Over the past four months, salmonella poisoning has left a stomach-churning trail of diarrhea and vomiting in its wake, sickening at least 410 people in 43 states across the U.S. and leaving three dead.

On Friday, the Minnesota Department of Health announced a possible break in the case, revealing that the suspected culprit might be one of the fave foods of the nation's kids and adults alike: peanut butter. Officials believed that a five-pound tub of King Nut brand creamy peanut butter in a Minnesota nursing home might contain the bacteria to blame for the outbreak. The next day, the Ohio-based company recalled all peanut butter manufactured by the Peanut Corporation of America, including the Parnell's Pride brand. These  large containers are used in institutions such as schools and nursing homes, so none of the biggies in your pantry – Jif, Smuckers, Peter Pan or Skippy -- are likely to be affected.

Yesterday, Minnesota health officials released the results of serotyping tests showing that the strain  of salmonella discovered in the nursing home tub – the typhimurium type – was, in fact, a match with the one responsible for the outbreak.

There are some 40,000 cases of salmonella infection each year; about 600 of them are fatal.  Last spring, an outbreak that killed one and sickened some 1,300 people was linked to Mexican Serrano peppers. In the past 15 years, only two outbreaks have been  linked to peanut butter.  In 2006, more than 620 people were sickened in 47 states by salmonella in Peter Pan and Great Value peanut butter (none died), and in 1996, it struck more than 500 peanut-butter lovers in Australia.

We talked to Michael Doyle, director of the Center for Food Safety at the University of Georgia in Griffin, to find out how salmonella gets into peanut butter.  Following is an edited transcript of  our conversation with him:

How does salmonella get into peanut butter?

Feces from some animal is a strong possibility. A leak in the roof, for example, caused one of the early outbreaks. How salmonella got into the water that was on the roof, no one knows for sure. Maybe birds, for instance, which accumulate around peanut butter processing plants.

The roasting of peanuts is the only step that will kill the salmonella. If contamination occurs after the roasting process, the game is over and salmonella is going to survive. Studies have shown that salmonella can survive for many months in peanut butter once it's present. Fatty foods are also more protective of salmonella, so when it gets into the acid of the stomach -- which is our first line of defense -- it may not get destroyed.  Peanut butter, being a highly fatty food, could survive better.



Why have these outbreaks only happened in the past 15 years?

Some of these processing plants are quite dated, and that may be part of the problem.  They just haven't been maintained.  Thirty years ago when they were built, they didn't have leaks like that.

Is there any way to destroy the bacteria once it's in there?

Not by current procedures.

Theoretically, you could irradiate it. It's not an approved process. And because it’s a high-fat product, you'd get a lot of off odors because of lipid oxidation.  I'm not sure radiation would be good approach. 

We have done thermal inactivation studies on trying to kill salmonella in peanut butter. But even when you get up to 190 degrees Fahrenheit (90 degrees Celsius), it takes many minutes and might affect the integrity of the product.  Heating may not be an easy fix.

So, how can you keep salmonella out of peanut butter in the future?

The key is to have a rigid system in place that does not allow contamination by water or other vectors after the roasting process.  Water in a peanut butter processing plant is like putting gasoline on a fire.  It will not only spread the salmonella, but the salmonella will grow when water is present.  Salmonella is not likely to grow in a dry environment.