How to Tell If Milk Has Gone Bad
The reliable ways to tell if milk has gone bad — smell, texture, taste test — and why the sell-by date on the carton isn't the whole story.
By Askento Editorial Team · 3 min read · Apr 1, 2026
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Milk often gets thrown out on the date alone, or kept a little too long. Here's how to actually tell.
The Sell-By Date Isn't an Expiry Date
The date printed on milk cartons is usually a "sell-by" or "best before" date — not a "use by" date. It indicates when the store should pull it from shelves, not when it becomes unsafe to drink.
How long past that date? Sources differ. U.S. Dairy says properly refrigerated milk is typically good for 5 to 7 days after the sell-by date; Clemson Extension is more cautious, at one to five days. Both assume the milk has been kept cold the whole time, so let smell and texture make the final call.
How to Actually Tell
Smell it. This is the most reliable test. Fresh milk has very little smell, or a faint milky scent. Bad milk smells sour, sharp, or distinctly "off." Trust your nose — if it smells sour, it is.
Look at it. Pour a small amount into a glass. Fresh milk is smooth and consistent. U.S. Dairy lists curdling, clumping and a thicker-than-usual texture as signs of spoilage.
Taste, only as a last check. U.S. Dairy lists a rancid taste among the signs of spoilage, and says to discard milk that shows any of them. If it already smells or looks off, skip the taste and throw it out.
Why Milk Spoils
Lactic acid bacteria naturally present in milk multiply over time, producing lactic acid that causes the sour taste and eventually the curdling. This happens faster at warmer temperatures.
Making Milk Last Longer
- Keep it cold. Store milk at the back of the fridge, not in the door — the door temperature swings every time it opens. U.S. Dairy recommends a fridge setting of 36 to 38°F; Clemson says below 40°F.
- Don't leave it out. Milk can sit out for up to two hours, or one hour when it's above 90°F, according to U.S. Dairy.
- Don't pour leftovers back. Clemson advises against returning unused milk that has been sitting out to its original container.
What About Sour Milk?
Milk that smells sour has started to spoil. The sources above say to discard milk with a sour smell, off-flavour or curdled texture, so we don't recommend drinking it or cooking with it.
Ultra-Pasteurised (UHT) Milk
Shelf-stable UHT milk can sit unopened at room temperature, depending on the packaging; check the label. Once opened, refrigerate it and treat it like fresh milk: Clemson says to use it within several days. The smell and appearance tests apply equally.
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