curdling

thumb|A cheesemaker checks the set of milk curd after vegetable rennet was added to milk Curdling is the breaking of an emulsion or colloid into large parts of different composition through the physio-chemical processes of flocculation, creaming, and coalescence. Curdling is purposeful in the production of cheese curd and tofu, but undesirable in the production of a sauce, cheese fondue or a custard.

== Method == thumb|A curd knife is used to cut milk curd into small cubes thumb|A pan of curdled milk In curdling, the pH of the milk decreases and becomes more acidic. Independently floating casein molecules attract one another, forming "curdles" that float in a translucent whey. At warmer temperatures, the clumping reaction occurs more quickly than at colder temperature. Curdling occurs naturally if cows' milk is left open in a warm environment to air for a few days.