Date of Graduation

5-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Food Science (PhD)

Degree Level

Graduate

Department

Food Science

Advisor/Mentor

Acuff, Jennifer C.

Committee Member

Gibson, Krusten E.

Second Committee Member

Adams, Richard

Third Committee Member

Kwon, Young Min

Keywords

Environmental factors; Harborage; Hygiene; Low-water activity; Salmonella

Abstract

Low-moisture foods (LMFs) were not historically seen as safety risks due to their ability to inhibit pathogen growth, but evidence shows pathogens can survive in LMFs long-term, causing outbreaks and recalls. Previous studies have lacked the ability to replicate real-world contamination scenarios in LMF processing environments. This research aimed to develop inoculation methods to form LMF persistent bacterial populations (LMF PBPs) and assess pathogen responses to environmental factors, culturing methods (broth vs. lawn), and matrix type and ratio. Inoculation methods tested were “direct” (combining bacterial culture and powder on a stainless-steel coupon) and “slurry” (premixing culture and powder). Matrices included three food types (nonfat dry milk powder [NFDM], peanut butter powder [PB], and chicken powder [CP]), a nonfood matrix (silicon dioxide [SiO2]), a nonfood matrix (silicon dioxide [SiO2]), and desiccated Salmonella without a matrix (Sal). Salmonella Tennessee (K4643), linked to peanut butter outbreaks, was used. The direct method involves sprinkling NFDM and SiO2 (0.1 or 0.4 g) onto the coupons inoculated with 0.5 mL of Salmonella. The slurry method (0.5 mL culture to 0.4 g powder) gave consistent results, showing lower Salmonella reduction (1.5–1.6 log CFU/cm2) in NFDM and culture alone LMF PBPs. It was chosen for further tests comparing liquid- and plate-grown cultures. Proceeding with the plate-grown cells using the lawn-based methods, temperature and relative humidity (RH) levels were selected to examine the impacts of environmental factors, in addition to comparing nutrient composition of matrices and storage time on the reductions of Salmonella in LMF PBPs. The coupons were stored for seven or 28 days at 30, 50, or 70% RH at 25 or 35 °C and sampled on predetermined days. Lower reductions were observed for Salmonella LMF PBP made using lawn-grown (0.3–2.7 log CFU/cm2) compared to broth (1.0–5.1 log CFU/cm2) for all food matrices. For both culturing methods, Salmonella in LMF PBP with SiO2 (2.7–5.1 log CFU/cm2) reduced significantly (p < 0.05) more than the food matrices (0.7–2.2 log CFU/cm2) and Sal without a matrix (0.3–1.5 log CFU/cm2). The RH levels, temperature, matrix, and their interactions significantly (p < 0.05) affected the reductions of Salmonella. The significantly greater reductions as the RH increased were specifically observed in LMF PBPs with SiO2. Temperature and RH had no significant effect on Salmonella LMF PBPs in NFDM and Sal. However, matrix type and storage days significantly impacted log reductions (p < 0.05) of Salmonella LMF PBP. Salmonella reductions in LMF PBP with PB were significantly (0.5 ± 0.3 log CFU/cm2) lower compared to the reductions observed on other food matrices (1.9-3.6 ± 0.1 log CFU/cm2). The findings indicate that the LMF PBP method produces a stable population of Salmonella, which is important for evaluating cleaning and sanitation interventions where significant log reductions need to be demonstrated. Future research, using the methods established in the present study, should explore the long-term impact of varying environmental conditions, such as fluctuating humidity or cleaning and sanitation, on the persistence of LMF PBPs formed using the slurry method.

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