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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Some oils are more prone to going rancid than others, as they are exposed to oxygen in air and light. Hazelnuts get smelly relatively quickly even in butter form. Peanut oil is indeed more stable.
When nuts are roasted under heat, some water is evaporated off, which makes it impossible for any organism to grow on the product. Stored in jar, the butter has a small contact area with oxygen, only from the top surface, and only the pocket beneath the lid while sealed.
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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