STRATEGIES TO CONTROL FOODBORNE PATHOGENS ON FRESH APPLES

Lina Sheng

Consumption of fresh fruits and vegetables keeps increasing due to their health-beneficial effect. However, fresh produce has caused more illnesses and outbreaks than any other single food category. Listeria monocytogenes and Shiga toxin-producing Escherichia coli (STEC) are important foodborne pathogens frequently involved in fresh produce outbreaks and recalls. This dissertation comprehensively evaluated the antimicrobial efficacy of both chemical and plant-derived natural antimicrobials against these pathogens on fresh apples and further explored the potential mechanisms. Our storage study showed a limited reduction of L. monocytogenes inoculated on whole apples during 12-week cold storage regardless of storage temperature, inoculation level, apple variety, or agricultural practice. Therefore, the apple industry cannot rely on cold storage alone to control this pathogen. Additional interventions are needed to eradicate Listeria on fresh apples. Gaseous ozone, continuously applied at very low doses of ~87 ppb, reduced L. innocua, a non-pathogenic surrogate of L. monocytogenes, by more than 5.0 Log10 CFU/apple after 30-week storage in controlled atmosphere without any negative influence on apple fruit quality. Chlorine or chlorine-based sanitizers at industrial commonly used concentration of ~100 ppm free available chlorine, prevented L. monocytogenes cross-contamination from fruit-to-water and fruit-to-fruit, while only resulted in ~1.0 Log10 CFU/apple reduction of L. monocytogenes on apples. We further assessed the antimicrobial efficacy of plant-derived natural antimicrobial, cinnamon oil, against STEC. Cinnamon oil at 0.025% (v/v) effectively inhibited the growth of STEC including E. coli O157:H7 and CDC “top-six” non-O157 STECs in medium setting. A 2-min wash with 0.125% (v/v) cinnamon oil and 100 ppm chlorine reduced non-O157 STECs by ~2.0 Log10 CFU/apple, while 0.25-0.50% (v/v) cinnamon oil showed significantly superior inactivation efficacy than chlorine. In addition, C. cassia oil at sub-lethal concentrations suppressed the production of Shiga toxin, which is an important virulence factor of STEC, and inhibited Shiga toxin phage induction in multiple ways. In summary, foodborne pathogens contaminated on fresh produce are difficult to eliminate. The current industrial chemical intervention or storage practice are not enough to ensure fresh apple microbial safety. Low dose continuous gaseous ozone application during long-term storage or plant-derived antimicrobial wash might provide the potential intervention

STRATEGIES TO CONTROL FOODBORNE PATHOGENS ON FRESH APPLES

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