Ultrasound Enhanced Sanitizer Efficacy in Reduction of Escherichia coli O157:H7 Population on Spinach Leaves

The use of ultrasound to enhance the efficacy of selected sanitizers in reduction of Escherichia coli O157:H7 populations on spinach was investigated. Spot-inoculated spinach samples were treated with water, chlorine, acidified sodium chlorite (ASC), peroxyacetic acid (POAA), and acidic electrolyzed water with and without ultrasound (21.2 kHz) for 2 min at room temperature. The effects of ultrasound treatment time and acoustic energy density (AED) were evaluated at an ASC concentration of 200 mg/L. The effect of ASC concentration, with a fixed AED of 200 W/L, was also examined. Microbial analysis indicated that ASC reduced E. coli O157:H7 population by 2.2 log cycles over that of water wash, while the reduction from other sanitizers was about 1 log cycle. Ultrasonication significantly enhanced the reduction of E. coli cells on spinach for all treatments by 0.7 to 1.1 log cycle over that of washes with sanitizer alone. An increase in the ASC concentration enhanced the efficacy of the combined treatment of ASC and ultrasonication, especially at ASC concentrations of < 300 mg/L. Increasing the ultrasound treatment time from 0 to 4 min and AED from 0 to 500 W/L were both effective in increasing the effectiveness of the ASC and ultrasound combined treatments. In addition, E. coli O157:H7 inoculated on the underside of spinach leaves (rough side) were more difficult to remove than those inoculated on the upper side (smooth side).