Effects of direct and in-package pulsed light treatment on inactivation of E. coli O157:H7 and reduction of microbial loads in Romaine lettuce

The microbial safety of fresh produce continues to be a real concern. Novel nonthermal technologies are required to lessen the risk of pathogen contamination. The objective of this research was to develop and evaluate the effects of direct and in-package pulsed light (PL) treatment on the survival of E. coli O157:H7 in Romaine lettuce. Treatment influence on reduction of background microbial loads of Romaine lettuce was also explored. A three strain-cocktail of E. coli O157:H7 was selected for inoculum preparation for their link with foodborne outbreaks. Surface inoculated Romaine lettuce pieces (2.5 × 2.5 cm) was subjected to pulsed light treatment for a maximum of 1 min (63 J/cm²). Polyethylene (PE) films of 0.00254, 0.00508 and 0.00762 cm thickness with ample UV transmission (54–83%) ability were used for packaging. PL treatment of 10 s, equivalent to a dose of 10.5 J/cm², was considered optimum beyond which wilting of leaves was observed. Both direct and in-package treatment provided greater than 1 log reduction of the pathogen in 1 s (1.05 J/cm²). Direct treatment resulted in 2.68 ± 0.37 log CFU/g reduction of E. coli O157:H7 at optimal dose, whereas log reductions were decreased to 2.52 ± 0.19, 2.31 ± 0.34 and 2.18 ± 0.25 log CFU/g for Romaine lettuce in 0.00254, 0.00508 and 0.00762 cm thickness packaging enclosures respectively. The decrease in log reductions was not significantly (P > 0.05) affected by film thickness. No significant difference (P > 0.05) in E. coli O157:H7 decontamination efficacy between packaged and unpackaged Romaine lettuce was observed due to PL treatment. The initial total aerobic bacteria and mold and yeast populations were also reduced significantly (P < 0.05) by >1 log, due to the treatment. Overall, the results of this work demonstrate that PL treatment may be used to enhance microbial safety and reduce post processing contamination of packaged Romaine lettuce.