Purpose: The aim of this study was to assess the relationship between organic load and various physicochemical wash water parameters on the efficacy of sodium hypochlorite with or without two different acidifiers against Escherichia coliO157:H7 during pilot-scale production of fresh-cut iceberg lettuce.
Methods: Flume tank water containing 10% (w/v) blended iceberg lettuce and a commercial chlorine-based produce sanitizer (XY-12, Ecolab, St. Paul, MN) at a concentration of 50 ppm free chlorine (pH 8.1) was adjusted to pH 6.5 using two acidifiers - SmartWash (SW) (New Leaf Food Safety Solutions, Salinas, CA) or citric acid (CA). Sanitizer efficacy against a 4-strain, avirulent, GFP-labeled E. coli O157:H7 cocktail was then assessed in triplicate trials by processing dip-inoculated (5.4 kg) followed by uninoculated (3 x 5.4 kg each) heads of iceberg lettuce at 10 min intervals in a pilot-scale processing line from shredding through centrifugal drying with sanitizer-free water (pH 7.3) serving as the control. Shredded lettuce (25 g) and water samples (50 ml) were collected every 30 s from the 3.3-m flume tank along with additional water samples at 2-min intervals between batches. Wash water was also assessed for various physicochemical parameters, most importantly pH and oxidation/reduction potential (ORP). All samples were appropriately neutralized, diluted and surface-plated on TSAYE + ampicillin with or without membrane filtration to enumerate E. coliO157:H7.
Results: All three sanitizer treatments were more effective (P < 0.05) than water alone, with chlorine, chlorine + SW, and chlorine + CA respectively reducing E. coli O157:H7 populations 0.43, 1.54, and 1.58 logs on lettuce. Average E. coli O157:H7 levels in the water after processing were significantly lower (P < 0.05) using chlorine + CA (0.53 log CFU/ml) or chlorine + SW (-0.04 log CFU/ml) at pH 6.5 than chlorine alone (2.96 log CFU/ml) at pH 8.09. Oxidation/reduction potential was significantly higher using CA and SW (P < 0.05) than chlorine alone.
Significance: Both SmartWash and citric acid effectively increased the efficacy of sodium hypochlorite against E. coli O157:H7 populations in wash water containing a 10% organic load, thereby decreasing the extent of cross contamination. However, sanitizer use alone cannot ensure end product safety.