Purdue University Graduate School
STWu-phD-dissertation_20200425.pdf (2.94 MB)

Assessing Listeria monocytogenes contamination risk using predictive risk models and food safety culture management in retail environments

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posted on 2020-04-28, 14:06 authored by Tongyu WuTongyu Wu

Retail environments are critical transmission points for Listeria monocytogenes to humans. Past studies have shown L. monocytogenes contamination varies widely across retail environments. L. monocytogenes can transmit among environmental surfaces and subsequently from environment to food via cross-contamination. Modified SSOPs (sanitation standard operating procedures) have been shown to have limited impact on reducing L. monocytogenes prevalence in retail deli environments. Food safety culture and climate, such as beliefs, values, and hygiene behaviors, have been identified as factors impacting food safety performance and microbial outputs. Handwashing and its compliance are among the most prominent personal hygiene aspects subjected to investigation in the past decade, illustrating hygiene behavior as a risk factor and an important consideration to ensure food safety. Additionally, effective management and well-designed infrastructure, such as vertical and lateral communication, employees’ training, accountability, and equipment designed to prevent cross-contamination, have also been described as critical contributors to a sustainable food safety program. However, given such a deadly foodborne pathogen as L. monocytogenes, the correlation between food safety culture and its prevalence remains unknown. We hypothesized that there was a relationship among food safety culture management, infrastructure, and L. monocytogenes prevalence at retail. Our goal is to identify additional risk factors on L. monocytogenes control, develop feasible recommendations, and direct resources to enhance food safety.

In the present dissertation, we developed and implemented a predictive risk model, along with employee- and management-implemented SSOPs, in 50 deli establishments across six U.S. states to evaluate and control L. monocytogenes contamination risk and prevalence (Chapter 2). The predictive risk model, based on logistic regression, uses five environmental sites to predict L. monocytogenes prevalence in the entire deli environment. It identified 13 high-risk stores, seven of which were confirmed during subsequent monthly sampling. We found that deep clean intervention reduced L. monocytogenes prevalence on non-food contact surfaces both immediately after the intervention and during follow-up, with marginal significance (αadj=0.0125). The employee- and management-implemented deep clean can control L. monocytogenes prevalence in retail delis; the predictive risk model, though conservative, will require further validations and can be useful for surveillance purposes.

Complementary to the above study, we tackled the L. monocytogenes challenge via food safety culture and climate approach (Chapter 3). Concurrently to the monthly environmental sampling, we distributed food safety culture and climate survey to the 50 stores, with one manager and up to five associates from each establishment, over a 12-month period and overlapped with before, after, and follow-up deep clean. We found that stores with lower L. monocytogenes contamination risk had better food safety culture, including greater sense of commitment to food safety program (padj=0.0317) and more complete training (padj=0.0117). Deep clean improved managers’ (padj=0.0243) and associates’ (padj=0.0057) commitment to food safety. This study indicates that food safety culture and climate are crucial component in building a viable, sustainable food safety program.

Another survey tool was used to evaluate infrastructure designs, management strategies, and sanitation practices in relation to L. monocytogenes control in retail produce environments (Chapter 4). We distributed the survey to 30 retail produce departments across seven U.S. states. Hand hygiene, minimizing cross-contamination, and maximizing equipment cleanability were the most prominent factors in L. monocytogenes control.


Purdue University Graduate School Andrews Fellowship

The Foundation for Meat and Poultry Research and Education

USDA NIFA Hatch project 2016-67017-24459

USDA-NIFA-AFRI 2015-05673

Neogen Corp (Lansing, MI)


Degree Type

  • Doctor of Philosophy


  • Food Science

Campus location

  • West Lafayette

Advisor/Supervisor/Committee Chair

Haley F. Oliver

Additional Committee Member 2

Stephen Lindemann

Additional Committee Member 3

Rebekah Klein-Pejšová

Additional Committee Member 4

Bruce Craig

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