Physicochemical Parameters as Predictors of Sanitizer Efficacy against Escherichia coli O157:H7 in Leafy Green Wash Water Containing Sanitizers and Organic Load

Introduction: Sodium hypochlorite is routinely used in flume tanks during leafy green processing to minimize cross-contamination. However, given the increasing number of leafy-green-associated recalls and outbreaks, improved methods are needed for commercial monitoring of sanitizer efficacy containing high organic loads.

Purpose: Consequently, this study aimed to 1) determine the ability of acidified and non-acidified sodium hypochlorite to reduce Escherichia coli O157:H7 populations in simulated processing water in a bench-top model and 2) assess the relationship between various physicochemical parameters and organic load of the wash water on Escherichia coli O157:H7 inactivation.

Methods: A spigoted glass carboy was used for efficacy testing of a chlorine-based produce sanitizer (XY-12, Ecolab, St. Paul, MN) containing 50 ppm available chlorine at  pH 7.51 or 6.50 (acidified with T-128, New Leaf Food Safety Solutions, Salinas, CA or citric acid (CA)) in triplicate against a 4-strain avirulent, GFP-labeled E. coli O157:H7 cocktail in 4-l of  wash water containing 0, 2.5, 5 or 10% (w/v) blended iceberg lettuce, with sanitizer-free water serving as the control. Mesh bags containing 25 g of dip-inoculated (6.10 log CFU/g) iceberg lettuce were exposed to the wash water for 90 s after which five water  samples (50 ml) were collected through the spigot at 2 min intervals, immediately neutralized, appropriately diluted and surface-plated on TSAYE + amp with or without membrane filtration to quantify E. coli O157:H7, with non-neutralized samples also assessed for Chemical Oxygen Demand (COD), Oxidation/Reduction Potential (ORP), total solids, turbidity, and maximum filterable volume (MFV) using a 0.45 µm membrane.

Results: E. coli O157:H7 populations were significantly lower (P < 0.05) in wash water containing 0 (-2.00 to -1.36 log CFU/ml) as opposed to a 2.5, 5, or 10% organic load (1.87 to 3.74 log CFU/ml) at all sampling times for all three treatments. Using a 2.5% organic load, E. coli O157:H7 inactivation rates were -0.10, -0.20, and -0.14 log CFU/ml per min for chlorine, chlorine + CA, and chlorine + T-128, respectively, with these rates significantly impacted (P < 0.05) by COD, ORP, total solids, turbidity, and MFV for all three chlorine treatments.

Significance: Organic load reduced the efficacy of all three chlorine treatments against E. coli O157:H7, increasing pathogen persistence in the water. Based on these findings, COD, ORP, total solids, turbidity, and MFV can be useful predictors of sanitizer efficacy, giving leafy greens processors a means of better assuring end-product safety.