Purpose: This study evaluated ACP treatment uniformity and efficacy when applied to bulk Romaine lettuce leaves, inoculated with E. coli O157:H7and stacked in a commercial plastic clamshell container. Color, surface morphology, carbon dioxide (CO2) generation, and weight loss of the leaves were also determined for leaves stacked in different positions in the container.
Methods: Romaine lettuce samples, washed and inoculated with E. coli O157:H7 (~6 log CFU/g lettuce), were packed in a model bulk packaging configuration: three rows and 3 or 5 layers (container: 14.8 × 12.8 × 2.7 cm). Packaged lettuce was treated with ACP (gap distance: 3.0 cm) at 61.2 kV (1.3 A) for 10 min. Leaves were removed in order and analyzed for E. coliO157:H7 inhibition, color (Hunter Lab), and weight loss of the samples (three replications). Results were correlated with the leaf position within the bulk stack, i.e., proximal to or distal from the electrodes in this prototype.
Results: ACP reduced E. coli O157:H7 on packaged lettuce by 0.4±0.2 - 0.8±0.2 log CFU/g lettuce, with no significant correlation to where the leaves were located in each stacked configuration (P > 0.05). Respiration, leaf weight, and leaf color (L*, a*, and b* values) were not significantly different (P > 0.05) for leaves from any position in the bulk stack, regardless of the number of layers.
Significance: ACP is a promising method for inactivation of E. coli O157:H7 on Romaine lettuce leaves within commercial packages as a terminal processing step. Container headspace volume and orientation of lettuce with respect to discharge electrodes are key areas for further research and development for this technology.