acroyear | spoiler alert

LiveScience.com: Life's Little Mysteries - Can Peanut Butter Go Bad?:

You get a hankering for a peanut butter and jelly sandwich, and after rooting around in the pantry you discover a neglected plastic jar of PB — score! But you’re a little worried about how long it has sat back there consorting with the pretzels. You unscrew the brightly colored lid and peek inside — no green or white fuzz — and a timid sampling confirms it still tastes good. It’s time to slap together a big, sloppy PB&J.

Peanut butter is gooey and delicious, yet it can remain at room temperature for months without spoiling. Low moisture levels and high oil content keep this butter from going bad for quite some time, but don’t go ignoring that expiration date just yet. Peanut butter can go rancid in about a year and lose its flavor. While fungi and bacteria won’t ruin your peanut butter, oxygenation eventually will.

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Instead, rancidification is a chemical process in which oxygen breaks down the molecular structures of lipids (the technical term for fats) and changes the flavor and odor of food in rather unappetizing ways. As peanut butter is exposed to more and more oxygen over its lifetime, it becomes likelier to undergo this decay.

Fortunately, peanut butter is blessed with high amounts of the natural antioxidant vitamin E. This nutrient helps stave off oxidation and prolongs PB’s shelf life, according to Richard Faulks, a senior scientist at the Institute of Food Research in Norwich, England. Even so, rancidification typically sets in about nine to 12 months after purchase, hence the “Best If Used By” that appears on the container.

Health-wise, however, rancid peanut butter is not something to really worry about. “It won’t hurt you if you eat it — it will just taste bad,” says Maribeth Cousin, a professor of food science at Purdue University in Indiana. “Some people actually eat rancid food without knowing it, depending upon their individual taste buds.”