Date of Award
Doctor of Philosophy
Veterinary Microbiology and Preventive Medicine
James S. Dickson
Survival and recovery of Listeria monocytogenes in the ready-to-eat (RTE) meat processing environment were studied, from environmental contamination to intervention. This pathogen survived in simulated dust using sand for more than 70 d. Longer daily survival rate and better recovery were obtained when inoculated sand samples were stored at lower temperatures with higher relative humidity levels. L. mononocytogenes survived and grew once in contact with RTE meats. No significant differences (P > 0.05) in attachment were observed for the different selected RTE meats. Desiccated cells of L. monocytogenes were not observed to go into viable but non-culturable (VBNC) state well indicating that current methods of using nonselective media, tryptic soy agar with 0.6% yeast extract, is adequate in recovering the majority of the viable cells. Addition of pyruvate was comparable but anaerobic incubation had less recovery. Five individual strains and a mixed cocktail of all five showed no significant differences (P > 0.05) in attachment characteristics. L. monocytogenes (86%) attached strongly to RTE meats within 5 min. No significant differences (P > 0.05) were observed in cell surface hydrophobicity and cell surface charge among strains. However, using cationized ferritin, culture age affected the cell surface charge. Processed meats have a relatively hydrophobic surface as observed from contact angle measurements. Once L. monocytogenes was inoculated onto the surfaces, the angles were altered. Low dose irradiation to provide safety while minimizing organoleptic changes is possible. A dose of 1.5 kGy was established for 3-log reduction while 2.5 kGy for 5-log reduction of L. monocytogenes contaminated RTE meats based on nonselective media. Preliminary results showed no growth in meats irradiated at 4.0 kGy. Survivors were observed for irradiated meats at 2.0 kGy stored at 10°C after the second week. No growth was observed in samples irradiated at 2.0 kGy stored at 4°C until the fifth week. A sensory evaluation was conducted with meats irradiated at 1.5 kGy. Consumers could not detect any differences (P > 0.05) between irradiated and nonirradiated frankfurters but differences (P < 0.05) were detected between irradiated and nonirradiated sliced meats.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu
Chi Ching Sally Foong
Foong, Chi Ching Sally, "The ecology of Listeria monocytogenes in ready-to-eat (RTE) meats " (2003). Retrospective Theses and Dissertations. 581.