Occasionally I buy shelled nuts and I noticed that once I open the original package they tend to go off fairly quick, maybe in a couple of weeks. But peanut butter? That lasts months and they same is true for almost paste. And to be more precise I’m talking about 100% peanut butter so (hopefully) a product without any additional preservatives, not even salt. What’s happening here?
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If you notice how they go off, it’s probably not mold or rot, but an off taste we call rancid. Rancid flavors are the result of oxidation/hydrolysis of fats, which is generally the result of exposure to air. A nut has a pretty small volume to its relatively large surface area, whereas a nut butter is a dense paste that only has a thin surface exposed to air. Most of the butter is protected by that film, and the film itself is protected by the jar.
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