Date of Award


Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)


Food Science and Human Nutrition


Alfred A. Bushway

Second Committee Member

Beth Calder

Third Committee Member

Vivian Wu


Foodborne illness is a major public health concern that costs the United States billions of dollars every year and results in millions of Americans getting sick. Three bacterial foodborne pathogens responsible for many of these illnesses are Salmonella, Escherichia coli (E. coli) O157:H7, and Listeria monocytogenes (L. mono). Outbreaks of foodborne illness in which acidified foods such as mayonnaise have been implicated have brought into question the perceived safety of these and other low-pH food products. Since prior research has focused on dairy-based salad dressings, this thesis examined the survival of Salmonella, E. coli 0157:H7, and L. mono in oil and vinegar-based dressings. Two vinaigrettes, a fig-based (fig) and strawberry-based (berry) dressing, were provided by a commercial manufacturer. The dressings were adjusted to pH 3.3 and pH 3.9 and held at room temperature (20-25°C) for the 14 day duration of this study. A three serotype or strain mixture of each pathogen was used to inoculate dressing samples at high (~5 log CFU/ml) and low (~2 log CFU/ml) levels. Microbiological analyses were performed on days 0 (within 1 hour post inoculation), 1, 3, 7, 10, and 14. Enrichment procedures were performed on samples in which no growth was observed. Additionally, all day 14 samples were subjected to enrichment. The type of dressing, pH, initial inoculum level, and the combined effects of these factors had significant (p<0.05) effects on pathogen survivability in dressings. In general, a faster die-off of pathogens was observed in fig dressing over berry. Pathogen death occurred more rapidly in samples adjusted to pH 3.3 than those adjusted to pH 3.9. The fig dressing had 1.73% titratable acidity calculated as acetic acid compared to 1.08% titratable acidity calculated as acetic acid in the berry dressing. Acetic acid has been shown to be bactericidal; therefore the higher percentage of acetic acid was probably responsible for the observed differences in pathogen survivability between the two dressings. Of the three pathogens used in this study, Salmonella was most rapidly inactivated, with no viable cells recovered within 1 to 24 hours in most samples. Only 1 or 2 colonies of E. coli 0157:H7 were recovered in 2 samples on day 10. L. mono displayed the greatest survivability in the dressings, with viable cells still recovered by direct plating in all berry samples on day 14. Results of this study demonstrate the efficacy of low pH and acetic acid content in controlling pathogen growth in oil and vinegar-based salad dressings. However, this research shows that L. mono exhibits a tolerance to the low pH environments present in these products and viable cells are able to persist for extended periods of time, suggesting that L. mono could become an issue for acidified food processors. Future research should investigate this trend to support the safety of acidified foods.