Efficacy of decontamination and a reduced risk of cross-contamination during ultrasound-assisted washing of fresh produce

Contamination of fresh produce with bacteria can occur during cultivation, harvest and post-harvest processing and the current sanitation approaches for post-harvest processing are not adequate for inactivation of pathogenic or spoilage bacteria that may be attached to the surface of fresh produce. This study evaluated the role of ultrasound with and without surfactants and sanitizers to improve decontamination of fresh produce and the impact of these treatments on the quality of fresh produce. Gram-positive bacterial strain Listeria innocua, and gram-negative bacterial strains Escherichia coli O157:H7 and Pseudomonas fluorescens were tested in this study. The reduction in bacterial concentration from inoculated leaf samples increased with an increase in ultrasound treatment time until 10 min for all the three strains. When compared to ultrasound treatment alone, combination of ultrasound treatment with surfactants did not significantly increase the removal of E. coli and L. innocua from the lettuce surfaces, however, the removal efficacy of P. fluorescens increased significantly under the same treatment conditions. The results of simultaneous washing of multiple leaves using ultrasound showed the net bacterial reduction from lettuce leaves decreased by approximately 0.5 log CFU/cm2 for all the selected strains compared to treatment of a single leaf. Similarly, combination of ultrasound with surfactants for simultaneous treatment of multiple leaves did not improve removal of E. coli and L. innocua from the surface of lettuce leaves, but improved removal of P. fluorescens by approximately 1 log unit when compared to ultrasound treatment alone. The evaluation of the risk of potential cross-contamination during ultrasound washing suggested ultrasound could significantly reduce the bacterial attachment to the uninoculated lettuce surface. Combination of ultrasound with adequate active free chlorine could prevent the transfer of E. coli and L. innocua cells from the inoculated leaf sample to uninoculated leaves. Evaluation of lettuce quality attributes in our study found that exposure to ultrasound treatment for less than 10 min had no significant effect on the quality of fresh lettuce. Thus, ultrasound treatment has a potential to be an alternative for maintaining the safety and quality of fresh produce during post-harvest processing.