ARS | Publication request: Refrigerated storage of Escherichia coli O157:H7-inoculated Romaine lettuce and baby spinach reduces efficacy of irradiation and sodium hypochlorite washes

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: March 10, 2008
Publication Date: June 27, 2008
Citation: Niemira,B. 2008. Refrigerated storage of Escherichia coli O157:H7-inoculated Romaine lettuce and baby spinach reduces efficacy of irradiation and sodium hypochlorite washes [abstract].Institute of Food Technologists Annual Meeting.New Orleans,LA. p.1.

Technical Abstract: Contamination of leafy green vegetables with Escherichia coli O157:H7 is a source of ongoing concern for consumers. Bacteria in biofilms are relatively resistant to chemical treatments, but little is known about the response of leaf surface biofilms to irradiation. Leaves of Romaine lettuce and baby spinach were dip inoculated in a cocktail of three strains of E. coli O157:H7 and stored at 4C for various times (0, 24, 48, 72h) to allow biofilms to form. After each time, the leaves were treated with either a 3-minute wash with a sodium hypochlorite solution (0, 300 or 600ppm) or increasing doses of irradiation (0, 0.25, 0.5, 0.75 or 1.0 kGy). Viable bacteria were recovered and enumerated. Chlorine washes were generally only moderately effective, and resulted in maximal reductions of 1.3 log cfu/g for baby spinach and 1.8 log cfu/g for Romaine. Increasing time in storage prior to chemical treatment had no effect on spinach, but reduced the efficacy of 600ppm applied to Romaine. For both types of leaves, allowing time for biofilm formation reduced the efficacy of irradiation. D10 values (the dose required for a 1 log reduction) significantly increased with increasing storage time, up to 48h post-inoculation. From 0h of storage, D10 increased from 0.19 kGy to a maximum of 0.40-0.43 kGy for Romaine and 0.52-0.54 kGy for spinach. These results indicate that the biofilm habitat can reduce the efficacy of irradiation in eliminating pathogens from leafy vegetables.