Peanut Butter

Unopened peanut butter keeps 6–24 months in the pantry. Opened commercial peanut butter keeps about 3 months in the pantry or 6–9 months refrigerated.

Natural peanut butter — the kind with oil separation and no added stabilizers — is different: it should be refrigerated after opening and keeps 3–6 months there. Without hydrogenated oils to hold it together, the exposed oil goes rancid noticeably faster at room temperature.

Store the jar upside down for a day or two after buying natural peanut butter; it redistributes the oil and makes the first stir much easier.

Refrigerated peanut butter is stiff and hard to spread. Take it out ten minutes ahead, or keep a small working jar at room temperature.

Peanut butter freezes but there is rarely any point — it keeps well enough without.

Peanut butter is one of the most shelf-stable foods in a kitchen, and the reason is water activity.

It contains almost no free water. Microbes need water to grow, and peanut butter's water activity is around 0.2–0.3 — far below the roughly 0.85 that bacteria require and the 0.6 that even molds need. This is why an opened jar sits in a cupboard for months without any preservative doing the work.

That same property produced one of the more counterintuitive facts in food safety: because Salmonella cannot grow in peanut butter but can survive in it for a very long time, and because the high fat content protects bacteria from heat, peanut butter has been the vehicle for several significant US outbreaks — most notably the 2008–2009 Peanut Corporation of America recall. Contamination comes from processing, not from your jar.

What actually limits peanut butter is oxidative rancidity of the oil. Peanut oil is largely monounsaturated and reasonably stable, but it still reacts with oxygen over time, producing a sharp, bitter, paint-like smell and taste. Heat and light accelerate it.

Oil separation is normal in natural peanut butter and is not spoilage — stir it back in.

Discard on a rancid smell, mold, or any visible moisture in the jar.