Fresh milk should have a consistent, thin texture and a mild smell. If the expiry date on the carton has not yet passed, the milk should still be suitable for consumption. If milk is spoiled, it may smell sour or begin to thicken into a lumpy, yogurt-like consistency.
All types of milk should be stored in the refrigerator at a temperature of 40 degrees Fahrenheit or below. Cartons of milk purchased in grocery stores come with a printed expiry date. Because the expiry date on milk is regulated by the state rather than federally, there may be some variation in how long milk stays fresh after this date. In general, the best-before date should be sooner than 21 days after the milk was extracted.
Most milk available in grocery stores is pasteurized, meaning it is heated prior to selling in order to kill potentially harmful bacteria. Pasteurization is not a perfect process, and these strains of bacteria eventually multiply even when the milk is refrigerated. If the refrigerator is not cold enough, the bacteria grow back more quickly. Drinking milk directly from the carton may also promote faster spoilage because bacteria from the human mouth are introduced into the milk.
For individuals with healthy immune systems, milk that shows no detectable sign of spoilage is likely safe for five to seven days past the printed expiry date. Babies, the elderly and those with weaker immune systems should avoid milk that is past its expiry date.