Curd is a dairy product obtained by curdling (coagulating) milk with rennet or an edible acidic substance such as lemon juice or vinegar and then draining off the liquid portion (called whey). Milk that has been left to sour (raw milk alone or pasteurised milk with added lactic acid bacteria) will also naturally produce curds, and sour milk cheese is produced this way. The increased acidity causes the milk proteins (casein) to tangle into solid masses, or "curds". The rest, which contains only whey proteins, is the whey. In cows' milk, 80% of the proteins are caseins. Curd products vary by region and include cottage cheese, quark (both curdled by bacteria and sometimes also rennet) and paneer (curdled with lemon juice).