Date of Award


Degree Type


Degree Name

Master of Science


Food Science and Technology

Major Professor

P. Michael Davidson

Committee Members

David Golden, Doris D'Souza


Microbial surrogates are defined as non-pathogenic organisms with behavioral and/or survival characteristics correlating to those of specific pathogens. The purpose of this study was to determine the thermal inactivation kinetics (D and z-values) of Lactobacillus bulgaricus ATCC 11842, Lactobacillus fructivorans ATCC 8288, Lactobacillus plantarum ATCC 4008, Listeria innocua ATCC 33091, ATCC 51742, and 11/97, and Staphylococcus carnosus CS-299 as potential surrogates for hepatitis A virus (HAV; D72°C = 0.9 min). Additionally, the effect of sub-lethal heat on increasing the thermotolerance of L. bulgaricus was investigated. All strains were tested in phosphate buffered saline (PBS) as a preliminary step and, contingent upon the survival characteristics, further tested in ultra-high temperature (UHT) 2% fat milk and chopped spinach. Thermal inactivation experiments in PBS, UHT milk, and chopped spinach were performed at a temperature range of 58-70°C for 0-60 min. For sub-lethal heating, L. bulgaricus was subjected to 45 or 50°C for 30 min prior to inoculation of milk and subsequent heating at 70°C. L. fructivorans, L. plantarum, and all L. innocua strains had insufficient thermal resistance to be used for HAV surrogates. In contrast, D-values for L. bulgaricus in UHT milk were 9.98, 2.68, and 0.45 min, at 65, 67, and 70˚C, respectively, with a z-value of 3.7°C. When subjected to sub-lethal heating, the D70˚C values for the 45 or 50˚C treatment were 0.34 or 0.48 min, respectively. Thus sub-lethal heating did not increase thermal resistance. D-values for L. bulgaricus in spinach were 43.29, 10.13, 3.20, 0.91 min for 63, 65, 67, 70°C. S. carnosus had D-values in PBS of 3.46, 1.27, and 0.47 min at 65, 67, and 70°C, respectively, and a z-value of 5.84°C. In milk S. carnosus D-values were 4.60, 1.94, and 0.46 min at 65, 67, and 70°C, respectively and in spinach D-values of 4.61, 2.42, 0.73 min were found at 65, 67, and 70°C, respectively. While D-values of L. bulgaricus and S. carnosus were lower than that of HAV, they may have potential value at high cell concentrations in validation studies for inactivation of HAV.

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