Date of Award
Doctor of Philosophy
Food Science and Human Nutrition
James S. Dickson
Alan A. Dispirito
Several key factors we investigated in order to improve the safety of foods and to better understand the survival of microorganisms when exposed to stress. Pork patties were irradiated at 0, 0.5, 1, and 2KGy, and stored at 0°C. The shelf life of these irradiated pork patties (aerobically packaged and vacuum packaged; fresh and temperature abused) were studied and predictive models were developed based on generation time and lag phase extension. Next, the physiological response of Salmonella typhimurium ATCC 14028 to media composition and pH was investigated by studying the changes in the cell membrane fatty acids, cell surface charge and ability to attach to beef tissue. Three different media, nutrient broth, tryptic soy broth and tryptic soy broth supplemented with 0.5% yeast extract and 0.1% pyruvate at pH 5.0, 6.0, 7.0, 8.0, and 9.0 were studied. Media-dependent responses of S. typhimurium revealed a more diverse fatty acid profile when grown in nutrient broth and qualitative differences were observed in the presence of C 18:1 and C 18:2 with lower pH. Scanning- and transmission-electron microscopy revealed morphological changes in viable cells grown at pH 5.0 and 9.0;Quantitative estimations of microbial populations in a food product are fundamental to assessing microbial quality of that food product. In this study we have used flow cytometer to rapidly estimate numbers of fluorescently stained microorganism and to identify injured sub populations. In order to identify a suitable fluorescent stain, we evaluated rhodamine 123, carboxy fluorescein diacetate, fluorescein diacetate, propidium iodide, and a commercially available stain, BacLight viability kit. Escherichia coli, Staphyloccus aureus and Listeria innocua were exposed to heat, acid, irradiation and starvation and stained with these dyes. Among the dyes studied, BacLight viability kit gave positive results in certain cases and it appears that viability staining with these vital stains was dependent upon the type of stress undergone by bacteria and may not be suitable for all kind of stresses. Finally, a simple column filtration using sand particles was developed to clarify meat homogenate suitable enough to quantify bacterial population in spoiling pork patties, using flow cytometry. By this method, bacterial cells in meat suspensions were stained with BacLight viability kit and were quantified rapidly.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Raveendran Jolarpettai Venugopal
Venugopal, Raveendran Jolarpettai, "Survival and growth characteristics of bacteria in irradiated meat " (1997). Retrospective Theses and Dissertations. 11752.