Milk curd forms when milk curdles. During curdling, chemical processes cause casein, the milk proteins, to clump and form curds. People can deliberately curdle milk by using agents such as rennet, which is involved in the making of cheese, or acids, such as vinegar and lemon juice. Milk also curdles naturally when it spoils. The liquid that remains when milk has curdled is called whey.
Most natural milk curdling occurs because of proteases, enzymes already present in milk, plus those introduced through bacterial contamination. Exposure to light also degrades methionine and cystine, two amino acids in milk, and contributes to curdling. Cheese makers discovered that adding rennet, enzymes produced in the stomachs of ruminants, causes milk to curdle quickly because of the protease chymosin. The curds could then be made into cheese and yogurt. Because supplies of animal rennet can become limited, and for the sake of lacto-vegetarians, cheese makers developed vegetable rennets – dried caper leaves and fig juice – and microbial rennets, molds that are fermented then purified. Genetic engineering makes it possible to produce chymosin by inserting rennet-producing genes into fungi and yeasts and then fermenting them. This fermentation-produced chymosin is identical to chymosin produced in animals and has been widely available since 1990.