Curdling is the breaking of an

emulsion An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Althoug ...

colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend ...

into large parts of different composition through the physio-chemical processes of

flocculation Flocculation, in the field of chemistry, is a process by which colloidal particles come out of suspension to sediment under the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The action differs from pr ...

, creaming, and

coalescence Coalescence may refer to: * Coalescence (chemistry), the process by which two or more separate masses of miscible substances seem to "pull" each other together should they make the slightest contact * Coalescence (computer science), the merging of ...

. Curdling is purposeful in the production of

cheese curd Cheese curds are moist pieces of curdled milk, eaten either alone or as a snack, or used in prepared dishes. They are consumed throughout the northern United States and Canada. Notably, cheese curds are popular in Quebec, as part of the dish p ...

and

tofu Tofu (), also known as bean curd in English, is a food prepared by coagulating soy milk and then pressing the resulting curds into solid white blocks of varying softness; it can be ''silken'', ''soft'', ''firm'', ''extra firm'' or ''super firm ...

; undesirable in the production of a

sauce In cooking, a sauce is a liquid, cream, or semi-solid food, served on or used in preparing other foods. Most sauces are not normally consumed by themselves; they add flavor, moisture, and visual appeal to a dish. ''Sauce'' is a French word t ...

, cheese fondue or a

custard Custard is a variety of culinary preparations based on sweetened milk, cheese, or cream cooked with egg or egg yolk to thicken it, and sometimes also flour, corn starch, or gelatin. Depending on the recipe, custard may vary in consistency fro ...

Method

In curdling, the pH of the milk decreases and becomes more acidic. Independently floating

casein Casein ( , from Latin ''caseus'' "cheese") is a family of related phosphoproteins (CSN1S1, αS1, aS2, CSN2, β, K-casein, κ) that are commonly found in mammalian milk, comprising about 80% of the proteins in cow's milk and between 20% and 60% of ...

molecules attract one another, forming "curdles" that float in a

translucent In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale (one in which the dimensions a ...

whey Whey is the liquid remaining after milk has been curdled and strained. It is a byproduct of the manufacturing of cheese or casein and has several commercial uses. Sweet whey is a byproduct resulting from the manufacture of rennet types of hard ...

. 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.

Cheese and tofu

Milk Milk is a white liquid food produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals (including breastfed human infants) before they are able to digestion, digest solid food. Immune factors and immune ...

and

soy milk Soy milk (simplified Chinese: 豆浆; traditional Chinese: 豆漿) also known as soya milk or soymilk, is a plant-based drink produced by soaking and grinding soybeans, boiling the mixture, and filtering out remaining particulates. It is a sta ...

are curdled intentionally to make

cheese Cheese is a dairy product produced in wide ranges of flavors, textures, and forms by coagulation of the milk protein casein. It comprises proteins and fat from milk, usually the milk of cows, buffalo, goats, or sheep. During production, ...

and

by the addition of

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...

s (typically

rennet Rennet () is a complex set of enzymes produced in the stomachs of ruminant mammals. Chymosin, its key component, is a protease enzyme that curdles the casein in milk. In addition to chymosin, rennet contains other enzymes, such as pepsin and a ...

acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...

s (including

lemon juice The lemon (''Citrus limon'') is a species of small evergreen trees in the flowering plant family Rutaceae, native to Asia, primarily Northeast India (Assam), Northern Myanmar or China. The tree's ellipsoidal yellow fruit is used for culina ...

), or various

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...

s (

magnesium chloride Magnesium chloride is the family of inorganic compounds with the formula , where x can range from 0 to 12. These salts are colorless or white solids that are highly soluble in water. These compounds and their solutions, both of which occur in natu ...

calcium chloride Calcium chloride is an inorganic compound, a salt with the chemical formula . It is a white crystalline solid at room temperature, and it is highly soluble in water. It can be created by neutralising hydrochloric acid with calcium hydroxide. Ca ...

, or

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywall. ...

); the resulting

curd Curd is obtained by coagulating milk in a sequential process called curdling. It can be a final dairy product or the first stage in cheesemaking. The coagulation can be caused by adding rennet or any edible acidic substance such as l ...

s are then pressed.

Egg sauces

In hot preparations emulsified with

egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the a ...

s like

hollandaise Hollandaise sauce ( or ; ), also called Dutch sauce, is a mixture of egg yolk, melted butter, and lemon juice (or a white wine or vinegar reduction). It is usually seasoned with salt, and either white pepper or cayenne pepper. It is well known ...

and

, curdling is the undesirable result of overheating the sauce. Sauces which contain starch curdle with more difficulty. In cold sauces like

mayonnaise Mayonnaise (; ), colloquially referred to as "mayo" , is a thick, cold, and creamy sauce or dressing commonly used on sandwiches, hamburgers, composed salads, and French fries. It also forms the base for various other sauces, such as tartar ...

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 Yogurt (; , from tr, yoğurt, also spelled yoghurt, yogourt or yoghourt) is a food produced by bacterial Fermentation (food), fermentation of milk. The bacteria used to make yogurt are known as ''yogurt cultures''. Fermentation of sugars in t ...

, overheating often causes curdling. The higher the fat content, the less likely curdling is.

Strained yogurt Strained yogurt, Greek yogurt, yogurt cheese, sack yogurt, or kerned yogurt is yogurt that has been strained to remove most of its whey, resulting in a thicker consistency than normal unstrained yogurt, while still preserving the distinctive so ...

used in sauces also curdles only with difficulty.

Coffee

When a plant based milk such as

soya milk Soya may refer to: Food * Soya bean, or soybean, a species of legume native to East Asia, widely grown for its edible bean * Soya sauce, see soy sauce, a fermented sauce made from soybeans, roasted grain, water and salt Places * Sōya District ...

is added to coffee, curdling can sometimes occur. To help prevent this manufacturers sometimes add acidity regulators.

Cheesecake

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

Bibliography

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

References

{{Cooking techniques Cooking techniques Culinary terminology