Yogurt is one of those foods that can still look mostly fine right up until it is not. The good news is that you do not need to taste it to know whether it has started to turn. A quick check of the jar or cup can tell you a lot, and in the case of dairy, that is exactly the habit to build.
What to check before you open it
Start with the lid. If it is puffed up, domed, or feels under pressure, treat that as a warning sign. Yogurt should not build up gas the way a fermented beverage might. A swollen lid can mean unwanted bacterial activity has started inside the package, and that is a reason to throw it away.
Next, look at the surface once you open it. A little whey separation is normal in plain yogurt, especially if it has been sitting a while. Clear or slightly yellow liquid on top does not automatically mean spoilage. Just stir it back in if the yogurt otherwise smells clean and looks normal. But if the separation comes with bubbling, fizzing, or a frothy look, that is no longer a harmless change.
Signs that mean discard it
- Mold: any fuzzy spot, colored patch, or speck that looks green, blue, pink, or black means the whole container goes.
- Rancid or sharp smell: yogurt should smell pleasantly tangy, not rotten, yeasty, or unusually pungent.
- Gas or hissing: an audible release when you open it, or a foamy top, is a bad sign.
- Changed texture: slimy, curdled in an odd way, or stringy yogurt is not worth saving.
- Damaged seal: if the lid was already loose, leaking, or broken, do not gamble on it.
There is an important distinction between normal sourness and a spoiled smell. Yogurt is naturally tangy, and some separation is part of its personality. What you are looking for is a clear shift: the smell becomes aggressive rather than fresh, the texture goes from creamy to weird, or the container shows pressure. Those are the moments to stop treating it like a snack and start treating it like waste.
What is normal and what is not
Normal: a little whey on top, a thicker layer underneath, and a clean sour-milk aroma. Also normal: yogurt that is slightly more tart near the date on the package, as long as the container has been refrigerated properly and nothing else looks off.
Not normal: mold of any kind, a bulging lid, bubbles, gas, or a smell that makes you pull back. If you see any of those, do not scrape off the top and eat the rest. With yogurt, spoilage can spread beyond the part you can see.
Best habit of all: keep yogurt cold, seal it tightly after opening, and use a clean spoon every time. That small routine does more to keep yogurt safe than any sniff test ever will.