Yogurt

Yogurt keeps 1–2 weeks refrigerated, usually including a week or so past its printed date, and freezes for 1–2 months.

Keep it on a shelf rather than in the door, and close the lid tightly. Yogurt readily absorbs refrigerator odors, and once opened it also loses its protection from airborne mold spores.

Do not eat straight from the tub if you plan to keep the rest. A used spoon introduces bacteria from your mouth into an otherwise controlled environment, and that is what turns two weeks into five days. Decant a portion instead.

Freezing does not kill the live cultures, though it reduces their numbers, and the texture turns grainy on thawing as the proteins separate. Frozen-then-thawed yogurt is best in smoothies, baking, or marinades rather than eaten plain.

Yogurt is unusual on this site in that it is already, deliberately, a controlled bacterial culture — and that culture is what protects it.

The starter bacteria ferment lactose into lactic acid, dropping the pH to around 4.0–4.6. That acidity is genuinely inhibitory: most foodborne pathogens cannot grow below about pH 4.6, which is why yogurt lasts two weeks where plain milk lasts a week. The live cultures also actively compete with incoming organisms for nutrients.

What acidity does not stop is mold, which tolerates acid conditions perfectly well. Mold is the realistic spoilage risk for yogurt, and it is the reason the advice differs from the one for hard cheese: any visible mold means discard the entire container. Yogurt is soft and high-moisture, so mold filaments spread invisibly well beyond the fuzzy spot you can see, and some molds produce mycotoxins that diffuse into the surrounding food. Scraping the top off does not work here.

Liquid on the surface is not spoilage. That is whey, the watery fraction separating out from the curd — entirely normal, especially after the tub has been jostled. Stir it back in, or pour it off if you prefer a thicker result.

The other real signals are a smell that has gone yeasty or alcoholic rather than cleanly tart, and a fizzy or bubbling texture indicating wild yeast has taken hold.