Decontamination of Escherichia coli O157:H7 on green onions using pulsed light (PL) and PL-surfactant-sanitizer combinations

Imported green onion has been associated with three large outbreaks in the USA. Contamination has been found on both domestic and imported green onions. The objective of our study was to investigate Escherichia coli O157:H7 inactivation efficacy of pulsed light (PL) as well as its combination with surfactant and/or sanitizers on green onions. Green onions were cut into two segments, stems and leaves, to represent two different matrixes. Stems were more difficult to be decontaminated. Spot and dip inoculation methods were compared and dipped inoculated green onions were found to be more difficult to be decontaminated. Results showed that 5 s dry PL (samples were not immersed in water during PL treatment) and 60 s wet PL (samples were immersed in water and stirred during PL treatment) treatments provided promising inactivation efficacy (> 4 log10 CFU/g) for spot inoculated stems and leaves. For dip inoculated green onions, 60 s wet PL treatment was comparable with 100 ppm chlorine washing, demonstrating that PL could be used as an alternative to chlorine. To further increase the degree of microbial inactivation, combined treatments were applied. PL combined with surfactant (SDS) was found to be more effective than single treatments of PL, SDS, chlorine, citric acid, thymol, and hydrogen peroxide, and binary combined treatments of PL with one of those chemicals. Addition of chlorine or hydrogen peroxide to the PL-SDS combination did not further enhanced its microbial inactivation efficacy. The combination of PL and 1000 ppm of SDS reduced the E. coli O157:H7 populations dip inoculated on the stems and leaves of green onions by 1.4 and 3.1 log10 CFU/g, respectively. Our findings suggest that PL could potentially be used for decontamination of E. coli O157:H7 on green onions, with wet PL added with SDS being the most effective PL treatment.