The heat resistance of Listeria monocytogenes was determined in 0.1 M KH2PO4 buffer at three temperatures (50, 55, and 60°C), three pH levels (5, 6, and 7), and three NaCl concentrations (0, 2, and 4%). Survival curves were fit using nonlinear regression with a modified Gompertz equation. The Gompertz equation is capable of fitting survival curves which are linear, those which display an initial lag region followed by a linear region, and those which are sigmoidal. Parameter estimates were used to describe the lag region, death rate, and the tailing region of a survival curve. These estimates were also used to predict single and interactive effects of temperature, pH, and percentage of NaCl on the log surviving fraction (LSF) of bacteria. Interactions among these variables significantly (P < .05) affected the LSF. Generally, increased pH or NaCl concentration lead to an increased (P < .05) LSF, where as increased time or temperature lead to a decreased (P < .05) LSF. All multiple factor interactions significantly (P < .05) affected the LSF. These interactions differed depending on the heating medium and the region of the survival curve. The correlation of observed LSF and predicted LSF (R2 = .89) indicated that the Gompertz equation was in close agreement with the observations. This study demonstrated that the Gompertz equation and nonlinear regression can be used as an effective means to predict survival curve shape and response to heat of L. monocytogenes in many different environmental conditions.

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Reduction of Listeria monocytogenes Scott A on uninjured and injured surfaces of green peppers after 0.3- and 3-mg/ liter gaseous and aqueous ClO2 treatment and water washing for 10 min at 20 degrees C was studied. Growth of the L. monocytogenes untreated or treated with 0.6 mg/liter ClO2 gas for 30 min at 20 degrees C on green peppers also was investigated. A membrane-surface-plating method was used for resuscitation and enumeration of L monocytogenes treated with ClO2. The bacterial viability on pepper surfaces was visualized using confocal laser scanning microscopy (CLSM). Live and dead cells of L. monocytogenes were labeled with a fluorescein isothiocyanate-labeled antibody and propidium iodide, respectively. More than 6 log CFU/5 g L. monocytogenes on uninjured surfaces and about 3.5 log CFU/5 g on injured surfaces were inactivated by both 3-mg/liter and 0.6-mg/liter ClO2 gas treatments. The 3-mg/liter aqueous ClO2 treatment achieved 3.7- and 0.4-log reductions on uninjured and injured surfaces, respectively; whereas, water washing alone showed 1.4- and 0.4-log reductions, respectively. ClO2 gas treatment was the most effective in reducing L. monocytogenes on both uninjured and injured green pepper surfaces, when compared with aqueous ClO2 treatment and water washing. The significant difference (P < 0.05) between log reductions on uninjured and injured surfaces and the results from CLSM analysis suggested that injured surfaces protected more bacteria from sanitation treatments than did uninjured surfaces. Not only could L. monocytogenes grow on green pepper surfaces at 7 degrees C, bacteria that survived the 0.6-mg/liter ClO2 gas treatment also could grow.