Curdled milk is caused by multiple factors, often relating to the proteins found in milk.
Multiple factors, harmful and not, cause milk to curdle. Curdled milk is actually used in the process of cheese making and other dairy products, but you wouldn't want to pour it over your cereal or drink a big glass of it. Factors such as heat, age and added ingredients may cause milk to curdle. Add this to my Recipe Box.
Heat
Exposing milk to high temperatures, especially during cooking, will cause it to curdle. The cooking process that curdles milk is not harmful to consumers. High cooking temperatures (on the stove or in the oven) will cause the proteins found in milk to separate from the fats, thus curdling it. When cooking milk, leave it on a low temperature and stir frequently.
Age and Bacteria
Milk will curdle as it expires because of cultivating bacteria. As milk ages, bacteria convert its natural sugar-lactose to lactic acid, making it sour and curdle, according to "Milk Products" published on Homefamily.net. Milk is pasteurized to eliminate this harmful bacteria, which is naturally present in it. Leaving milk at room temperature or allowing it to sit without refrigeration allows ideal conditions for the growth of natural microorganisms, resulting in souring and curdling. Drinking milk that's curdled because of bacteria activity will result in a food-borne illness.
Adding Acidic Elements
Milk will curdle when lemon juice or any other acidic elements are added to it. Vinegar will have the same effect, thickening the milk. Some cooks use this method as a substitution for buttermilk, and isn't a harmful form of curdled milk. Acid causes a protein found in milk (casein) to curdle and form lactic acid, according to "Milk Products."
Making Cheese
Milk is intentionally curdled to make cheese. According to "Milk Products," the enzyme rennin causes the protein casein to curdle. The solid curds are separated from the liquid whey and used in the process of cheese-making.
Tags: found milk, Milk Products, according Milk, according Milk Products, casein curdle