INHIBITION OF LISTERIA MONOCYTOGENES BY LIQUID SMOKE AND ISOEUGENOL, A PHENOLIC COMPONENT FOUND IN SMOKE

The behavior of Listeria monocytogenes was monitored in wiener exudate or Tryptose Broth (TB) in the presence of liquid smoke (CharSol Supreme) or smoke ingredients (i.e., phenols and acetic acid). L. monocytogenes grew in untreated exudate held at 25C, but was inactivated in the presence of 0.2 and 0.6% liq'iid smoke (D-values of 36 and 4.5 h, respectively). Of 11 individual phenols tested, only isoeugenol exhibited antilisterial activity in TB at 37C. Growth of the pathogen in the presence of isoeugenol (100 ppm) was inhibited to a greater extent in TB acidified with acetic acid to pH 5.8 compared to pH 7.0. These studies confirm the potent antilisterial activity of liquid smoke and establish the potential of isoeugenol for controlling the growth of L. monocytogenes in certain processed meats .     

INHIBITION OF LISTERIA MONOCYTOGENES AND OTHER BACTERIA BY SODIUM DIACETATE

Growth and survival patterns of Listeria monocytogenes strain Scott A were studied in brain heart infusion broth containing sodium diacetate. Minimum inhibitory concentrations decreased with decrease in temperature, from 35 and 32 mM at 35C and 20C, respectively, to 28 mM at 5C. Broth pH containing 35, 32, and 28 mM sodium diacetate was 5.25, 5.40 and 5.60, respectively. Sodium diacetate was more effective than acetic acid alone in inhibiting the organism over the pH range of 5.0-6.0. Addition of 21 mM (0.3%) sodium diacetate to ground beef or beef slurry suppressed total aerobic counts during refrigerated storage. Although the meat pH decreased from 5.6 to 5.2 by the addition of the compound, a major part of the antimicrobial effect was attributed to the diacetate and not just pH. Sodium diacetate suppressed growth of three additional L. monocytogenes strains and strains of Escherichia coli, Pseudomonas fluorescens, Salmonella enteriditis and Shewanella putrefaciens. P. fragi, Yersinia enterocolitica, Enterococcus faecalis, Lactobacillus fermentis and Staphylococcus aureus were insensitive to the compound. Sodium diacetate has potential for use in controlling growth of listeriae in meat, poultry and fish products and suppressing growth of certain Gram-negative spoilage and pathogenic microorganisms.     

INHIBITION OF LISTERIA MONOCYTOGENES AND OTHER BACTERIA IN SPANISH SOFT CHEESE MADE WITH LACTOCOCCUS LACTIS SUBSP. DIACETYLACTIS

The inhibitory effects of a lactose-negative single-strain starter culture of Lactococcus lactis subsp. diacetylactis were evaluated in Spanish soft cheese (Queso fresco, pH 6.5) against Listeria monocytogenes, Escherichia coli O157:H7 , Enterobacter cloacae, Pseudomonas aeruginosa and Pseudomonas fluorescens. Queso Fresco made without starter culture and stored at 3 and 7C supported growth of all bacteria tested, with the exception of E. coli and P. aeruginosa which were unable to grow at 3C. When cheese was made with the starter culture and stored at 7C , L. monocytogenes, P. aeruginosa and E. coli did not grow, the growth of E. cloacae was delayed, and P. fluorescens grew uninhibited. In cheese stored at 3C, only P. fluorescens was able to grow.     

INHIBITION OF LISTERIA MONOCYTOGENES BY NATIVE MICROFLORA OF WHOLE CANTALOUPE1

Fresh‐cut cantalcupe has been recalled due to the possible presence of Listeria monocytogenes. Several studies have reported that naturally occurring microflora of vegetable surfaces may be antagonistic to pathogen attachment, growth or survival. To test this possibility for L. monocytogenes and cantaloupes, whole melon were treated with water, ethanol (70%) or chlorine (200 ppm) to reduce the native microflora on the melon surfaces. Treated or untreated melons were immersed in a six strain cocktail of L. monocytogenes (10⁷ CFU/mL) for 10 min and then allowed to dry for 1 h inside a biosafety cabinet followed by storage at 5, 10 and 20C for 15 days. Fresh‐cut pieces prepared from the treated or untreated melons and directly inoculated with L. monocytogenes (3.48 log CFU/g) were stored under the same conditions listed above. Populations of L. monocytogenes and five classes of native microflora were investigated. Growth of L. monocytogenes in sterile or nonsterile rind and fresh‐cut homogenates was also studied. The population of L. monocytogenes recovered from inoculated (10³ to 10⁸ CFU/mL) whole melons given no disinfection treatment or washed with water was significantly less (P < 0.05) than that recovered from melons treated with chlorine or EtOH. In general, populations of L. monocytogenes declined on the surface of treated and untreated whole melons and on fresh‐cut pieces over the 15 days storage period at the temperatures tested. However, the decline in pathogen populations was less rapid in the presence of reduced populations of native microflora. Higher populations of L. monocytogenes were attained in sterile tissue homogenates than in nonsterile homogenates. Addition of yeast and mold to sterile rind homogenates was highly inhibitory to growth and survival of the pathogen. The results of this study indicate that native microflora of whole cantaloupe inhibited attachment to rind surfaces as well as survival and growth of L. monocytogenes on cantaloupe surfaces and homogenized fresh‐cut pieces. Thus, L. monocytogenes recontamination of melons having a reduced level of native microflora following application of a disinfection treatment may be a food safety concern.     

INHIBITION OF LISTERIA MONOCYTOGENES BY CARNOBACTERIUM PISCICOLA IN VACUUM-PACKAGED COOKED CHICKEN AT REFRIGERATION TEMPERATURES

Listeria monocytogenes inhibition by bacteriocinogenic Carnobacterium piscicola DX or nonbacteriocinogenic C. piscicola 2818 was examined in vacuum-packed minimally processed chicken cubes with gravy at 4, 8 and 15C. C. piscicola DX and C. piscicola 2818 at 10⁴ CFU/g were coinoculated individually with a pool of three Listeria monocytogenes strains at 10² CFU/g. At 4 and 8C , C. piscicola DX inhibited growth of L. monocytogenes by 3 logs, significantly (P < 0.05) more than did C. piscicola 2818. At 15C both C. piscicola strains inhibited L. monocytogenes by one log cycle. The pH of all inoculated systems decreased from an initial pH of 6.14 to final values ranging from 6.06 to 5.65 depending on the inocula used. Bacteriocin was detected in the systems coinoculated with C. piscicola DX. These studies demonstrate that lower temperatures and bacteriocin production enhanced L. monocytogenes inhibition by C. piscicola     

INHIBITION OF LISTERIA MONOCYTOGENES AND OTHER BACTERIA BY A PLANT ESSENTIAL OIL (DMC) IN SPANISH SOFT CHEESE

The inhibitory effects of a mixture of plant essential oils (DMC) were tested in culture media and Spanish soft cheese. The minimum inhibitory concentration (MIC) was determined in plate count agar and tryptose broth, with pH adjusted to 6.5, against Listeria monocytogenes, Listeria innocua, Staphylococcus aureus, Lactobacillus brevis, Micrococcus luteus, Salmonella cholerasuis, Escherichia coli O157:H7 , Enterobacter cloacae, Pseudomonas aeruginosa and Pseudomonas fluorescens. The mixture of essential oils inhibited all Gram-positive bacteria tested at 40 ppm, but higher concentrations were needed to inhibit Gram-negative bacteria, and no inhibitory effect was found against Ps. fluorescens. The effects of DMC against L. monocytogenes and E. coli O157:H7 were evaluated in Spanish soft cheese (Queso Fresco, pH=6.5) stored at 7C. DMC had a bacteriostatic effect against L. monocytogenes at concentrations of 2500 ppm but was ineffective to control the growth of E. coli O157:H7 .     

THE INCIDENCE AND LEVEL OF LISTERIA SPP. AND LISTERIA MONOCYTOGENES CONTAMINATION IN PROCESSED POULTRY AT A POULTRY PROCESSING PLANT

To estimate the incidence and levels of Listeria spp. in an industrial poultry processing plant, samples of chicken breast meat, livers, surfaces of saws and tables, hands and gloves were analyzed. Forty percent of the breast samples presented Listeria: L. monocytogenes, L. innocua and L. grayi. Liver samples were contaminated by L. innocua and L. grayi. High levels of Listeria monocytogenes were found on saws (<30–2400 MPN/equipment) and tables (<30–11,000 MPN/equipment). Hands were contaminated by L. monocytogenes, L. innocua and L. grayi and gloves with L. innocua and L. grayi. The levels of Listeria on hands and gloves were low (<110 MPN/hand). L. monocytogenes serotypes were 1/2a, 1/2b, 1/2c and 4b. Overall, the study demonstrated the high prevalence of Listeria spp. and specifically L. monocytogenes in chicken breast meat, equipment and hands. Improvements and innovations at the poultry processing plant may effectively reduce final production contamination with Listeria.     

PREVALENCE OF LISTERIA MONOCYTOGENES IN SOME TURKISH FOODSTUFFS

In this study, the presence of Listeria monocytogenes was evaluated in basic groups of foods in Ankara, Turkey between 2001 and 2002. Eighty prepared ground meat, 100 white cheese and 100 parsley samples were obtained from different markets where they were offered for consumption. L. monocytogenes was detected according to the method of ISO Draft International Standard. Fraser Broth was used for enrichment process, and isolation of Listeria spp. was carried out by Oxford Agar. L. monocytogenes was then identified by biochemical and serological tests. L. monocytogenes was isolated from 13.75, 4 and 2% of 80 prepared ground beef, 100 white cheese and 100 parsley samples, respectively. The results indicate that these foodstuffs have risk of L. monocytogenes. Contamination of L. monocytogenes was not detected in packaged ground beef and white cheese samples. High incidence rate of L. monocytogenes was noted for the samples from open and local market.     

BEHAVIOR OF LISTERIA MONOCYTOGENES IN RECONSTITUTED INFANT CEREALS

Listeria monocytogenes may contaminate and grow in reconstituted infant cereals refrigerated or temperature abused before consumption. To assess this risk, a 5-strain composite of the pathogen was inoculated in rice, oatmeal, and wheat-rice-oatmeal infant cereals, hydrated (1/6, w/v) with apple juice, pasteurized milk (2% fat) or water, and stored at 4, 15, or 25C for 0, 8 and 24 h to simulate potential advance home preparation and abuse. L. monocytogenes survived with minimal (<0.6 log cfu per mL) or no growth in all cereal preparations at 4C, but it increased in cereals hydrated with water or milk by 2.4–3.3 and 5.3–6.4 log cfu per mL in 24 h at 15 and 25C, respectively. Reconstitution of cereals with water or milk supported growth (P < 0.05) even at 8 h of abuse. In cereals hydrated with apple juice and stored at 15 or 25C, populations of L. monocytogenes were maintained below 3.0 log cfu per mL at 24 h. These results indicate that refrigeration and/or reconstitution with an acid fruit juice may inhibit growth of L. monocytogenes in infant cereal meal preparations. However, in order to ensure prevention of growth, such meals should preferably be consumed soon after preparation or held at 4C for less than 8 h.     

SUCROSE AND ULTRA HIGH PRESSURE INACTIVATION OF SACCHAROMYCES CEREVISIAE AND LISTERIA INNOCUA

D‐values for ultra high pressure inactivation of Saccharomyces cerevisiae and Listeria innocua were evaluated in commercial applesauce with selected concentrations of soluble solids ranging from 13 to 60%. Applesauce with S. cerevisiae was compressed to 300 MPa and immediately decompressed (identified as 0 s) or held at 300 MPa for 30‐s intervals for 150 s at room temperature. Applesauce with L. innocua was treated at 375 MPa for 0 to 300 s at room temperature. D‐values for both microorganisms at soluble solids concentrations of 13, 20 and 30% were calculated. D‐values for inactivation of S. cerevisiae were 26.3, 29.9 and 46.5 s in applesauce adjusted with sucrose to 13, 20 and 30% soluble solids, respectively. D‐values for inactivation of L. innocua were 40.2, 55.2 and 120.6 s in applesauce also adjusted with sucrose to 13, 20 and 30% of soluble solids. Ultra high pressure treatment of commercial applesauce inoculated with S. cerevisiae or L. innocua adjusted to 40, 50 or 60% soluble solids reduced an initial inocula of 10⁷to 10⁶by less than 1 log. Soluble solids concentrations greater than 30% provide a baroprotective effect on inactivation by high pressure treatment. The use of ultra high pressure technology by the food industry to inactivate microorganisms may be limited in food models with high soluble solids concentrations. Soluble solids adjusted with sucrose concentrations greater than 30% protect against the destruction of microorganisms. An effective pressure treatment that ensures the microbial stability of foods depends on an understanding of the relationship between microorganisms and food components.     

FATE OF LISTERIA MONOCYTOGENES AND SALMONELLA SPP. IN THERMALLY PROCESSED COLOR-MODIFIED TURKEY MEAT

The growth/survival of Listeria monocytogenes and Salmonella spp. was evaluated in two thermally-processed turkey raw materials. Flaked turkey (FT) was prepared from boneless, skinless thighs. Portions were washed with sodium phosphate buffers to lighten tissue color and produce color-modified turkey (CMT). Raw materials were cooked, inoculated with pathogens, and held at 4 and 20C. Salmonella numbers declined in cooked samples stored at 4C for 21 days, but increased by approximately 6 logs in 2 days for the FT and CMT held at 20C. L. monocytogenes increased by approximately 5 logs in samples held for 14 days at 4C. By 2 days at 20C, L. monocytogenes increased more than 5 logs in both materials. Growth/survival of these pathogens did not differ (p > 0.05) between FT and CMT. The data suggest that CMT does not present any more health hazards than conventional turkey raw materials.     

SURVIVAL OF LISTERIA MONOCYTOGENES IN YOGURT WITH VARYING LEVELS OF FAT AND SOLIDS

Yogurts with varying levels of fat and solids were fermented with Lactobacillus bulgaricus and Streptococcus thermophilus in the laboratory to a titratable acidity (TA) of .09%. Each yogurt was seeded with one of three strains of Listeria monocytogenes at two levels and survival was monitored at 1–7, 14, 21 and 28 days during storage at 4°C. All strains survived longer in the skim milk/high solids yogurts which had a higher pH than the whole milk/low solids yogurt.
To determine the effects of pH, solids and fat, simulated yogurts, prepared by acidifying milk preparations to pH 4.2, 4.1 and 4.0, were inoculated with L. monocytogenes. Survival was affected by differences in pH and solids content and strain L. monocytogenes while fat content had no apparent effect .     

THERMAL RESISTANCE OF YERSINIA ENTEROCOLITICA AND LISTERIA MONOCYTOGENES IN MEAT AND POTATO SUBSTRATES

The heat resistance of a wild type and nalidixic acid resistant strain of Yersinia enterocolitica, and Listeria monocytogenes, was measured in meat (minced beef and minced beef homogenate) and potato substrates over the temperature range 50–60C. Comparisons of heat resistance were determined using D-values calculated using a linear survival model. The results showed that the wild-type strain of Y. enterocolitica was more heat resistant than the mutant (p<0.05). Under most conditions, the use of a nonselective/overlay recovery medium resulted in higher D-values compared to a selective recovery medium (p<0.05). Analysis of the data using a nonlinear survival model (D₁ and D₂ -values) suggested the presence of heat resistant subpopulations and was particularly evident for the mutant strain, and in potatoes compared to minced beef.