Why Does Milk Turn Sour So Quickly in the Fridge?

Milk can sour early because of temperature, breaks in the cold chain, time since opening, or contamination. Instead of assuming one cause, check your refrigerator with a thermometer.

Cold storage. Keep milk that requires refrigeration at 4 °C / 40 °F or below on a stable interior shelf rather than in the door. Unopened UHT milk may have different instructions; once opened, follow the label's refrigeration and use period. Keep the container closed and pour what you need into a clean glass.

Spoilage. Do not drink, taste, or use spontaneously soured, curdled, or suspicious milk in other recipes, including curd cheese. Boiling is not a way to rescue suspicious milk. Discard milk requiring refrigeration if it has been at room temperature for more than 2 hours, or more than 1 hour above 32 °C / 90 °F.

Planning. Buy what you can use and track the date. A later date is not a safety guarantee if storage instructions were not followed. If freezing, check the product's suitability; texture may change after thawing.