emerges as a cause of Listeria growth rebound.