American cheese is smooth and light yellow or orange in color. It is usually sold in blocks or squares. More than half of all cheese consumed in the United States is processed cheese of this kind. American cheese is essentially young cheddar cheese, made of pasteurized cows milk, which then goes through a shredding and heating process. Various other dairy ingredients, such as dyes and emulsifiers, are added to create a smooth, mild, odorless, meltable, and stable product. Cheese is a solid food made from the curdled milk of various mammalsâ€”most commonly cows but sometimes goats, sheep, or buffalo. Rennet is often used to induce coagulation in the milk, although some cheeses are curdled with acids such as vinegar or lemon juice, or with extracts of various species of Cynara (sometimes called vegetable rennet). Rennet is an enzyme traditionally obtained from the stomach lining of bovine calves, although sometimes a microbiological (laboratory-produced) substitute is used. Bacteria are added to cheese to reduce the pH, alter the texture, and develop flavor, and some cheeses also have molds, either on the outer skin or throughout. There are hundreds of types of cheese. Different styles and flavors of cheese are the results of using different species of bacteria and molds, different levels of milk fat, variations in length of aging, differing processing treatments (cheddaring, pulling, brining, mold wash) and different breeds of cows, sheep, or other mammals. Other factors include animal diet and the addition of flavoring agents such as herbs, spices, or smoking. Pasteurization of the milk may also affect flavor. Processed cheese (or process cheese) is a food product made from regular cheese and other unfermented dairy ingredients, plus emulsifiers, extra salt and food colourings. The best known processed cheese is orange in color and mild in flavor, with a medium-firm consistency; it is commonly known in the U.S. as American cheese and in Australia as Tasty Cheese. Many other flavors and textures of processed cheese are also made. Processed cheese has two technical advantages. Traditional cheesemaking inevitably produces 'scrap' pieces that would not be acceptable for supermarket display; production of processed cheese from cheese scrap allows the cheesemaker to 'add value' to otherwise unmarketable scrap. Also, some consumers actually prefer processed cheese for its smooth melting. With prolonged heating regular cheese will separate into a molten protein gel and liquid fat; processed cheese will not separate in this manner. The emulsifiers (typically sodium or potassium phosphate, tartrate, or citrate) in processed cheese reduce the tendency for tiny fat globules in the cheese to coalese and pool on the surface of the molten cheese. Because processed cheese does not separate when melted, it is used as an ingredient in a variety of dishes. It is a fairly popular condiment on hamburgers, as it does not run off, nor change in texture or taste, as it is heated. Processed cheese is sometimes sold in blocks, but more often sold packed in individual slices, with plastic wrappers or wax paper separating them. Due to the processing and additives, some varieties cannot legally be labeled as "cheese" in many countries, including the United States and Great Britain, and so are sold as "cheese food", "cheese spread", or "cheese product", depending primarily on the amount of cheese, moisture, and milkfat present in the final product.