Browsing by Author "Linton, Richard Howard"

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    Increase in heat resistance of Listeria monocytogenes Scott A by sublethal heat shock
    Linton, Richard Howard (Virginia Tech, 1991-02-05)
    Log phase cells of Listeria monocytogenes Scott A were heat shocked in Trypticase Soy + 0.6% Yeast Extract broth at 40, 44, and 45°C for 3, 10 and 20 min at each temperature, followed by heating at 55â C for 50 minutes in order to determine an optimum heat shock response. Most heat shock temperatures significantly increased thermal resistance (p < 0.05). Increasing heat shock temperature and time allowed the organism to survive much longer at 50 to 65°C than nonheat shocked cells. The optimal heat shock condition was 45°C for 20 min where D-values at 55°C increased 2.3 fold in non-selective agar and 1.6 fold in selective agar in comparison to non-heat shocked cells. However, cells heat shocked at 48°C for 10 min gave more consistent results; these cells were heated at 50, 55, 60, and 65°C to determine a z-value. Although D-values notably increased due to heat shocking, z-values remained constant regardless of the plating medium. When aerobically heat shocked cells (45°C for 10 min) were plated on a non-selective or a selective medium, a 1.4x increase in D-value was observed when enumerated under strictly anaerobic conditions. Aerobically heat shocked cells (48°C for 10 min) added to shrimp samples retained the increased heat resistance at 55°C when enumerated on a nonselective medium compared to the non-heat shocked cells. Heat shocking conditions may be created in pasteurization or minimal thermal processing of foods allowing increased heat resistance of pathogenic and spoilage microorganisms.
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    Use of the Gompertz equation to model non-linear survival curves and predict temperature, pH, and sodium chloride effects for Listeria monocytogenes Scott A
    Linton, Richard Howard (Virginia Tech, 1994-05-05)
    Numerous examples of non-linear survival curves, plotted as log surviving cells vs. time, for bacteria exposed to heat have been reported. Factors which may affect the shape of a survival curve and the heat resistance of bacteria include temperature, pH, and NaCl concentration. Many studies have examined the effect of these factors individually, but little information exists on the combined effects. The objective of this study was to mathematically model non-linear survival curves to account for these factors and their interactions simultaneously. Heat resistance of Listeria monocytogenes(L. monocytogenes) was determined in O.lM KH₂P0₄ buffer and in infant formula at three temperatures (50, 55, and 60 C), three pH levels (5, 6, and 7), and three NaCl concentrations (0, 2, 4%). Survival curves were fit using linear regression, non-linear regression with a modified logistic equation, and non-linear regression with a modified Gompertz equation.