Curdling

In curdling, the pH of the milk decreases and becomes more acidic.[1] Independently floating casein molecules attract one another, forming "curdles" that float in a translucent whey.[1] At warmer temperatures, the clumping reaction occurs more quickly than at colder temperature.[1] Curdling occurs naturally if cows' milk is left open in a warm environment to air for a few days.

Cheese Curd prior to pressing

Silky Tofu(Kinugoshi Tofu)

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 resulting curds are then pressed.[2]

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.

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.

When a plant based milk such as soya milk is added to coffee, curdling can sometimes occur. To help prevent this manufacturers sometimes add acidity regulators.[3]

When making cheesecake, if water is added to the cream cheese during the combining period, it will curdle.

• Harold McGee, On Food and Cooking: The Science And Lore Of The Kitchen, 1984–2004.
• Bethany Moncel "Why Does Milk Curdle". Foodreference.about.com.