Why doesn’t peanut butter go bad?

- Asks Eric from Baltimore

Even after months of neglect, peanut butter doesn't seem to spoil. [Credit: Adam T. Hadhazy]
By | Posted April 7, 2008
Posted in: Ever Wondered?, Featured, Life Science
Tags: ,

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.

The axiom that tasty foods are fatty foods is certainly true in the case of peanut butter. Label-lookers will notice that peanut butter is full of fat, sporting some 16 grams in a two-tablespoon serving. In addition to its natural peanut oil, peanut butters often contain other vegetable oils such as cottonseed, soybean and rapeseed. Since oil and water don’t mix, the more oil there is, the less room there is for water.

So despite feeling wet and greasy, peanut butter is actually extremely dry, with a moisture content of about 2 percent, according to Lydia Botham, the public affairs director for Land O’Lakes Inc. Without water, most bacteria and fungi can’t survive. Aridity is the chief way that peanut butter and other unrefrigerated comestibles, such as pasta and cereal, hold corruption at bay. For comparison purposes, butter from an udder has somewhere in the range of 17 percent water by volume, and must therefore be kept at a chilly temperature to retard bacterial and fungal proliferation.

But peanut butter’s dryness doesn’t guarantee immortality. In fact, the high fat content leaves PB vulnerable to a different type of food spoilage called rancidification. This isn’t the same as oil separation, when the fats disassociate from peanut particles and pool on top of the jar’s contents. This happens normally in peanut butter and can be resolved simply by stirring.

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.”

So maybe the peanut butter that you rescued from culinary limbo, which is now cementing your mouth closed as you crush that PB&J, is actually rotten and would taste disgusting to anybody else. As you reflect on the last few nibbles, you may ponder the biggest question about peanut butter, and one that science can’t answer: So which is better anyway, crunchy or smooth?

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  1. Wow!

    Good to know. I’ve often noticed that peanut butter usually doesn’t go bad. I wouldn’t have assumed it was because of a low moisture content. Fascinating.

    Now I’ll have a staple food to stock up on for the Rapture. Nuclear Holocaust be damned! Randification will be one less thing to worry about while I’m repopulating the species with my peanut buttery offspring!

    BJ, April 10, 2008 at 6:10 am
  2. I vote for crunchy. But in all seirousness the best is the kind at WF that comes straight from the peanuts themselves with a mere flip of a switch. Adam, nice article.

    Ashley, July 31, 2008 at 2:40 pm
  3. That would explain a lot (like why the jar of PB is still good even though it has been sitting around in the basement pantry for a year). Smooth gets my vote because it’s easier to spead

    Megan, September 4, 2008 at 6:07 pm
  4. I grew up with creamy, and was deprived of the goodness which is crunchy. But perhaps my love of crunchy is all in my head since eating it now is (at least in my mind) somehow making up for all those years I didn’t have it.

    Perry, December 31, 2008 at 2:26 pm
  5. This was an extremely interesting article. I woke up with the worse urge to eat a pb&j sandwich and I was just debating whether I should eat it or not because the pb has been sitting back there for months but now I know it won’t kill me so I will make sandwich :)

    vanessa, April 12, 2009 at 12:19 pm
  6. I’m just getting over 3 days of severe cramping and violent “rejection” from eating PB that isn’t 6 months old. It does, however, have white and dark brown fungi growing in spots. Wish I had turned on the light while preparing it. where did that come from?

    Crunchy.

    tuna, January 4, 2010 at 4:54 pm
  7. so you mean to tell me. that you can have pb for years unopen and the stuff is still good. . i think that is alot of crap. . if we can make food that doesnt go bad then why cant we make a car that wont brake. are send alot of pb over seas to help with the hungry problem. . so please answer that for me. .

    patsy, March 24, 2010 at 2:12 pm
  8. Patsy^ some people are ignorant because they don’t read or seek the facts. Then there is the people that read the facts are are too idiotic to understand. Patsy you are an idiot

    Sar, April 16, 2010 at 1:19 am
  9. The worst taste I have ever tasted is rancid peanut butter. I don’t know how old it was, but if it goes bad, you’ll have no doubt!

    Blue, April 13, 2011 at 10:00 pm
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