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

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 H 2 O 2 delivered at 40 ml/min, UV‐C dose 114 mJ/cm 2 , 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.