Addition of chemical sanitizers during commercial flume washing of leafy greens remains the sole microbial mitigation strategy. However, continued Escherichia coli O157:H7 outbreaks have raised concerns regarding recirculation of wash water, with the accumulation of organic load during processing leading to decreased efficacy of chlorine-based sanitizers. Reliable methods to quantify the impact of organic load on sanitizing efficacy do not yet exist.Initially, the efficacy of six different wash treatments (water alone, 50 ppm peroxyacetic acid, 50 ppm mixed peracid, or 50 ppm available chlorine either alone or acidified to pH 6.5 with citric acid (CA) or T-128) was assessed using 5.4 kg of iceberg lettuce inoculated to contain 106 CFU/g of a 4-strain non-toxigenic, GFP-labeled, ampicillin-resistant cocktail of E. coli O157:H7 in a pilot-scale leafy green processing line consisting of a commercial shredder, conveyor, flume tank, shaker table, and centrifugal dryer. Without an organic load in the water, none of the sanitizers were more effective (P ≤ 0.05) than water alone at reducing E. coli O157:H7 populations on lettuce, with reductions ranging from 0.8 to 1.4 log CFU/g. However, chlorine, chlorine + CA, and chlorine + T-128 were generally more effective (P ≤ 0.05) than the other treatments against E. coli O157:H7 in the flume water, with reductions of 3.8, 5.5, and 5.4 log CFU/ml after 90 s of processing, respectively. Thereafter, a novel and cost-effective carboy system was developed to assess the efficacy of the same five sanitizing agents against E. coli O157:H7 in wash water containing an organic load of 0 to 10% (w/v) blended lettuce. After iceberg lettuce previously inoculated to contain E. coli O157:H7 at 106 CFU/g was washed for 90 s, E. coli O157:H7 persistence was subsequently correlated to various physicochemical parameters of the wash water. Organic load negatively impacted the efficacy of chlorine, chlorine + CA, and chlorine + T-128 (P ≤ 0.05), with typical E. coli O157:H7 reductions of < 1 log CFU/ml after 10 min of exposure. However, the efficacy of peroxyacetic acid and mixed peracid was unaffected by organic load (P > 0.05), with average E. coli O157:H7 reductions of ~4.8 and ~5.5 log CFU/ml, respectively, after 10 min of exposure. Finally, efficacy of the same five sanitizer treatments was assessed against E. coli O157:H7 on iceberg lettuce, in wash water, and on surfaces of a pilot-scale processing line using flume water containing an organic load of 0 to 10% (w/v) blended lettuce. Organic load negatively impacted the efficacy of all three chlorine treatments (P ≤ 0.05), with typical E. coli O157:H7 reductions of > 5 log CFU/ml by the end of processing with no organic load in the wash water and 0.9 - 3.7 log CFU/ml with a 10% organic load. Organic load rarely impacted (P > 0.05) the efficacy of either peroxyacetic acid or mixed peracid, with typical reductions of > 5 log CFU/ml in wash water throughout processing for all organic loads. Sanitizer efficacy against E. coli O157:H7on lettuce was seldom impacted by organic load. In both the carboy system and the pilot-scale processing line, reduced sanitizer efficacy generally correlated to increases (P ≤ 0.05) in total solids, chemical oxygen demand and turbidity, and decreases (P ≤ 0.05) in maximum filterable volume, indicating that these tests may be effective alternatives to the industry standard of oxygen/reduction potential. These findings demonstrate that monitoring of both sanitizer concentration and wash water quality is critical to minimizing the likelihood of amplifying a previously isolated contamination event.
Read
