Submitted to: International Journal of Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/2009
Publication Date: 5/1/2009
Citation: Keskinen, L.A., Burke, A.M., Annous, B.A. 2009. Efficacy of chlorine, acidic electrolyzed water and aqueous chlorine dioxide solutions to decontaminate Escherichia coli O157:H7 from lettuce leaves. International Journal of Food Microbiology. 132:134-140. Interpretive Summary: Leafy green vegetables contaminated with Escherichia coli O157:H7 have been responsible for at least 8 outbreaks of illness since 2000. Since leafy green vegetables such as Romaine and Iceberg lettuce are consumed raw, processors of ready-to-eat, pre-packaged salads have frequently used sanitizing washes to reduce surface contamination. However, many of the salad producers implicated in outbreaks have been using chlorine washes but still had E. coli O157:H7 contaminating the finished product. Therefore, in this study 3 different commercially available sanitizing solutions (acidic electrolyzed water, acidified sodium chlorite, and aqueous chlorine dioxide) were evaluated for their effectiveness in reducing E. coli O157:H7 contamination of Iceberg or Romaine lettuce as compared to water or chlorine. Iceberg or Romaine lettuce was inoculated with approximately 100,000,000 E.coli O157:H7 bacteria per gram, stored overnight under refrigeration (4C), and treated the next day with the sanitizers for 2 minutes. After treatment, aqueous chlorine dioxide (100 and 200 ppm chlorite) and acidified sodium chlorite (100 and 200 ppm chlorite) reduced E. coli O157:H7 on Iceberg lettuce by up to 96%. This was significantly better than the reduction achieved by the levels of chlorine currently used in industry, which reduced E. coli O157:H7 by 78%. On Romaine lettuce, none of the sanitizers was capable of reducing numbers of E. coli O157:H7 by more than 90%. When samples were viewed by scanning electron microscopy, samples of Romaine lettuce that had been washed with chlorine were shown to have E. coli O157:H7 still present in damaged areas of the lettuce, or in clusters of bacteria, yeasts and molds, which appear to protect the E. coli O157:H7 from the sanitizers. Our results suggest that acidic electrolyzed water, acidified sodium chlorite and aqueous chlorine dioxide are as effective as chlorine for the treatment of Romaine lettuce and are more effective than chlorine for the treatment of Iceberg lettuce. These treatments may be good alternatives in cases where usage of chlorine is undesirable.
Technical Abstract: This study compared the efficacy of chlorine (20 – 200 ppm), acidic electrolyzed water (50 ppm chlorine, pH 2.6), acidified sodium chlorite (20 – 200 ppm chlorite ion concentration, Sanova), and aqueous chlorine dioxide (20 – 200 ppm chlorite ion concentration, TriNova) washes in reducing populations of Escherichia coli O157:H7 on artificially inoculated lettuce. Fresh-cut leaves of Romaine or Iceberg lettuce were inoculated by immersion in water containing E. coli O157:H7 (8 log CFU/ml) for 5 min and dried in a salad spinner. Leaves (25 g) were then washed for 2 min, immediately or following 24 h of storage at 4C. The washing treatments containing chlorite ion concentrations of 100 and 200 ppm were the most effective against E. coli O157:H7 populations on Iceberg lettuce, with log reductions as high as 1.25 log CFU/g and 1.05 log CFU/g for TriNova and Sanova wash treatments, respectively. All other wash treatments resulted in population reductions of less than 1 log CFU/g. Chlorine (200 ppm), TriNova, Sanova, and acidic electrolyzed water were all equally effective against E. coli O157:H7 on Romaine, with log reductions of ~1 log CFU/g. The 20 ppm chlorine wash was shown to be no more effective than deionized water at reducing populations of E. coli O157:H7 on Romaine and Iceberg lettuce. Scanning electron microscopy indicates that E. coli O157:H7 incorporated into biofilms and damaged lettuce tissue remains on the lettuce leaf, while individual cells on undamaged lettuce leaf surfaces are more likely to be washed away.