This study was undertaken to determine the effect of acid adaptation on the survival and growth of Escherichia coli in broth and fermented goat milk. In the first phase of the study, acid resistance of E. coli O157:H7 strain UT 10 and E. coli ATCC 25922 was determined in brain heart infusion broth at pH 7.4, 4.5 and 2.5. Variations due to acid stress in the counts of both strains were determined. Alterations in the fatty acid profile due to acid adaptation were also determined. Acid adaptation enhanced the survival of both strains at pH 4.5, but neither strain could survive after 4 h at pH 2.5. At optimum growth conditions (pH 7.4), E. coli ATCC 25922 exhibited increased viability over E. coli UT 10. At pH 4.5, E. coli UT 10 was more tolerant to low pH than E. coli ATCC 25922. An increase in saturated fatty acids (SAFAs) of both AA strains was observed, indicating the importance of lipid modification in enhancing survival at low pH. In the second phase of the study, acid resistance of E. coli O157:H7 strains UT 10 and UT 15 were determined in commercial fermented at 30 °C for 24 h and stored at 7 °C for 2 days and traditional Amasi fermented for 3 days at ambient temperature (ca 30 °C). E. coli O157:H7 counts in traditional Amasi could not be detected after 3 days of challenge while those in commercial Amasi were detected at 2.7 log10 cfu/ml after the same period. The survival of acid adapted (AA) and non-adapted (NA) strains was similar in traditional Amasi while in commercial Amasi, the NA strain survived significantly better than the AA strain. The results of this study indicate that E. coli O157:H7 can survive during fermentation and storage of fermented goat milk Amasi. They also show that survival of the pathogen in a broth model differs from that in a food system. Survival of E. coli O157:H7 in fermented milk is affected by fermentation time, pH and storage temperature. The food industry should therefore adapt their processing/ preservation procedures by taking the most acid tolerant pathogenic E. coli strains into consideration in order to ensure the safety of their products. / Dissertation (MSc(Agric))--University of Pretoria, 2009. / Food Science / unrestricted

Foodborne illnesses caused by Listeria monocytogenes have long been associated with ready-to-eat (RTE) meats contaminated after the primary thermal process has been applied. It is believed that brine solutions used to chill cooked RTE products may serve as a reservoir for L. monocytogenes becoming a potential point of post-processing contamination for RTE meats. Re-circulating ultraviolet light (UV) systems are being used to inactivate L. monocytogenes in chill brines; however very little has been reported on the dose response of healthy and stressed L. monocytogenes to UV in brine solutions. The objectives of this research were to determine 1) minimum dose of UV required to inactivate L. monocytogenes in distilled water, fresh brine, undiluted spent brine, and diluted spent brine, 2) if adaptation to food processing stresses affects the dose response, and 3) if the acquisition of antibiotic resistance mechanisms provides resistance to ultraviolet light 4) effect of stress adaptation on survival in brine solutions. After UV exposure, populations were reduced as follows from greatest to least: water > fresh brine > 5% spent brine > 35% spent brine > 55% spent brine > 100% spent brine (P ≤ 0.05). There were no population differences between acid stressed and antibiotic resistant or healthy and heat shocked (P > 0.05). However, acid-stressed and sulfanilamide-resistant were more resistant to UV light than healthy and heat shocked L. monocytogenes (P ≤ 0.05). Survival in brine solutions (no UV) followed the trend, from greatest to least (P ≤ 0.05): sulfanilamide-resistant > acid-stressed > healthy > heat-shocked. Population estimates decreased from initial inoculation to final sampling for each cell type suspended in spent brine (P ≤ 0.05), but only healthy and heat- shocked cells suspended in fresh brine were significantly reduced (P ≤ 0.05). Knowledge of UV dosing required to control L. monocytogenes in brines used during RTE meat processing, and a greater understanding of the interactions that may influence dose will aid manufacturers in establishing appropriate food safety interventions for these products. / Ph. D.