Cheese is a foodstuff made from the curdled milk of various animals, most frequently cows, but often goats, sheep, and water buffalo. Rennet is often used to induce milk to curdle, although some cheeses are curdled with acids like vinegar or lemon juice, or with extracts of various species of Cynara (sometimes called vegetable rennet). Rennet is an enzyme obtained traditionally from the stomach lining of bovine calves, but more frequently nowadays, a microbiological (laboratory produced) substitute is used. Bacteria are added to cheese to reduce the pH, alter texture, and develop flavor, and some cheeses also have molds, either on the outer skin or throughout. The natural color of cheeses range from off-white to yellow. In some parts of the world, such as Wisconsin USA, the milk fat is low in beta-carotene, making the cheese a paler yellow than normal. In this case it is common to add annatto plant dye as a coloring agent.
Some cheeses are made with the addition of herbs and spices. In some locations as a response to the loss of diversity in mass-produced cheeses, a cottage industry has grown up around home cheesemaking.
Different styles and flavors of cheese are the result of using different species of bacteria and molds, different levels of milk fat, variations in length of aging and differing processing treatments (cheddaring, pulling, brining, mold wash). Other factors include milk animal diet and the addition of herbs and spices to some cheeses.
Some controversy exists over the safety of cheese made by traditional methods using unpasteurized milk and over how pasteurization affects flavor.
