By now, the ConAgra (Peter Pan) peanut butter outbreak is old news. But it’s still on the mind of a lot of people. A couple of weeks ago, I was asked to settle an argument about whether it was safer to store an open jar of peanut butter in the refrigerator or at room temperature. I’ve done some digging into the scientific literature, and here’s my answer.
Salmonella cannot grow in peanut butter, but it is shielded from heat damage by the high fat and low water content of this food. Peanut butter is so protective that the typical processing temperatures of 70-75ºC (158º-167ºF) can’t kill Salmonella in the product. Even boosting the temperature to 90ºC (194ºF) doesn’t do the job. For comparison, USDA recommends that we cook raw poultry to an internal temperature of 165ºF (74ºC), which is more than enough to kill Salmonella in that food.
This might help, by the way, to explain ConAgra’s problem. Salmonella probably found a home somewhere on the production line that wasn’t easily accessible for cleaning (maybe one of the filling nozzles?). The Salmonella probably was shed sporadically into the peanut butter. Even if the microbe entered the peanut butter before the pasteurization step or while the peanut butter was still hot, the Salmonella would have been protected from heat by the high fat/low water environment.
I was able to find just one study that compared the survival of Salmonella in different types of peanut butter at room temperature (21ºC) and in the refrigerator (5ºC). Under the conditions of that study, Salmonella survive for at least 24 weeks in peanut butter stored in the fridge - except in the case of natural peanut butter, in which most of the Salmonella died off during the 24-week period. Storage at room temperature hastened the death of Salmonella in all of the peanut butter varieties, although there were still a few survivors hanging around at 24 weeks.
What does this mean for storing an open jar of peanut butter? Room temperature storage might be a bit safer, and it's certainly easier to spread room temperature peanut butter than rock-hard peanut butter right out of the fridge. But neither storage temperature will ensure a safe product. For better or for worse, we have to rely on ConAgra and other peanut butter manufacturers for that.