Milk

Opened milk keeps about one week refrigerated, and often a few days past the printed date if it is genuinely held at 40°F or below.

The most useful thing you can do is stop storing it in the door. Milk is more temperature- sensitive than almost anything else in the fridge, and the door is its warmest spot, cycling toward room temperature every time the fridge is opened. Moving milk to the back of a lower shelf routinely buys several extra days — the difference between 38°F and 43°F roughly halves its life.

Close the cap properly and return it promptly. Milk absorbs odors readily, and every minute on the counter is spent in the bacterial growth range.

Milk freezes for about three months. It separates and looks grainy on thawing — thaw in the refrigerator and shake hard, and it recovers well enough for cooking, baking, and most drinking. Leave headspace in the container, since milk expands as it freezes.

After a power outage, milk is perishable fridge food — see what to keep or discard when power returns. If floodwater reached cartons, see flood food safety.

Pasteurization heats milk enough to destroy pathogens, but it does not sterilize. A population of heat-tolerant and post-pasteurization bacteria survives, and refrigeration only slows them.

Those survivors are mostly lactic acid bacteria. They ferment lactose into lactic acid, which is what sours milk — dropping the pH until the casein proteins destabilize and curdle. That process is where yogurt, buttermilk, and cheese come from, so souring is fermentation rather than putrefaction. It is why sour pasteurized milk is unpleasant rather than acutely dangerous, and why the smell warns you long before you would get ill.

This is also why milk's shelf life is so sensitive to a few degrees. Bacterial growth rates roughly double for every 8–10°F increase, which is the whole reason the fridge door costs you days.

Ultra-pasteurized and UHT milk are heated far hotter, destroying nearly everything, which is why those cartons keep for months unopened and sometimes sit on an unrefrigerated shelf. Once opened, they follow the same one-week rule as ordinary milk.

Raw milk is a different risk category entirely. Without pasteurization it can carry Listeria, Salmonella, E. coli O157:H7, and Campylobacter, and it can do so while tasting completely normal. The CDC advises against drinking it, particularly for children, pregnant people, and anyone immunocompromised.

Milk cartons almost always carry a "sell by" date, which is an inventory instruction for the store rather than a safety deadline. Milk is typically fine for several days past it if it has been kept properly cold.

Because the date is set on the assumption of ideal handling, it tells you less than your own storage does. Milk kept in the fridge door can sour before its date; milk kept at the back of a cold shelf routinely outlasts it by a week.

Trust your nose here. Unlike meat, soured milk announces itself clearly and well before it becomes a real hazard.