Increase in Heat Resistance of Listeria monocytogenes Scott A by Sublethal Heat Shock

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Abstract
Log phase cells of Listeria monocytogenes Scott A were heat shocked in trypticase soy + 0.6% yeast extract broth at 40, 44, and 48°C for 3, 10, and 20 min, followed by heating at 55°C for 50 min in order to determine an optimum heat shock response. Most heat shocking temperatures significantly increased thermal resistance (p ≤ 0.05). Increasing heat shock temperature and time allowed the organism to survive much longer than nonheat shocked cells at 50 to 65°C. The optimal heat shock condition was 48°C for 20 min where D-values at 55°C increased 2.3-fold in nonselective agar and 1.6-fold in selective agar. However, cells heat shocked at 48°C for 10 min gave more consistent results; these cells were heat processed 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. Heat shocking at 48°C significantly increased D-value ratios for cells enumerated on nonselective vs. selective media. Heat shocking conditions may be created in pa...

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