Milk is an unusually nutritious food, but it's also an unusually perishable one. At room temperature it can spoil in a matter of hours, and it's all too easy to leave it on the table just a bit too long in the course of a normal day. As a result, almost everyone eventually has the experience of finding sour, lumpy milk in their glass or cereal bowl. Although soured milk can be an unpleasant surprise, it's important to remember that it's not truly spoiled.
Why It Sours
Milk's rich blend of protein, fat and sugars makes it a potent source of nutrition for microscopic life forms, as well as for calves and humans. Many bacteria, notably in the Lactobacillus and Staphylococcus families, have a specialized knack for reproducing vigorously in milk. They feed on the lactose present in the milk, converting it into mild lactic acid. It's that acid that gives soured milk its distinctive tang. The acidity has the additional effect of causing the milk's proteins to "clabber," or lump together into curds. Although these twin effects are unpleasant when unexpected, they're at the root of many cherished foods.
'Spoiled' on Purpose
Milk that's soured by bacterial action is "spoiled" as a fresh beverage, but the resulting acidity actually makes it inhospitable to most other bacteria. Yogurt, kefir, sour cream and other dairy products rely on this effect to prolong milk's usable life as a foodstuff. Some cheeses, notably soft, fresh cheeses, begin with a similar acidification, as does the cultured buttermilk sold at your supermarket. In the days before refrigeration, slightly "off" milk was a daily reality, and in fact many recipes relied on it as an ingredient.
The Baker's Friend
Soured milk came into its own as an ingredient in the 19th century, with the introduction of baking soda as a purified leavening agent. Baking soda, or sodium bicarbonate, is a mildly alkaline chemical that reacts in the presence of moisture and acidity to produce carbon dioxide gas. By incorporating soda and soured milk into recipes for biscuits, quick breads and flapjacks, home cooks could obtain a faster and more reliable rise than with sourdough. Many modern recipes perpetuate that tradition, calling for fresh milk to be artificially soured with lemon juice or vinegar in place of naturally soured milk.
Sour vs. Spoiled
The date stamped on your milk represents a sell-by date for the retailer, not a use-by date for the consumer, so it's only a rough guide. Well-chilled milk should remain fresh and drinkable past that date, though any warming can shorten its shelf life. Once it's past its prime, use your judgment. Milk that's slightly lumpy but still looks good, and has a cleanly acidic smell, is safe to use in your baking. Milk that has a powerful funk, or is mottled with yellow or pink hues, is genuinely spoiled by harmful bacteria and should be discarded.