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Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 5/28/2007 Publication Date: 11/3/2007 Citation: Niemira, B.A. 2007. Relative Efficacy of Sodium Hypochlorite Wash Versus Irradiation to Inactivate Escherichia coli O157:H7 Internalized in Leaves of Romaine Lettuce and Baby Spinach. Journal of Food Protection. 70(11):2526-2532. Interpretive Summary: 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 the human pathogen Escherichia coli O157:H7. The bacteria was pushed inside the leave with a vacuum perfusion process. The leaves were treated with a 3 minute water wash or with either a chemical or a physical antimicrobial treatment. The chemical treatment was a 3 minute wash with a one of two different levels of sodium hypochlorite solution: a commonly recommended concentration for produce washes (300 ppm) or double this concentration (600 ppm). The physical treatment was various doses of ionizing radiation, ranging from 0.25 to 1.5 kGy. After treatment, the leaves were suspended in a neutral buffer solution and crushed to recover and count the internalized bacteria. It was observed that the vacuum perfusion effectively forced bacteria into the leaf tissues. Washing with plain water was not effective at reducing the levels of the pathogen on either spinach or lettuce. For spinach leaf pieces, neither 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, both levels of the chemical treatment gave less than 90% reduction. Ionizing radiation, in contrast, significantly reduced the pathogen population. The level of kill was dependent on the dose applied, with reductions of 99.99% on romaine lettuce and 99.9% on spinach at the highest dose tested. The amount of radiation necessary to reduce the population by a given amount was higher for pathogen cells internalized in spinach leaves than for cells internalized in romaine lettuce leaves. 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. This information will be useful in addressing concerns related to human pathogens on produce by developing a process that can kill E. coli O157:H7 internalized in leafy vegetables. 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 intracellular spaces of the leaves. The leaves were treated with a 3 min water wash, a 3 min wash with a sodium hypochlorite sanitizing solution (300ppm or 600ppm), or various doses of ionizing radiation (0.25 - 1.5 kGy). Leaves were stomached to recover the internalized cells and survivors counted. 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, 300ppm and 600ppm 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. |
