For example, the source of the 2007 outbreak of salmonella in Peter Pan peanut butter turned out to be a leaky roof. Donnelly said vermin and birds lived in the roof, and a leak transferred their salmonella to the peanut products below.
"You have to test the lines themselves, but you also have to test floors and coolers and drains, and walls and roofs," said Donnelly. In the food safety industry, these types of contaminations are deemed "environmental."
The spokesman for PCA said the U.S. Food and Drug Administration found two such environmental sources of salmonella contamination in the Georgia plant.
The PCA peanut butter salmonella recall followed shortly on the heels of the 2007 ConAgra and Peter Pan recall. However, food safety experts said peanut butter has historically been a very safe food.
"It's not a common event," said Michael Doyle, a professor and director of the University of Georgia Center for Food Safety in Griffin, Ga., of salmonella contamination.
"We have had three outbreaks in the world now attributed to peanut butter," said Doyle. The first was in Australia a decade ago, followed by last year's outbreak in the United States and the current salmonella outbreak.
Donnelly worries that consumers will only hear the first reports of the ongoing investigation and not realize the dangers of this recall.
"When you think about salmonella, for most of the population, we're healthy enough that if we get it, it won't likely end in any symptoms," Donnelly said.
However with big cafeterias serving peanut butter in schools, nursing homes and hospitals, it's likely that any contaminated peanut butter will be fed to those most at risk: the sick, the elderly and the young.
"Therein lies the problem," she said.
Donnelly advises the public to pay attention to the food sources of grandparents and children, who may be at risk for salmonella infection.
"When you see recalls happening, in reality only about 20 percent of the recall product comes back to the plant," Donnelly said. "It wouldn't hurt to just check in with the food service director or the school or hospital to make sure they're aware of the recall."
The FDA keeps an updtated list, and a searchable site for recalled peanut butter products on its Web site.