Type of Document Master's Thesis Author Linton, Richard Howard URN etd-10222009-124928 Title Increase in heat resistance of Listeria monocytogenes Scott A by sublethal heat shock Degree Master of Science Department Food Science and Technology Advisory Committee
Advisor Name Title Pierson, Merle D. Committee Chair Bishop, J. Russell Committee Member Bunce, George Edwin Committee Member Keywords
- Listeria monocytogenes
Date of Defense 1991-02-05 Availability restricted Abstract
Log phase cells of Listeria monocytogenes Scott A were heat shocked in Trypticase Soy + 0.6% Yeast Extract broth at 40, 44, and 4S∘C for 3, 10 and 20 min at each temperature, followed by heating at 55∘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 4S∘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 (4S∘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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