Antimicrobial activity of lactic acid bacteria against Listeria monocytogenes on frankfurters formulated with and without lactate/diacetate

Contamination by Listeria monocytogenes has been a constant public health threat for the ready-to-eat (RTE) meat industry due to the potential for high mortalities from listeriosis. Lactic acid bacteria (LAB) have shown protective action against various pathogenic bacteria. The aim of this study was to evaluate the antilisterial activity of a combination of three LAB strains (Lactiguard®) on L. monocytogenes. The combination of the LAB was inhibitory to L. monocytogenes inoculated onto frankfurters not containing lactate/diacetate after 8 weeks of refrigerated storage (0.6 log reduction compared to L. monocytogenes only control), and when a cell free extract (CFS) of the LAB was added with LAB even more inhibition was obtained (1.2 log reduction compared with L. monocytogenes only). In frankfurters containing lactate/diacetate the LAB and the LAB plus CFS were more effective in reducing growth of L. monocytogenes after 8 weeks of refrigerated storage (2 and 3.3 log reductions respectively).     

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).     

Survey of naturally and conventionally cured commercial frankfurters, ham, and bacon for physio-chemical characteristics that affect bacterial growth

Natural and organic food regulations preclude the use of sodium nitrite/nitrate and other antimicrobials for processed meat products. Consequently, processors have begun to use natural nitrate/nitrite sources, such as celery juice/powder, sea salt, and turbinado sugar, to manufacture natural and organic products with cured meat characteristics but without sodium nitrite. The objective of this study was to compare physio-chemical characteristics that affect Clostridium perfringens and Listeria monocytogenes growth in naturally cured and traditionally cured commercial frankfurters, hams, and bacon. Correlations of specific product characteristics to pathogen growth varied between products and pathogens, though water activity, salt concentration, and product composition (moisture, protein and fat) were common intrinsic factors correlated to pathogen growth across products. Other frequently correlated traits were related to curing reactions such as % cured pigment. Residual nitrite and nitrate were significantly correlated to C. perfringens growth but only for the ham products.     

Effect of preinoculation growth media and fat levels on thermal inactivation of a serotype 4b strain of Listeria monocytogenes in frankfurter slurries

Preinoculation growth conditions and fat levels were evaluated for effects on the heat resistance of Listeria monocytogenes strain MFS 102 in formulated frankfurter slurries and on frankfurter surfaces. Comparison of linear inactivation rates (D-values) for cells heated in frankfurter slurry showed that growth conditions were significant (P<0.05) factors affecting subsequent thermal resistance. The average D(60 degrees C)-values for the five preinoculation growth media tested from most resistant to least heat resistant were: tryptic soy broth with 0.6% yeast extract (TSBYE) (2.2 min) and 8.5% fat slurry (2.2 min), followed by 23% fat slurry (1.7 min) and 11% fat slurry (1.7 min), and then TSYBE with quaternary ammonium compounds added (TSBYE+Q) (1 min). The fat level in the frankfurter heating media also had a significant (P<0.05) effect on the thermal death rate of L. monocytogenes. Cells heated in 8.5% fat slurry had a significantly higher (P<0.05) D(60 degrees C)-value (2.2 min) than those heated in 11% fat (1.0 min) and 23% fat slurry (0.9 min). Growth media (TSBYE, 8.5% fat slurry, and TSBYE+Q), and fat level (15% and 20%), however, were not significant factors (P>0.05) affecting thermal inactivation rates on frankfurter surfaces. Heat inactivation rates were consistently higher on frankfurter surfaces compared to similar treatments done in frankfurter slurry. On frankfurter surfaces, a 2.3- to 5.1-log(10) reduction was achieved after 15 min depending on frankfurter surface type. The time necessary to achieve a 3-log(10) reduction using post-processing pasteurization of frankfurters in a hot water-bath at 60 degrees C almost doubled for cells grown in TSBYE and heated in 23% fat frankfurter slurry (19.6 min) versus cells grown and heated in 8.5% fat frankfurter slurry (10.8 min).     

Spray application of liquid smoke to reduce or eliminate Listeria monocytogenes surface inoculated on frankfurters

In a simulated post process contamination scenario liquid smoke was sprayed on the frankfurters after peeling, and then inoculated with Listeria monocytogenes (Lm). Samples that did not receive a liquid smoke spray remained at approximately 2 log cfu/cm² during the 48 h of storage while the levels on the liquid smoke treated frankfurters continued to decline until they were below detection level (1 cfu/100 cm²). A shelf-life study lasting 140 days indicated that liquid smoke suppressed the growth of Lm for up to 130 days. An application of 2 or 3 ml liquid smoke at packaging resulted in at least a 1 log reduction of Lm within 12 h post packaging.     

Reduction of Listeria monocytogenes on frankfurters treated with lactic acid solutions of various temperatures

United States regulations require ready-to-eat meat and poultry processors to control Listeria monocytogenes using interventions which may include antimicrobials that reduce post-processing contamination by at least 1 log-cycle; if the treatment achieves ≥2 log reductions, the plant is subject to less frequent microbial testing. Lactic acid (LA) may be useful as a post-lethality intervention and its antimicrobial properties may increase with temperature of application. The aim of this study was to evaluate the effect of LA solution concentration and temperature on L. monocytogenes counts of inoculated frankfurters and to identify parameters (concentration, temperature, and time) that achieve 1 and 2 log-unit immediate reductions. Frankfurters were surface-inoculated with a 10-strain mixture of L. monocytogenes (4.4 ± 0.1 log CFU/cm²) and then immersed in distilled water or LA solutions (0–3%) of 4, 25, 40, or 55 °C for 0–120 s. A regression equation for L. monocytogenes reduction included significant (P < 0.05) effects by the terms of concentration, time, temperature, and the interaction of concentration and temperature; other tested parameters (other interactions, quadratic and cubic terms), within the experimental range examined, did not affect (P ≥ 0.05) the extent of reduction. Results indicated that the effectiveness of LA against L. monocytogenes, in addition to concentration, increased with solution temperature (in the range of 0.6–2.8 log CFU/cm²). The developed equation may allow processors to vary conditions of treatment with LA to achieve a 1 or 2 log-unit reduction of the pathogen and comply with United States regulations.     

Effect of fat content on survival of Listeria monocytogenes during simulated digestion of inoculated beef frankfurters stored at 7 °C

Potential effects of the fat content of frankfurters on the gastrointestinal survival of Listeria monocytogenes were investigated. At various stages of storage (7 °C, up to 55 days), inoculated frankfurters of low (4.5%) and high (32.5%) fat content were exposed to a dynamic gastrointestinal model (37 °C) and L. monocytogenes counts were determined at intervals during exposure in each gastrointestinal compartment (gastric, GC; intestinal, IC). Bacterial survival curves in each compartment were fitted with the Baranyi and Roberts mathematical model. L. monocytogenes populations on low- and high-fat frankfurters exceeded 8.0 log CFU/g at 39 and 55 days of storage, respectively. Major declines in populations occurred after 60 min on low-fat frankfurters in the GC, with reductions of 2.6 to >7.2 log CFU/g at 120 min on days 1 and 39 of storage, respectively. L. monocytogenes reductions in high-fat frankfurters ranged from 1.6 (day-1) to 5.2 (day-55) log CFU/g. Gastric inactivation rates were 0.080–0.194 and 0.030–0.097 log CFU/g/min for low- and high-fat samples, respectively. Since gastric emptying began while the gastric pH was >5, initial counts (enumerated 30 min after ingestion) reaching the IC depended on initial contamination levels on each product, which increased during storage. Subsequent reductions during the intestinal challenge were 0.1–1.4 log CFU/g. Findings indicated protective effects of fat against gastric destruction of L. monocytogenes. However, since the effects of fat were observed mainly at later stages of gastric exposure, they did not influence numbers of viable cells reaching the IC.     

Prevalence of Salmonellae, Listeria monocytogenes, and fecal coliforms in bulk tank milk on US dairies

The objective of this study was to determine the prevalence of Salmonella, Listeria monocytogenes, and fecal coliforms in bulk tank milk in the United States. As part of the NAHMS Dairy 2002 survey, 861 bulk tank milk samples were collected from farms in 21 states. Milk was directly plated on selective agars for direct bacterial enumeration and was enriched in selective broths to increase detection sensitivity. Somatic cell counts (SCC) and standard plate counts (SPC) were also determined. Coliforms were detected in 95% (818 of 860) of the samples, and the average SCC was 295,000 cells/mL. Twenty-two samples (2.6%) were culture-positive for Salmonella, and 9 serotypes were identified: Montevideo (n = 7), Newport (n = 4), Muenster (n = 2), Meleagridis (n = 2), Cerro (n = 2), 44:Z36 (Z38) (n = 2), Dublin (n = 1), Anatum (n = 1), and 9, 12:nonmo-tile (n = 1). Listeria monocytogenes was isolated from 56 (6.5%) samples, and serotyping of these isolates yielded 5 serotypes (1/2a, 1/2b, 3b, 4b, and 4c). Of the L. monocytogenes isolates, 93% were serotypes 1/2a, 1/2b, and 4b, the most common human clinical isolates. Regional differences in L. monocytogenes and Salmonella prevalence were observed, but more studies are needed to determine the validity of these differences. There were no apparent relationships between SCC or SPC and incidence of Salmonella or L. monocytogenes. Although the prevalence of L. monocytogenes and Salmonella was low, these pathogens represent a potential risk to consumers of raw milk and raw milk products.     

Review--Persistence of Listeria monocytogenes in food industry equipment and premises

To understand why Listeria monocytogenes may persist in food industry equipment and premises, notably at low temperature, scientific studies have so far focused on adhesion potential, biofilm forming ability, resistance to desiccation, acid and heat, tolerance to increased sublethal concentration of disinfectants or resistance to lethal concentrations. Evidence from studies in processing plants shows that the factors associated with the presence of L. monocytogenes are those that favor growth. Interestingly, most conditions promoting bacterial growth were shown, in laboratory assays, to decrease adhesion of L. monocytogenes cells. Good growth conditions can be found in so-called harborage sites, i.e. shelters due to unhygienic design of equipment and premises or unhygienic or damaged materials. These sites are hard to eliminate. A conceptual model of persistence/no persistence based on the relative weight of growth vs. outcome of cleaning and disinfection is suggested. It shows that a minimum initial bacterial load is necessary for bacteria to persist in a harborage site and that when a low initial bacterial charge is applied, early cleaning and disinfection is the only way to avoid persistence. We conclude by proposing that there are no strains of L. monocytogenes with unique properties that lead to persistence, but harborage sites in food industry premises and equipment where L. monocytogenes can persist.     

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.     

Effects of potential antimicrobial ingredients used to control Listeriamonocytogenes on quality of natural casing frankfurters

Listeriosis concerns have led to increased antimicrobial use; reformulation may affect quality. The objective was to determine the effects of potential antimicrobials on quality of natural casing frankfurters. Eleven treatments were used: no antimicrobials (control); 3.0% sodium lactate and sodium diacetate blend (SL+SD); 1.0%, 2.5%, and 3.5% buffered sodium citrate (SC); 1.0%, 2.5%, and 3.5% buffered sodium citrate and sodium diacetate blend (SC+SD); and 1.0%, 2.5%, and 5.0% whey protein peptides (WP). Trained and consumer panelists analyzed sensory characteristics. Cooking yield was unaffected by treatment (P>0.05). Addition of SC and SC+SD decreased pH (P<0.05). Texture profile analysis was conducted. Hunter color values and growth of aerobic bacteria and lactic acid bacteria were monitored over 15 weeks of storage. No samples reached spoilage. SL+SD 3% caused few changes; WP caused changes perceived as negative; and SC and SC+SD caused changes often perceived as positive.     

Multiple-locus variable number of tandem repeat analysis (MLVA) of Listeria monocytogenes directly in food samples

Listeria monocytogenes is the etiologic agent of listeriosis responsible for severe and fatal infections in humans. Listeria contamination occurs quite often in a wide range of foods due to its ubiquitous nature. Isolates need to be characterized to a strain level for accurate diagnosis of Listeria infection, epidemiological studies, investigation of outbreaks and effective prevention and control of food-borne listeriosis. The purpose of this research was to evaluate the multiple-locus variable number of tandem repeat analysis (MLVA) for sub-typing L. monocytogenes isolates in pure cultures and in food matrices. Two multiplex PCR assays were formulated to amplify six specific loci using fluorescently-labeled primers; and the amplicons were analyzed by capillary electrophoresis. The MLVA method resulted in 34 unique DNA fingerprint patterns from 46 L. monocytogenes isolates of 10 serotypes which had 29 or 30 PFGE patterns with a single restriction enzyme and 34 AFLP patterns. The MLVA patterns of the 46 isolates remained unchanged in the presence of pre-enriched food matrices including sausage, ham, chicken, milk and lettuce. The MLVA method successfully typed L. monocytogenes strains spiked in cheese, roast beef, egg salad and vegetable samples after 48 h enrichment at the initial inoculation levels of 1-5 CFU per 25 g of food or higher. The limits of detection (typing) of the MLVA method were 10³-10⁴ CFU/mL of pre-enriched food broth when evaluated using post-spiked sausage, ham, chicken, milk and lettuce samples. The MLVA method was simple, highly discriminatory, and easy to perform with portable (numerical) results. To our knowledge, this is the first report that describes the application of the MLVA method directly to food samples and demonstrates the possibility to obtain rapid and accurate subtyping results before an isolate is obtained.     

The inhibitory effect of natural microflora of food on growth of Listeria monocytogenes in enrichment broths

The aims of this study were to (i) compare the inhibitory effects of the natural microflora of different foods on the growth of Listeria monocytogenes during enrichment in selective and non-selective broths; (ii) to isolate and identify components of the microflora of the most inhibitory food; and (iii) to determine which of these components was most inhibitory to growth of L. monocytogenes in co-culture studies. Growth of an antibiotic-resistant marker strain of L. monocytogenes was examined during enrichment of a range of different foods in Tryptone Soya Broth (TSB), Half Fraser Broth (HFB) and Oxoid Novel Enrichment (ONE) Broth. Inhibition of L. monocytogenes was greatest in the presence of minced beef, salami and soft cheese and least with prepared fresh salad and chicken pâté. For any particular food the numbers of L. monocytogenes present after 24 h enrichment in different broths increased in the order: TSB, HFB and ONE Broth. Numbers of L. monocytogenes recovered after enrichment in TSB were inversely related to the initial aerobic plate count (APC) in the food but with only a moderate coefficient of determination (R²) of 0.51 implying that microbial numbers and the composition of the microflora both influenced the degree of inhibition of L. monocytogenes. In HFB and ONE Broth the relationship between APC and final L. monocytogenes counts was weaker. The microflora of TSB after 24 h enrichment of minced beef consisted of lactic acid bacteria, Brochothrix thermosphacta, Pseudomonas spp., Enterobacteriaceae, and enterococci. In co-culture studies of L. monocytogenes with different components of the microflora in TSB, the lactic acid bacteria were the most inhibitory followed by the Enterobacteriaceae. The least inhibitory organisms were Pseudomonas sp., enterococci and B. thermosphacta. In HFB and ONE Broth the growth of Gram-negative organisms was inhibited but lactic acid bacteria still reached high numbers after 24 h. A more detailed study of the growth of low numbers of L. monocytogenes during enrichment of minced beef in TSB revealed that growth of L. monocytogenes ceased at a cell concentration of about 10² cfu/ml when lactic acid bacteria entered stationary phase. However in ONE Broth growth of lactic acid bacteria was slower than in TSB with a longer lag time allowing L. monocytogenes to achieve much higher numbers before lactic acid bacteria reached stationary phase. This work has identified the relative inhibitory effects of different components of a natural food microflora and shown that the ability of low numbers of L. monocytogenes to achieve high cell concentrations is highly dependent on the extent to which enrichment media are able to inhibit or delay growth of the more effective competitors.     

Staphylococcus aureus and Listeria monocytogenes in Norwegian raw milk cheese production

The aim of this study was to survey the presence of Staphylococcus aureus and Listeria monocytogenes during the cheese making process in small-scale raw milk cheese production in Norway.The prevalence of S. aureus in bovine and caprine raw milk samples was 47.3% and 98.8%, respectively. An increase in contamination during the first 2-3 h resulted in a 73.6% prevalence of contamination in the bovine curd, and 23 out of 38 S. aureus-negative bovine milk samples gave rise to S. aureus-positive curds. The highest contamination levels of S. aureus were reached in both caprine and bovine cheese after 5-6 h (after the first pressing). There was no contamination of L. monocytogenes in caprine cheeses and only one (1.4%) contaminated bovine cheese.This work has increased our knowledge about S. aureus and L. monocytogenes contamination during the process of raw milk cheese production and gives an account of the hygiene status during the manufacture of Norwegian raw milk cheeses.     

Assessing biofilm formation by Listeria monocytogenes strains

When a microtitre plate assay was used to quantify biofilm production by Listeria monocytogenes strains following growth in Tryptone Soy Broth (TSB) for 48 h at 20 degrees C, 127 of 138 strains (92.0%) were classified as weak, 9 of 138 strains (6.5%) as moderate and only 2 of 138 strains (1.5%) as strong biofilm formers. The strains included environmental, animal, food (persistent and sporadic strains) and clinical isolates previously typed using esterase electrophoresis (ESE) and multi-locus enzyme electrophoresis (MEE). Strains from different sources produced similar quantities of biofilm, whereas biofilm production by ESE type II strains, irrespective of source, was greater than that observed for other ESE types. No correlation between MEE type and biofilm production was observed. A Petri dish assay which allowed parallel quantification and microscopic examination of biofilms was used to examine biofilm formation by selected L. monocytogenes strains during growth in TSB for 14 days at 20 degrees C. Results from these assays showed that following prolonged incubation, some L. monocytogenes strains categorized as weak biofilm formers by the 48 h microtitre assay, were able to form biofilms similar in terms of quantity and structure to those produced by strains classified as strong or medium biofilm formers. Results from 14-day Petri dish assays confirmed 48 h microtitre assays regarding greater biofilm production by ESE type II strains compared to other ESE types of L. monocytogenes. Biofilm production was similar for ESE type II persistent and sporadic food isolates but reduced for ESE type II clinical strains.     

Combined effect of packaging atmosphere and storage temperature on growth of Listeria monocytogenes on ready-to-eat shrimp

Cooked, peeled, and deveined shrimp were inoculated with a 5 strain mixture of Listeria monocytogenes and packaged in air, vacuum, and a 100% carbon dioxide modified atmosphere. The packaged shrimp were then stored at 3, 7, and 12 degrees C for 15 days to monitor the growth of L. monocytogenes and psychrotrophic bacteria. Uninoculated shrimp were also subjected to sensory evaluation by a trained panel to measure odor and appearance over the storage period. Results demonstrated that shrimp packaged in CO(2) and stored at 3 degrees C did not permit growth of L. monocytogenes during the 15-day storage period, while all other packaging/temperature combinations allowed for multiplication of the bacterium. Carbon dioxide packaging also resulted in the slowest growth of psychrotrophic bacteria and resulted in shrimp having acceptable sensory odor and appearance scores at the end of storage. When strict temperature control is difficult, such as during processing, transportation, retail display, or home use, additional antimicrobial hurdles may be necessary to ensure safety.     

Molecular Subtyping and Tracking of Listeria monocytogenes in Latin-Style Fresh-Cheese Processing Plants

Latin-style fresh cheeses, which have been linked to at least 2 human listeriosis outbreaks in the United States, are considered to be high-risk foods for Listeria monocytogenes contamination. We evaluated L. monocytogenes contamination patterns in 3 Latin-style fresh-cheese processing plants to gain a better understanding of L. monocytogenes contamination sources in the manufacture of these cheeses. Over a 6-mo period, 246 environmental samples were collected and analyzed for L. monocytogenes using both the Food and Drug Administration (FDA) method and the Biosynth L. monocytogenes detection system (LMDS). Finished cheese samples from the same plants (n = 111) were also analyzed by the FDA method, which was modified to include L. monocytogenes plating medium (LMPM) and the L. monocytogenes confirmatory plating medium (LMCM) used in the LMDS method. Listeria monocytogenes was detected in 6.3% of cheese and 11.0% of environmental samples. Crates, drains, and floor samples showed the highest contamination rates, with 55.6, 30.0, and 20.6% L. monocytogenes positive samples, respectively. Finished products and food contact surfaces were positive in only one plant. The FDA method showed a higher sensitivity than the LMDS method for detection of L. monocytogenes from environmental samples. The addition of LMPM and LMCM media did not further enhance the performance of the FDA method for L. monocytogenes detection from finished products. Molecular subtyping (PCR-based allelic analysis of the virulence genes actA and hly and automated ribotyping) was used to track contamination patterns. Ribotype DUP-1044A, which had previously been linked to a 1998 multistate human listeriosis outbreak in the United States, was the most commonly identified subtype (20/36 isolates) and was isolated from 2 plants. This ribotype was persistent and widespread in one factory, where it was also responsible for the contamination of finished products. We hypothesize that this ribotype may represent a clonal group with a specific ability to persist in food processing environments. While previous listeriosis outbreaks were linked to Latin-style fresh cheeses made from unpasteurized milk, the presence of this organism in pasteurized cheese products illustrates that persistent environmental contamination also represents an important source of finished product contamination.     

Effects and interactions of sodium lactate, sodium diacetate, and pediocin on the thermal inactivation of starved Listeria monocytogenes on bologna

The effects and interactions of temperature (56.3-60 °C), sodium lactate (SL; 0-4.8%), sodium diacetate (SDA; 0-2.5%), and pediocin (0-10,000 AU) on starved Listeria monocytogenes (10⁷ CFU/g) on bologna were investigated. Bologna slices containing SL and SDA in the formulation were dipped in pediocin, surface inoculated, and treated at various temperatures using combinations of parameters determined by central composite design. D-values were calculated. The observed D-values ranged from 2.8 min at 60 °C to 24.61 min at 56.3 °C. Injury ranged from 9.1 to 76% under various conditions. The observed D-values were analyzed using second order response surface regression for temperature, SL, SDA, and pediocin, and a predictive model was developed. Predicted D-values were calculated and ranged from 3.7 to 19 min for various combinations of parameters. Temperature alone reduced the predicted D-values from 33.96 min at 56.3 °C to 11.51 min at 60 °C. Addition of SL showed a protective effect. Other combination treatments either reduced or increased D-values depending on temperature. The combination of SL and SDA was effective at lower temperatures, however, higher levels of SDA at higher temperatures made the organism more heat resistant. Pediocin (up to 5000 AU) with increasing temperature and SDA reduced D-values. Depending on temperature and concentration, the interactions between various additives can affect thermal inactivation of L. monocytogenes on bologna. Starvation rendered L. monocytogenes more susceptible to heat and additives.     

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.