Life Science

Why doesn’t peanut butter go bad?

- Asks Eric from Baltimore

April 7, 2008
Even after months of neglect, peanut butter doesn't seem to spoil. [Credit: Adam T. Hadhazy]
Even after months of neglect, peanut butter doesn't seem to spoil. [Credit: Adam T. Hadhazy]

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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BJ says:


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!

Ashley says:

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.

Megan says:

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

Perry says:

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.

vanessa says:

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 :)

tuna says:

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?


patsy says:

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. .

Sar says:

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

Blue says:

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!

Fantastic job, I was doing a google search and your internet site came up for homes for sale in Altamonte Springs, FL but anyway, I have enjoyed reading it, maintain it up!
Magasin d’usine coach

Randy Herman says:

I like crunchy more because I love peanuts naturally from the shell,and crunchy has little bits of natural peanuts in it

Steven says:

I know the comments are 5 years old but I never care about that. :)

I wonder if the person who had mold in the peanut butter was reusing the jelly knife with the peanut butter. Introducing moisture.

I seen a chart via google that showed something called water activity a term related to moisture content. It listed jams and jellies at .8 and cheese at .85. Seems jelly is more susceptible to mold than peanut butter and need to be careful double dipping the knife when making PB&J sandwiches or may reduce the shelf life of the peanut butter.

Rosie says:

Smooth is my fav I was eating some out of date peanut butter, smooth ov course

Charles Wickwire says:

I need to know how long you can eat it out of the jar :)

Joe says:

Steven, great point!

Joe says:

By the way, i just ate a scoop out of a jar of peanut butter that I found in the back of my cupboard. It’s 15 months past its “best by” date and although the lid has been closed, the factory seal was broken on it long ago.

The peanut butter tasted delicious. No mold and no rancid flavor whatsoever.

I’m going to keep this jar of peanut butter around for while and see just how long it’ll last.

Claire says:

I opened a completely sealed jar of crunchy that was 8 yrs old and found the creamy part tasted ok, but the chopped peanuts tasted funky. Hunh! Guess that’s what “rancid” tastes like.

mountain man says:

Tuna….that’s black mold (all fussy stuff)…air leak…loose lid or crack somewhere in the container for sure. p.s.-most starts on top of jars and lids, so rinse lids on mayo and such in frig. and wipe jar tops with clean sponge or p. towel. Tip-like cheese, if you can remove the mold safely away from the rest of the product you’re ok, but small jars are tough. Plastic say, sour cream containers: clean top and transfer to new container note…air is the enemy!

David says:

But this article does not explain why peanut butter resists going bad (resists rancidity) so well, while roasted peanuts in the shell and shelled peanuts go bad, go rancid, very fast. It is difficult to find roasted peanuts in the shell and shelled peanuts that are not stale or rancid. They go bad in 1-2-3 months, depending on packaging and storage temperature.

Even peanuts sealed in a can go bad. I can’t recall buying peanuts in the can that were not rancid.

I would like to know the answer to that question?

Peanuts in the shell and shelled peanuts in the jar or can are far more often bad, than good. I am lucky if I find half decent peanuts shelled or in the shell, once a year in Canada. And I love peanuts.

I found it best to buy raw peanuts sold for bird feed, and roast them in the oven the best. But you sure have to be careful, to get them just right. And it is a hassle that I would rather would not do.

Hippy Tea says:

David – you make a good point, all nuts seem to be prone to going rancid. I’d guess it’s because a bag of peanuts has much more air space, in between the nuts as well as on top, so they are more exposed to the air.

This article is great, it has answered something that has puzzled me for a while. As for the crunchy Vs smooth debate – crunchy every time! Love the little crunchy nut pieces.

Someone asked why we don’t send peanut butter to people who are starving – actually we do, there’s a product called Plumpy Nut which is peanut butter fortified with vitamins and minerals, and it’s used as an emergency food for people who are starving or malnourished, as it’s got a huge number of calories, lots of fat and protein, and is easy to eat.

Mary Lynne says:

We ate a jar of PB last year that was dated 2006, tasted great when we opened it up, like we bought it that day. This year I opened a jar from 2012, it was rancid. Most PB with varying dates have been fine, the only bad one I have found was the one described above dated 2012.

Mary Lynne says:

I forgot to state I only buy the creamy, never the crunchy!

Fan of peanut butter lol says:

I know, I know, a decade late to the party! And let me tell you, I have eaten A LOT of peanut butter and marmalade/cherry/blueberry/blackberry/raspberry/grape/cookie butter/chocolate/fig jellies and jams and preserves these last 10 years. A connoisseur really.

I became a vegetarian and a vegan in that time(not too difficult, I’ll always have peanut butter after all) and in that change of eating habit, I became a bit more conscious of the environment and wanted to swap all plastic to glass.

You know what that means… the loathsome realization of not eating my chunky(smooth is for perverts, weirdos, and freaks, generally those that eat smooth can be construed as nothing less than a genuine criminal) Jif peanut butter!!! THE HORROR!!! It was nothing less than an existential crisis. Why go on living?

But in that madness, I stood fast, and I tried one of the glass jar peanut butters, with all that icky oil and weird labels n’ all that. I used to associate glass peanut butter with something the extremely geriatric ate or those “weird” families when visiting a friend.

Come to find out, I love the various and strange glass jar peanut butters, stirring the oil, the whole ritual! But I reckoned, as an uneducated and ignorant consumer, that would expedite the process of rotting and becoming rancid(though to be fair I eat the stuff so fast I’d never get the opportunity to find out if we’re being honest!), but turns out I am delightfully wrong!

Glad to know I can eat this stuff over almost a year!


It is so cool the oil peanut butter lasts longer!!! I love that! Also, glass jar peanut butter tastes the best!!! And if it isn’t chunky… may god forgive your eternally dammed soul!!!!

Cheryl Laing says:

This was a great article in terms of resources for starving populations. Its shelf life and not needing refrigeration, as well as being nutrient dense makes it an ideal food for very poor people. I wish I had known this sooner when I lived for many years in Africa. We have so much PB in USA….how much of it is being donated?

Mike hammer says:

I had a plastic jar of peanut butter and it looked like little bugs or worms were in it.when I put a light on it they seemed to rise up too the surface !!?what could it be?

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