How to Tell If Milk Has Gone Bad
The reliable ways to know if your milk is still good — and why the date on the carton isn't the whole story.
3 min read · Updated 2026-04-01
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Milk is one of the most wasted foods in most households — often thrown out while still good, or occasionally used past its prime. 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.
Properly refrigerated milk is typically good for 5–7 days past the sell-by date. Many people throw out perfectly good milk based on the date alone.
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. Spoiled milk looks chunky, clumpy, or has visible separation. Yellowing can also indicate spoilage (though some separation is normal if milk has been sitting — give it a shake first).
Taste a small amount. If it smells borderline, taste a teaspoon. Sour milk tastes distinctly sour or "sharp." Fresh milk tastes clean and mildly sweet.
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 fluctuates every time it opens.
- Don't leave it out. Milk left at room temperature for more than 2 hours has noticeably faster spoilage.
- Keep the lid on. Milk absorbs odours from the fridge and oxidises when exposed to air.
- Don't return unused milk to the carton — once poured, bacteria from your glass can contaminate the whole carton.
What About Sour Milk?
Sour but not visibly spoiled milk is safe to use in cooking — particularly in baking. It works as a buttermilk substitute in pancakes, scones, cakes, and quick breads. The acidity reacts with baking soda to create lift. Don't drink it straight, but don't waste it either.
Ultra-Pasteurised (UHT) Milk
UHT milk (sold in shelf-stable cartons) has a shelf life of months when unopened. Once opened, it behaves like regular milk — good for 7–10 days refrigerated. The smell and appearance tests apply equally.