Curdling is the breaking of an

emulsion An emulsion is a mixture of two or more liquids that are normally Miscibility, 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 colloi ...

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 exte ...

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

flocculation In colloidal chemistry, flocculation is a process by which colloidal particles come out of Suspension (chemistry), suspension to sediment in the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The actio ...

, creaming, and

coalescence Coalesce, coalescence or coalescent can refer to: Chemistry and physics * 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 contac ...

. Curdling is purposeful in the production of

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

and

tofu or bean curd is a food prepared by Coagulation (milk), coagulating soy milk and then pressing the resulting curds into solid white blocks of varying softness: ''silken'', ''soft'', ''firm'', and ''extra (or super) firm''. It originated in Chin ...

; 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 flavour, texture, and visual appeal to a dish. ''Sauce'' is a French wor ...

, cheese fondue or a

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

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 light scattering by particles, scattering of light. On a macroscopic scale ...

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 c ...

. 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 lactating mammals. It is the primary source of nutrition for young mammals (including breastfeeding, breastfed human infants) before they are able to digestion, digest solid food. ...

and

soy milk Soy milk (or soymilk), also known as soya milk, is a plant-based milk produced by soaking and grinding soybeans, boiling the mixture, and filtering out remaining particulates. It is a stable emulsion of oil, water, and protein. Its original ...

are curdled intentionally to make

cheese Cheese is a type of dairy product produced in a range 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 prod ...

and

by the addition of

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

s (typically

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

acid An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...

s (including

lemon juice The lemon (''Citrus'' × ''limon'') is a species of small evergreen tree in the ''Citrus'' genus of the flowering plant family Rutaceae. A true lemon is a hybrid of the citron and the bitter orange. Its origins are uncertain, but some ...

), or various

salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...

s (

magnesium chloride Magnesium chloride is an inorganic compound with the formula . It forms hydrates , where ''n'' can range from 1 to 12. These salts are colorless or white solids that are highly soluble in water. These compounds and their solutions, both of which ...

calcium chloride Calcium chloride is an inorganic compound, a Salt (chemistry), 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 cal ...

, or

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, 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, drywall and blackboard or sidewalk ...

); the resulting

curd Curd is obtained by Denaturation (biochemistry), 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, a Kefir cheese, ...

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 ...

s like hollandaise 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, creamy sauce with a rich and tangy taste that is commonly used on sandwiches, hamburgers, Salad#Bound salads, bound salads, and French fries. It also forms the base for various o ...

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 , ; also spelled yoghurt, yogourt or yoghourt) is a food produced by bacterial Fermentation (food), fermentation of milk. Fermentation of sugars in the milk by these bacteria produces lactic acid, which acts on milk protein to ...

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

Strained yogurt Strained yogurt, Greek or Greek-style yogurt, yogurt cheese, sack yogurt, kerned yogurt or labneh is yogurt that has been strained to remove most of its whey, resulting in a thicker consistency than normal unstrained yogurt, while still preser ...

used in sauces also curdles only with difficulty.

Coffee

When a plant based milk such as

soya milk Soy milk (or soymilk), also known as soya milk, is a plant-based milk produced by soaking and grinding soybeans, boiling the mixture, and filtering out remaining particulates. It is a stable emulsion of oil, water, and protein. Its original ...

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

Cheesecake

When making

cheesecake Cheesecake is a dessert made with a soft fresh cheese (typically cottage cheese, cream cheese, quark or ricotta), eggs, and sugar. It may have a crust or base made from crushed cookies (or digestive biscuits), graham crackers, pastry, ...

, 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