In the spinach outbreak 19 months ago, the E. coli was traced to a cattle ranch in San Benito County, and was probably carried to the spinach field by feral pigs or by water.
The bagged spinach had been triple-rinsed at a Natural Selection Foods/Earthbound Farm plant in San Juan Bautista. The company now tests all greens for pathogens when they arrive at the plant and after they are processed.
Industry leaders such as Earthbound Farm have been searching for foolproof sanitation methods. But they are wary of irradiation. One obstacle is that irradiated foods cannot be certified organic under USDA standards.
"If it were proven effective at eradicating pathogens while preserving the freshness and nutritional integrity of the food without causing adverse environmental effects, then the National Organic Standards Board might reconsider those rules. But I don't think we have all those answers yet," said Will Daniels, an Earthbound Farm vice president in charge of food safety.
Robert Mandrell, research leader at the USDA's Produce Safety and Microbiology Research Unit in Albany, Calif., said consumers and producers might be more willing to accept irradiation if illnesses linked to leafy greens continued to grow.
"Irradiation is perhaps one of the few intervention steps that indeed can penetrate the leaf surface and kill microorganisms," said Mandrell, who was not involved in the study.
The researchers used special vacuums to force high levels of bacteria into the leaves of baby spinach and romaine, then irradiated some batches and treated others in a three-minute chlorine rinse. At least 99.9% of the E. coli, salmonella and listeria were wiped out by the irradiation, which was equally effective at killing them inside and outside the leaves. In comparison, the rinses killed 90% of the bacteria on the leaves, and none inside.
Pathogens can be absorbed by leaves through roots or stems, and also might get inside leaves when they are cut and processed. No one knows how common internal bacteria are or whether they were involved in any disease outbreaks.
"The question of the public health significance of internalized food-borne pathogens is yet to be answered," said Michael Doyle, director of the Center for Food Safety at the University of Georgia.
Some experts say aggregates, or concentrated, tightly adhering bunches of bacteria on leaf surfaces, could be triggering the illnesses. Such clusters are hard to kill with rinses, but irradiation is very effective at killing them, Mandrell said.
One potential drawback found in the study is that it took twice as much radiation to kill the bacteria inside the leaves as the amount required to kill the surface bacteria.
Food safety researchers who are skeptical about irradiation worry that doses high enough to uniformly kill pathogens could cause cell damage that shortens shelf life. Tests with irradiated greens have shown some texture changes, but no severe damage.
Niemira said irradiation would not replace conventional safety measures in fields and processing plants, but could augment them. "Irradiation is not going to kill everything on every product all the time," he said. "It's not a magic wand or a silver bullet. It's intended as another tool."
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marla.cone@latimes.com
