Combination of Ultrasound and Near Neutral Electrolyzed (NEO) Water in Reducing Escherichia coli O157:H7 and Salmonella Typhimurium DT 104 Inoculated on Romaine Lettuce Leaves

Introduction: Increased consumption of fresh produce has led to increased reports of foodborne illnesses associated with them, which may largely be attributed to the lack of effective cleaning treatment before consumption. Leafy greens (lettuce, cabbage, etc.) form a significant proportion of these reported cases.

Purpose: The aim of the study was to ascertain the effects of a combined ultrasound and NEO water (155 mg/l chlorine at pH 6.5) treatment on the reduction of Escherichia coli O157:H7 and Salmonella Typhimurium DT 104 inoculated on romaine lettuce leaves.

Methods: Whole lettuce leaves (80 g) were spot-inoculated with 100 μl of a mixture of 5 strains of either E. coli O157:H7 or Salmonella Typhimurium DT 104 and treated in a beaker containing 1.6 l of deionized (DI) or NEO water with ultrasound. Different treatment times (5, 10 and 15 min) and ultrasound power (130 and 210 W) were tested. Treatments without ultrasound served as a control and six samples of each treatment were analyzed.

Results:  NEO water use (compared to DI water), longer duration of treatment and higher ultrasound power resulted in significantly increased reductions of the pathogens (P < 0.05). The 15 min-210 W, NEO water combination led to the highest reductions, with 4.4 and 4.3 log reductions of E. coli O157:H7 and Salmonella Typhimurium, respectively, compared to < 2.8 log reductions for treatments without ultrasound for both pathogens. Treatments with NEO water completely inactivated pathogens in wash solutions however washing with DI water did not.

Significance:  Results obtained in this study suggest that applying ultrasound to NEO water treatments led to additional reductions in the pathogens of concern inoculated on romaine lettuce and therefore this technique can be applied to ensure produce safety.