Honors Theses

Date of Award


Document Type

Undergraduate Thesis



First Advisor

Colin Jackson

Relational Format



Outbreaks of foodborne illness are of concern to many Americans. While pasteurization of ingredients and freezing eliminates most microbiological hazards, ice cream can act as a vehicle for pathogen transmission and be a cause of foodborne disease. This study examined the bacterial content of seven nationally distributed brands of ice cream and one local farmer's market brand. The aims of the study were to (1) determine if there was a higher bacterial content in chocolate and strawberry ice cream compared to vanilla ice cream, potentially because of more ingredients and more opportunities for bacterial contamination in those flavors, and (2) determine if the local farmer's market ice cream contained a greater number of bacteria than the commercial brands, possibly because of reduced processing and less regulation. Ice cream samples were incubated on tryptic soy agar and milk agar plates, the number of colony forming units was determined, and representative cultures from each sample were identified using 16S rRNA gene sequencing. While more bacteria were found in chocolate ice cream than strawberry or vanilla, high variability in bacterial counts between brands meant that differences between flavors were not statistically significant (p=0.15-0.20). In terms of brand, the greatest number of bacteria were found in samples from the farmer's market and the bargain bulk brand, both of which gave counts of >30 million bacteria per pint container. The lowest bacterial counts were found in brands that had plastic seals around the rim of the container, suggesting that packaging style is an important factor. Bacillus cereus, a common cause of food poisoning, was identified in all brands except for the farmer's market brand, while Escherichia coli was identified in three brands.

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