Deog-Hwan Oh

Antimicrobial activity of ethanol, glycerol monolaurate or lactic acid against Listeria monocytogenes☆

Minimal inhibitory concentrations (MIC) and antimicrobial effects of glycerol monolaurate (monolaurin), ethanol and lactic acid, either alone or in combination, against Listeria monocytogenes in tryptic soy broth were determined. Ethanol at concentrations up to 1.25% did not inhibit growth, but growth was strongly inhibited in the presence of 5% ethanol. MIC values of monolaurin and ethanol alone were 10 micrograms/ml (0.001%) and 50,000 micrograms/ml (5%), respectively. However, MIC values were not changed when monolaurin was combined with ethanol. When 5 micrograms/ml monolaurin was combined with 5% ethanol, the inhibitory effect of the combination was similar to the most active compound alone after 24 h incubation. These data indicate little interaction between monolaurin and ethanol against L. monocytogenes. MIC value of lactic acid alone was 5000 micrograms/ml (0.5%), but was lower when 1.25% ethanol was combined with 0.25% lactic acid. When 2.5% ethanol was combined with 0.25% lactic acid, the combination did not increase the inhibitory effect of the most active single compound alone. This result also indicates that there was little interaction between ethanol and lactic acid.

Monolaurin and acetic acid inactivation of Listeria monocytogenes attached to stainless steel.

Individual and combined antimicrobial effects of monolaurin and acetic acid on Listeria monocytogenes planktonic cells or stainless-steel-adherent cells were determined in order to evaluate cell viability during a 25-min exposure period at 25 degrees C. A 10(7)-colony-forming units (CFU)/ml population of planktonic cells was completely inactivated by the synergistic combination of 1% acetic acid with 50 or 100 microg/ml of monolaurin within 25 or 20 min, respectively. Either compound alone caused partial but incomplete inactivation within the same time periods. A population of 10(5) CFU/cm2 of 1-day adherent cells on stainless steel was completely inactivated within 25 min, but with the highest concentrations of the combined chemicals, i.e., 1% acetic acid and 100 microg/ml of monolaurin. The combined chemical treatment again synergistically produced greater inhibition. A 10(6)-CFU/cm2 population of 7-day adherent cells was not completely inactivated within 25 min of exposure, although counts did decline. The results demonstrate increased resistance of attached L. monocytogenes to acetic acid and monolaurin and show that resistance increased with culture age. Combinations of organic acids and monolaurin might be considered as sanitizers of food contact surfaces, but activities of such combinations are likely to be less than other commonly used sanitizers.

Effect of pH on the Minimum Inhibitory Concentration of Monolaurin Against Listeria monocytogenes1

The effect of pH on the minimum inhibitory concentration (MIC) of glycerol monolaurate (monolaurin) against four strains of Listeria monocytogenes at 35°C in tryptic soy broth supplemented with 0.6% yeast extract was investigated. Our results demonstrate that the MIC of monolaurin was lower than MIC values reported for other common food antimicrobials such as potassium sorbate, tertiary butylhydroquinone, propyl paraben, and butylated hydroxyanisole. The MIC of monolaurin was reduced by decreasing the pH value of the medium. A 3-fold MIC reduction occurred when the pH decreased from pH 7.0 (10 μg/ml) to pH 5.0 (3 μg/ml).

Destruction of Listeria monocytogenes Biofilms on Stainless Steel Using Monolaurin and Heat

Individual and combined antimicrobial effects of monolaurin and heat on planktonic, 1-day adherent, or 7-day adherent cells of Listeria monocytogenes were determined to evaluate biofilm removal from stainless steel. Planktonic cells were more sensitive to heat and monolaurin than were cells attached to stainless steel. Young (1-day) biofilm cells were more sensitive to each treatment than were old (7-day) biofilm cells. Adherent cells were destroyed by 50 μg/ml monolaurin combined with heating at 65°C for 5 min. Cells in a rich nutrient environment were more resistant to treatment than were cells in a depleted nutrient environment. Results demonstrate the usefulness of combining chemical and physical treatments to control L. monocytogenes biofilm problems in the food industry.

Prevalence, Antibiotic Susceptibility, and Virulence Factors of Yersinia enterocolitica and Related Species from Ready-to-Eat Vegetables Available in Korea

A total of 673 ready-to-eat vegetable samples were collected in Korea from 2001 to 2002 and analyzed for the presence of Yersinia spp. We analyzed biotypes, serotypes, and susceptibility to 12 antibiotics and tested for virulence genes of pathogenic Yersinia enterocolitica isolates by PCR assay. Among the samples, 27 (4.0%) were found to be contaminated with Yersinia spp. Among the 27 strains of Yersinia spp. isolates, 18 strains (66.7%) of Y. enterocolitica, 5 strains (18.5%) of Y. frederiksenii, 3 strains (11.1%) of Y. intermedia, and 1 strain (3.7%) of Y. kristensenii were identified. According to the serotypes of Y. enterocolitica isolates, O:3 (11.1%) and O:5 (11.1%) were the most predominant, followed by O:8 (5.6%) and others (72.2%). For biotypes of Y. enterocolitica isolates, 1A (77.8%) was the most predominant, followed by 3B (11.1%), 3 (5.6%), and 5A (5.6%). Also, an antibiotic susceptibility test showed that Y. enterocolitica isolates were very susceptible to the antibiotics tested but highly resistant to ampicillin (94%), cephalothin (100%), and carbenicillin (83%). PCR assays with specific primers derived from yst and ail genes of Y. enterocolitica were applied to confirm the presence of pathogenic Y. enterocolitica. Among the 18 strains of Y. enterocolitica isolates, only 3 strains (O:3/1A, UT/3B, and UT/1A isolated from Chinese cabbage, onion, and spinach, respectively) were shown to have a virulence gene.

Effect of monolaurin and lactic acid on Listeria monocytogenes attached to catfish fillets

The purpose of this study was to determine the effects of monolaurin and lactic acid, singly or combined, on Listeria monocytogenes attached to catfish fillets. Skinless catfish fillets were inoculated with L. monocytogenes and dip treated in monolaurin and/or lactic acid solution for various time periods. Results showed that monolaurin up to 400 micrograms/ml had no influence on counts. Conversely, lactic acid-treated fillets had reduced counts compared to controls. Dipping in 0.85, 1.70, or 2.55% lactic acid for 30 min reduced counts by 0.9, 1.4, or 1.3 logs, respectively. Extending the dipping time to 60 min resulted in little additional decrease in counts. Combining monolaurin with lactic acid yielded results similar to lactic acid alone. Hence, population reduction ability resides with lactic acid and not monolaurin.

Influence of Temperature, pH, and Glycerol Monolaurate on Growth and Survival of Listeria monocytogenes

The effect of temperature, pH, and monolaurin on the growth and survival of Listeria monocytogenes in tryptic soy broth with yeast extract (TSBYE) was investigated. TSBYE medium containing 5, 6, 7, 8, or 9 μg/ml monolaurin was adjusted to pH 5.0, 5.5, or 7.0, inoculated with L. monocytogenes , and incubated at 7, 15, or 35°C. The effect of sublethal concentrations of monolaurin was measured by calculating generation times. Results indicated that temperature and pH were very important factors influencing the efficacy of monolaurin against L. monocytogenes . Bactericidal effects were observed at pH 5.0 but not observed at pH 5.5 or pH 7.0. Further, lethal effects of monolaurin increased as temperature increased at constant pH. Conversely, bacteriostatic effects on growth increased as temperature decreased at constant pH. At constant temperature the bactericidal and bacteriostatic effects of monolaurin increased as the pH of the medium decreased. Generation time of the organism as significantly (P < 0.05) influenced by pH, temperature, concentration of monolaurin, or their combined interactions. Thus, the results indicate that pH and temperature interact to affect the antimicrobial potential of monolaurin against L. monocytogenes .

Incidence and characterization of Listeria spp. from foods available in Korea.

A total of 410 domestic Korean food samples were analyzed for the presence of Listeria spp. by the conventional U.S. Department of Agriculture protocol, and presumptive strains were identified by morphological, cultural and biochemical tests according to Bergeys manual and confirmed by API-Listeria kit. Among the total 410 food samples, 46 samples (11.2%) were found to be contaminated with Listeria species. Among the 46 strains of Listeria spp. isolates, 8 strains (17.42%) for Listeria monocytogenes, 3 strains (6.5%) for Listeria seeligeri, 33 strains (71.7%) for Listeria innocua, and 2 strains (4.4%) for Listeria welshimeri were identified, respectively. Also, only beef, chicken, pork, frozen foods, and sausage were contaminated with L. monocytogenes, and the other products were free of L. monocytogenes. Of 46 Listeria spp. isolates, L. innocua (71.7%) was the most predominantly isolated in a variety of foods compared to other Listeria spp. An in vitro virulence assay for Listeria spp. using myeloma and hybridoma cells from murine and human sources was performed. The result showed that only L. monocytogenes killed approximately 95 to 100% hybridoma cells after 6 h and the other Listeria species, such as L. innocua, L. seeligeri, and L. welshimeri strains had about 0 to 10% lethal effect on hybridoma cells. Also, an antibiotic susceptibility test showed that Listeria spp. isolates were very susceptible to the antibiotics tested, except for nalidixic acid. Also, serotyping results showed 75% of L. monocytogenes isolates from beef, chicken, and frozen pizza belonged to serotype 1 and 25% from sausage were type 4.

Quality Enhancement of Vaccum Packaged Waxy Corns by Far Infrared Ray Drying

This study was conducted to determine the effect of far infrared ray drying on the microbial and quality changes of vacuum packaged waxy corns, such as microbial growth, rehydration, color differences, weight loss and hardness during the storage at and for 7 months. After far infrared ray drying for 6 hours at , 2.32 log CFU/g of total microbial counts in raw waxy corns was enumerated, but no microorganism was detected in steamed or sugar-treated waxy corns. However, no microorganism was observed in all treatments except for control samples until 3 month storage at , whereas steamed and sugar-treated waxy corns showed 2 and 2.7 log reduction compared to that of control after 3 month storage. Yeasts and molds were more resistant than bacteria against far infrared ray drying at the same conditions. Similar results were observed in storage. The degree of gelatinization in raw waxy corns far infrared ray drying changed from 98% to 96.2% after 7 month storage at , whereas steamed waxy corns with far infrared ray drying changed from 81.14% to 58.73%. Water contents in sugar-treated waxy corns with far infrared ray drying gradually reduced compared to steamed waxy corns as drying time increased. The L values in raw waxy corns far infrared ray drying increased as drying time increased, but L values in steamed or sugar-treated waxy corns significantly reduced after 12 hour far infrared ray drying. Hardness in raw waxy corns was higher than in steamed or sugar-treated waxy corns before storage, but similar hardness was observed between raw- and sugar-treated waxy corns after 9 hour drying. This results showed that the microbial reduction, the enhancement of shelf life and quality establishment of steamed or sugar-treated waxy corns could be maximized by using far infrared ray drying.

Comparison of Forced-air Cooling with Static-air Cooling on the Microbiological Quality of Cooked Blue Crabs1

Microbiological analyses were made on samples of cooked blue crab taken immediately after debacking and either forced-air cooling or static-air cooling. Forced-air cooling had significantly lower (P<0.05) total coliform and fecal coliform counts, 2.51 and 2.30 log10 MPN/100 g, compared with those of static-air cooling, 2.83 and 2.60 log10 MPN/100 g. All treatments had less than 2.30 log10 MPN/100 g Escherichia coli . Staphylococcus aureus counts in the forced-air cooled crabs were approximately 4-fold lower than counts in static-air cooled crabs. The aerobic plate counts and psychrotrophic plate counts were significantly lower (P<0.01) by 1.04 and 0.81 log10 CFU/g, respectively, by forced-air cooling compared to static-air cooling. Thermocouple temperature readings were used to determine differences in cooling rates between forced-air and static-air cooling. After 1.5 h of cooling, the initial precooled crabmeat temperature of 34°C (93°F) was reduced by forced-air cooling and static-air cooling to 4°C (40°F) and 20°C (67°F), respectively. The rates of cooling using forced-air and static-air were significantly different (P<0.01).