Inactivation of Escherichia coli on Romaine Lettuce Using a Gas-Phase Hydroxyl-Radical Process: From Laboratory Scale to Commercial Processing

Abstract

The following reports on the efficacy of a gas-phase hydroxyl radical-based process for decontaminating shredded lettuce on a laboratory and simulated commercial scale. The process is based on the ultraviolet light at 254 nm UV-C-mediated degradation of hydrogen peroxide mist and ozone gas to generate antimicrobial hydroxyl radicals. Escherichia coli K12 was applied as a surrogate for E. coli O157:H7, and at laboratory scale, the hydroxyl-radical process (1.5% vol/vol H₂O₂ delivered at 40 ml/min, UV-C dose 114 mJ/cm², 20 ppm ozone, 29°C chamber temperature, and 30 s residence time) could support a 1.63 ± 0.61 log CFU reduction. This is compared to the 0.57 ± 0.18 log CFU reduction obtained for a chlorine-based wash. In scale-up, batches (2-10 kg) of E. coli inoculated romaine lettuce were passed through sequential hydroxyl-radical reactors. Here, the units were elevated to create a cascade effect, with the hydrogen peroxide mist being introduced as an intermister between the reactors. It was found that the three units placed in sequence with intermisters supported a 2.05 ± 0.10 log CFU reduction of E. coli, thereby verifying that homogenous treatment had been achieved. Additional trials operated the hydroxyl-radical process at 4°C without loss of performance. The hydroxyl-radical process was not negatively affected by applying a pretreatment wash. The study has demonstrated that the hydroxyl-radical process can be applied as an alternative to postharvest wash to enhance the food safety of romaine lettuce.