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

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 pasteurization or minimal thermal processing of food allowing increased heat resistance of pathogenic and spoilage microorganisms.