UC Davis

Microbial contamination of fresh produce is one of the key risk factors that can lead to a nationwide outbreak of foodborne diseases. Microbial pathogens can be transmitted to fresh produce by cross-contamination during postharvest processing, which has been traced to the contamination of food contact surfaces and the growth of biofilms on these surfaces. Thus, there is an unmet need to understand the dynamics of bacterial transfer from food contact surfaces to fresh produce and improve decontamination processes to reduce cross-contamination. To address this need, this dissertation is focused on: (i) quantitative analysis of cross-contamination between fresh produce and food contact surfaces; (ii) development of antimicrobial food contact surfaces and evaluation of their efficacy in reducing cross-contamination; (iii) mechanistic mathematical modeling of targeted antimicrobial delivery systems for improved biofilm inactivation; and (iv) advancing a fundamental understanding of barriers to decontamination of fresh produce.In this research, a reproducible methodology was elaborated to simulate cross-contamination with varying physical contact conditions. Using this method, bacterial transfer between fresh produce and food contact surfaces with different hydrophobicity was quantified. Moreover, antimicrobial plastics were developed for food contact surfaces using chlorine-based sanitizer, and their effectiveness in reducing cross-contamination was evaluated. A poly(vinyl alcohol-co-ethylene) film was selected as a model hydrophilic plastic, and chlorine was chemically bound to these films with N-halamine precursors. A polypropylene coupon was selected as a model hydrophobic plastic, and food-grade coatings were deposited on these coupons to charge chlorine using biobased N-halamine precursors. Furthermore, a mathematical modeling framework was established to understand decontamination processes. Biofilm inactivation was modeled for a conventional sanitizer (free chlorine) and biobased targeted delivery systems (chlorine stabilized with yeast microparticles) based on mass transport and biochemical reaction kinetics. A system of partial differential equations was computed using COMSOL Multiphysics® software, and the simulation results were validated with experimental data. Similarly, microbial inactivation on a leaf surface was modeled for the same conventional sanitizer and the biobased targeted delivery systems, using topomimetic leaf replicasts prepared by microfabrication. The results of these investigations using a combination of experimental and mathematical modeling approaches illustrate the followings: (i) Cross-contamination of fresh produce was a rapid process (< 5 s), which exponentially increased with the contact time, and the applied contact force resulted in a linear increase in bacterial transfer from a contaminated leaf to other surfaces; (ii) Development of antimicrobial plastics with chemically bound chlorine or food-grade coatings both significantly reduced the risk of cross-contamination between fresh produce and food contact surfaces; (iii) Design of biobased targeted delivery systems for chlorine-based sanitizers significantly enhanced the antibiofilm activity. The simulation results showed that this enhancement was attributed to propertied of yeast microparticles, which improved chemical stability of chlorine and provided binding affinity to deliver the stabilized chlorine to target biofilms; (iv) Microscale topographical features and compositions of a leaf surface could be critical barriers to decontamination of fresh produce using conventional sanitizers. Effective decontamination may require novel sanitizer compositions that can target microbes in the macroscale crevices along protruding veins or microscale grooves of a leaf surface. Overall, this research demonstrates that microbial contamination of fresh produce can be reduced by immobilizing antimicrobials on food contact surfaces using N-halamine formulations with enhanced chemical stability. Furthermore, the research mechanistically illustrates the ability of biobased targeted delivery systems in improving decontamination of biofilms or fresh produce. The combination of experimental and mathematical modeling approaches proposed in this dissertation will advance the development of novel antimicrobial compositions and their enhanced delivery to the target microbes.