Half-and-Half

Opened half-and-half keeps 3–4 days refrigerated per FoodKeeper — considerably shorter than milk, and short enough to surprise people. Follow the package date if it is sooner.

Keep it at the back of a cold shelf, not in the door. Given how short the window is, those few degrees matter more here than almost anywhere else in the fridge.

Buy the smallest container you will actually use. Half-and-half is one of the most commonly wasted items in a refrigerator precisely because the package sizes outrun the window.

Ultra-pasteurized half-and-half is different and worth looking for: heated to a much higher temperature, it keeps far longer unopened and somewhat longer opened. Single-serve shelf-stable cups are UHT-processed, which is why they sit unrefrigerated at coffee counters.

Freezing is possible for a few months but breaks the emulsion — the thawed product is grainy and best used in cooking.

Half-and-half is roughly half milk and half cream, at about 10–12% fat, and it sits in an awkward spot for shelf life.

It has more fat than milk, which means more surface area of fat globules, more emulsion to destabilize, and more substrate for lipase enzymes that break fat into free fatty acids. But it does not have enough fat to gain the protection that butter gets from having its water dispersed in isolated droplets. The result is a product with milk's vulnerabilities and some of cream's as well.

The failure mode you will see first is curdling in hot coffee. That happens because coffee is acidic and hot, and as half-and-half ages its own lactic acid bacteria lower its pH further — until the proteins are close enough to their isoelectric point that the additional acid and heat make them clump instantly. Curdling in coffee is often the earliest practical signal that a carton has turned, sometimes before it smells wrong.

Beyond that, the signals are conventional: a sour smell, visible thickening or lumps in the carton, and any yellowing.

Because the window is only 3–4 days and the early signs are subtle, this is a case for going by the date you opened it rather than by inspection.