Food Safety from Farm to Fork: Microbial Attachment and Resilience on Spinach Surfaces

Foodborne illnesses lead to 400,000 deaths annually worldwide, and microbial contamination of fresh produce continues to pose a risk to human health in even the most developed countries. In the United States, 1 in 6 Americans are impacted by foodborne illness every year as the large number of steps that food goes through from farm to fork makes effectively tracing the source of a foodborne outbreak all the more difficult. Modern food production and distribution systems create complex and dynamic environments in which bacteria may attach and proliferate on produce surfaces, which has created a challenge for developing scientifically rigorous food safety regulations. Therefore, this doctoral research aimed to investigate the effects of various environmental conditions on bacterial adhesion, detachment, and disinfection on leafy greens using colloidal transport theory and fundamental research models. Overall, the results of this dissertation work indicate that the likelihood of bacterial attachment onto produce surfaces will vary throughout the watering, washing, and rinsing processes that take place between harvest and consumption. Employing a suite of environmentally relevant bacteria, the kinetics of bacterial attachment and removal from spinach epicuticle surfaces was investigated using a parallel-plate flow cell that allows for simulated gentle rinsing and real-time observation and enumeration of cells. Environmental factors like restricted nutrient availability and complex water chemistry increased attachment rates of foodborne bacteria to the leaf surface, which was attributed to decreases in cell surface charge and changes in extracellular polymers in favor of proteins. The introduction of copper oxide nanoparticles enhanced adhesion and minimized detachment of bacteria from spinach leaf surfaces, while nano-titanium dioxide had the opposite effect, highlighting potential unintended consequences of their application as pesticides or fertilizers in agricultural waters. By modeling standard chlorine disinfection rinses, leaf surface roughness was found to reduce the effectiveness of bleach in removing and inactivating pathogens adhered to spinach leaves. An alternative, enzyme-based disinfectant was produced and shown to increase bacterial detachment from the spinach surface and prevent biofilm formation on polycarbonate. Further, this doctoral research addresses the gap between scientists and policymakers through two projects that identify and explore mechanisms for translating scientific data into public policy change for promoting sustainable diets in the developing world and managing diverse, statewide water data in California. As a whole, this work has elucidated technical and political challenges and opportunities in ensuring food safety and security that warrant further research to inform scientifically rigorous policy and regulations