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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.
Looking particularly at the food preparation aspect of curdling rather than scientific, curdling is used to clump proteins in different types of milk. This occurs due to the protein molecules joining together after a change in pH. These clumps help form cheese curds, however this can also occur in non-cheese making procedures, being sauces or cheesecakes.
Milk and soy milk are curdled intentionally to make cheese and tofu by the addition of enzymes (typically rennet), acids (including lemon juice), or various salts (magnesium chloride, calcium chloride, or gypsum); the curds are then pressed.
In cold sauces like mayonnaise as well as in hot sauces, too large a ratio of fat to egg may also cause curdling.
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.