Curdling


In cookery, curdling is the breaking of an emulsion or colloid into large parts of different composition through the physico-chemical processes of flocculation, creaming, and coalescence. Curdling is intentional and desirable in making cheese and tofu; unintentional and undesirable in making a sauce or a custard. Curdling occurs naturally in cows' milk, if it is left open to air for a few days in a warm environment.
Milk is composed of several compounds, primarily fat, protein, and sugar. The protein in milk is normally suspended in a colloidal solution, which means that the small protein molecules float around freely and independently. These floating protein molecules refract light and contribute to the white appearance of milk. Normally these protein molecules repel each other, allowing them to float about without clumping, but when the pH of their solution changes, they can attract one another and form clumps. This is what happens when milk curdles, as the pH drops and becomes more acidic, the protein molecules attract one another and become "curdles" floating in a solution of translucent whey. This clumping reaction happens more swiftly at warmer temperatures than it does at cold temperatures.

Cheese and tofu

and soy milk are curdled intentionally to make cheese and tofu by the addition of enzymes, acids, or various salts ; the curds are then pressed.

Egg sauces

In hot preparations emulsified with eggs like hollandaise and custard, curdling is the undesirable result of overheating the sauce. Sauces which contain starch curdle with more difficulty.
In cold sauces like mayonnaise as well as in hot sauces, too large a ratio of fat to egg may also cause curdling.

Milk sauces

In sauces which include milk or yogurt, overheating often causes curdling. The higher the fat content, the less likely curdling is. Strained yogurt used in sauces also curdles only with difficulty.

Cooking

Curdling can occur in different cooking processes. Cheesecake is one such process. If water is added to the cream cheese during the combining period, it will curdle.