Feta

Opened feta keeps about 1–2 weeks refrigerated when stored in brine, and notably less when it is not. It does not freeze well.

Keep it in its brine. This is the single thing that matters. If you bought feta in a brine-filled tub, leave it there and make sure the cheese stays submerged. If you bought a vacuum-sealed block, make your own brine — roughly a teaspoon of salt per cup of water — and cover the cheese in a sealed container.

Feta stored dry, wrapped in plastic, dries out at the edges, sours quickly, and molds within days. Feta kept properly submerged easily reaches the top of its window and often beyond.

If the brine becomes cloudy or slimy, discard the cheese along with it. Change or top up the brine every few days if you are keeping the block for a while.

Freezing crumbles the texture into something dry and chalky.

Feta is preserved by salt and acid, which is a genuinely old technology and works well — but the brine is part of the product, not packaging to be discarded.

The brine does three things at once. It keeps the salt concentration in the cheese high enough to suppress bacterial growth. It excludes air, and therefore mold spores, from the cheese surface. And it stops the cheese drying out, since a dried surface cracks and gives contaminants a way in.

Take the cheese out of the brine and all three protections disappear at once. This is why the same block of feta can last two weeks or four days depending entirely on how it is stored.

Feta's acidity — around pH 4.4 — puts it below the threshold at which most foodborne pathogens can grow, which is a meaningful safety margin. Traditional Greek feta is made from sheep's or goat's milk and is legally protected as such; the cow's-milk versions sold widely in the US behave much the same for storage purposes.

Signs it has gone: mold of any color on the surface or floating in the brine, brine that has turned cloudy, ropy, or viscous, a sour smell distinct from the normal salty tang, and a slimy rather than firm-crumbly texture. Discard the whole container in any of those cases.