Inactivation of Escherichia Coli O157:H7 Internalized in Romaine Lettuce and Baby Spinach Leaves:Sodium Hypochlorite Wash vs. Irradiation

Authors: Niemira, Brendan

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 2/23/2007
Publication Date: N/A
Citation: N/A

Interpretive Summary:

Technical Abstract: Pathogenic bacteria which are internalized in leaf tissues are protected from the antimicrobial effects of surface treatments. Ionizing radiation is known to penetrate foods, but the efficacy of the process against internalized bacteria is unknown. Leaves of romaine lettuce and baby spinach were cut into pieces, submerged in a cocktail mixture of three isolates of Escherichia coli O157:H7 and subjected to a vacuum perfusion process to force internalize the bacterial cells into the intercellular spaces of the leaves. The leaves were treated with a 3 min water wash, a 3 min wash with a sodium hypochlorite sanitizing solution (300 ppm or 600 ppm), or various doses of ionizing radiation (0.25 - 1.5 kGy). Leaves were stomached to recover the internalized cells and survivors enumerated. The vacuum perfusion effectively forced bacteria into the leaf tissues. For spinach leaf pieces, neither water nor either of the sodium hypochlorite washes resulted in significant reductions of E. coli O157:H7 cells relative to the untreated control. For romaine lettuce leaf pieces, 300 ppm and 600 ppm each gave less than 1 log reduction, while water wash was not effective. Ionizing radiation, in contrast, significantly reduced the pathogen population in a dose-dependent manner, with reductions of 4 log units (romaine lettuce) or 3 log units (spinach) at the highest dose tested. The D10 value (the amount of irradiation necessary to reduce the population by 1 log) was higher for E. coli O157:H7 cells internalized in spinach leaves (0.45 kGy) than for cells internalized in romaine lettuce leaves (0.39 kGy). This study has shown that, unlike chemical sanitizers, ionizing radiation effectively eliminates internalized E. coli O157:H7 cells from leafy green vegetables, and that the pathogen is significantly less sensitive to radiation in spinach leaves than in romaine lettuce leaves.