How did my peanut butter get contaminated with Salmonella?

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Feb. 20 2007 7:05 PM

Peanut Butter and Salmonella

Who put bacteria in my PB&J?

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Peanut Butter. Click image to expand.
Peanut butter

Hundreds of people have contracted salmonella poisoning from contaminated jars of peanut butter, the FDA announced last week. ConAgra, the manufacturer responsible for the outbreak, will shell out more than $50 million to recall all of the Peter Pan and Wal-Mart "Great Value" brand peanut butter made at its plant in Sylvester, Ga. Wait, can you really get salmonella from peanut butter?

Daniel Engber Daniel Engber

Daniel Engber (@danengber) is a columnist for Slate. Send him an email at danengber@yahoo.com.

Yes. Poultry, meat, and eggs provide the most common source of salmonella infection and the associated disease "salmonellosis." The bacteria live in animals like cattle and birds and can easily be passed along in raw animal products. But the feces of infected animals can also contaminate many other foods, like fresh fruits and vegetables. Last week, Dole recalled cantaloupes that contained the bacteria, and in 2004, Roma tomatoes were contaminated.

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Peanut butter happens to be a pretty safe food when it comes to microorganisms. That's because the nuts are blanched, roasted, and ground up at temperatures high enough to kill any salmonella bacteria that might have gotten into the raw ingredients. But the germs can still contaminate the product in the "post-processing" phase of production—when the finished product is loaded into jars and labeled for sale. The only other known outbreak of peanut butter-related salmonellosis occurred in Australia in the mid-1990s: Post-processing contamination with fecal matter was the likely culprit.

Both the Australian and the recent American peanut butter outbreaks involved unusual strains of the bacteria—Salmonella Mbandaka and Salmonella Tennessee. The genus Salmonella comprises more than 2,000 different kinds of microbe, some more dangerous than others. The most deadly, Salmonella Typhi, affects the developing world and causes Typhoid fever. In the United States, the most common are Typhimurium and Enteritidis, both of which turn up with some regularity in the poultry supply. Like Tennessee, many strains are named after the locations where they were first discovered. Arizonae, for example, often turns up in fauna typical of the Southwest. (People who eat rattlesnakes or keep iguanas for pets are at particularly high risk.) Other varieties include Salmonella Saintpaul, Salmonella Jerusalem, and Salmonella Newjersey.

Bonus Explainer: Are Peter Pan and Great Value peanut butter the same, since they're made in the same plant? Not necessarily. While the recipes could be exactly the same, they could also be somewhat different. (Great Value, for example, might have fewer crunchies.) A large food manufacturer like ConAgra uses its facilities to produce its own regular, branded products (like Peter Pan peanut butter), but it can also contract out production of "private label" brands under the specifications of giant supermarket chains like Wal-Mart.

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Explainer thanks Michael Doyle of the University of Georgia and Dane Twining of the Private Label Manufacturers Association.