Butter

Butter keeps 1–2 months refrigerated and 6–9 months frozen, which is a long window by dairy standards.

Wrap it well. Butter is mostly fat, and fat is an excellent absorber of odors — unwrapped butter next to an onion will taste like onion within a day. Keep it in its foil wrapper inside a covered dish, or in the butter compartment if your fridge has a lidded one.

Salted butter can sit at room temperature in a covered dish for a day or two, and in a butter bell or crock for a little longer. Salt lowers water activity and inhibits bacterial growth in the small aqueous fraction. Unsalted butter should be refrigerated — it lacks that protection and goes rancid noticeably faster.

Butter freezes exceptionally well with no texture penalty. Freeze it in its original wrapper inside a freezer bag, and cut it into usable portions first.

Butter is roughly 80% fat, 16% water, and a few percent milk solids, and each fraction fails differently.

The water and milk solids are where bacteria can grow, but they exist as microscopic droplets dispersed through a continuous fat phase — physically isolated from each other. A bacterium in one droplet cannot reach the next. That isolation, plus salt in salted butter, is why butter keeps for months while the cream it was made from keeps for days.

The fat fails by oxidation rather than biology. Exposed to oxygen, light, and warmth, butterfat develops the sharp, waxy, crayon-like smell of rancidity. This is a chemical reaction, so refrigeration slows it and freezing slows it further, but nothing stops it. Rancid butter is unpleasant rather than dangerous, though the oxidation products are not something to seek out.

Light accelerates it considerably, which is a real argument for the foil wrapper over a clear dish.

A darkened, translucent surface layer on a stick of butter is oxidation and drying — trim it and use the rest. Actual mold means discard, since butter's soft high-fat body lets it spread. Clarified butter and ghee last far longer than either, because removing the water and milk solids removes the parts that spoil.