Curd is a dairy product obtained by coagulating milk in a process called curdling. The coagulation can be caused by adding rennet or any edible acidic substance such as lemon juice or vinegar, and then allowing it to coagulate. The increased acidity causes the milk proteins (casein) to tangle into solid masses, or curds. Milk that has been left to sour (raw milk alone or pasteurized milk with added lactic acid bacteria) will also naturally produce curds, and sour milk cheeses are produced this way.
Producing cheese curds is one of the first steps in cheesemaking; the curds are pressed and drained to varying amounts for different styles of cheese and different secondary agents (molds for blue cheeses, etc.) are introduced before the desired aging finishes the cheese. The remaining liquid, which contains only whey proteins, is the whey. In cow’s milk, 90 percent of the proteins are caseins.
There are two methods to make curd, with rennet and with acid. Using acid, like lemon juice, to make curd releases the lactose into the water. Thus the solid curd formed from this method is good for people with lactose intolerance. This type of curd is known as Chhena in India.Using rennet to make curd, attaches the lactose to the solid coagulated proteins. Thus it is not recommended for people with lactose intolerance. This type of curd is the commercial cheese available in supermarkets, such as Cheddar, Mozzarella, Swiz, Parmesan. Vegetarian rennet, usually from Withania coagulans, is used to make paneer in India.
Curd products vary by region and include cottage cheese, curd cheese (both curdled by bacteria and sometimes also rennet), farmer cheese, pot cheese, queso blanco, and paneer. The word can also refer to a non-dairy substance of similar appearance or consistency, though in these cases a modifier or the word curdled is generally used.