Curds are a dairy product

Dairy product

Dairy products are generally defined as foods produced from cow's or domestic buffalo's milk. They are usually high-energy-yielding food products. A production plant for such processing is called a dairy or a dairy factory. Raw milk for processing comes mainly from cows, and, to a lesser extent,...

 obtained by curdling (coagulating) milk

Milk

Milk is a white liquid produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals before they are able to digest other types of food. Early-lactation milk contains colostrum, which carries the mother's antibodies to the baby and can reduce the risk of many...

 with rennet

Rennet

Rennet is a complex of enzymes produced in any mammalian stomach to digest the mother's milk, and is often used in the production of cheese. Rennet contains many enzymes, including a proteolytic enzyme that coagulates the milk, causing it to separate into solids and liquid...

 or an edible acid

Acid

An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

ic substance such as lemon juice

Lemon juice

The lemon fruit, from a citrus plant, provides a useful liquid when squeezed. Lemon juice, either in natural strength or concentrated, is sold as a bottled product, usually with the addition of preservatives and a small amount of lemon oil.-Uses:...

 or vinegar

Vinegar

Vinegar is a liquid substance consisting mainly of acetic acid and water, the acetic acid being produced through the fermentation of ethanol by acetic acid bacteria. Commercial vinegar is produced either by fast or slow fermentation processes. Slow methods generally are used with traditional...

, and then draining off the liquid portion. The increased acidity causes the milk proteins (casein

Casein

Casein is the name for a family of related phosphoprotein proteins . These proteins are commonly found in mammalian milk, making up 80% of the proteins in cow milk and between 60% and 65% of the proteins in human milk....

) to tangle into solid masses, or curds. The remaining liquid, which contains only whey protein

Whey protein

Whey protein is a mixture of globular proteins isolated from whey, the liquid material created as a by-product of cheese production. Some preclinical studies in rodents have suggested that whey protein may possess anti-inflammatory or anti-cancer properties; however, human data is lacking...

s, is the whey

Whey

Whey or Milk Serum is the liquid remaining after milk has been curdled and strained. It is a by-product of the manufacture of cheese or casein and has several commercial uses. Sweet whey is manufactured during the making of rennet types of hard cheese like cheddar or Swiss cheese...

. In cow's milk, 80% of the proteins are caseins. Milk that has been left to sour (raw milk alone or pasteurized milk with added lactic acid bacteria

Lactic acid bacteria

The lactic acid bacteria comprise a clade of Gram-positive, low-GC, acid-tolerant, generally non-sporulating, non-respiring rod or cocci that are associated by their common metabolic and physiological characteristics. These bacteria, usually found in decomposing plants and lactic products, produce...

 or yeast) will also naturally produce curds, and sour milk cheese

Sour milk cheese

Sour milk cheese is a cheese that has been curdled by natural souring or by the addition of lactic acid bacteria, such as Cottage cheese. Sour milk cheese does not use rennet for coagulation....

 is produced this way.
Curd products vary by region and include cottage cheese

Cottage cheese

Cottage cheese is a cheese curd product with a mild flavor. It is drained, but not pressed, so some whey remains and the individual curds remain loose. The curd is usually washed to remove acidity, giving sweet curd cheese. It is not aged or colored. Different styles of cottage cheese are made from...

, quark (both curdled by bacteria and sometimes also rennet) and paneer

Paneer

Paneer is a fresh cheese common in South Asian cuisine. It is of Indian origin. In eastern parts of India, it is generally called Chhena...

 (curdled with lemon juice).