Casing or sausage casing is the material that contains and encloses the filling of a sausage. Casings are typically divided into two categories, natural and artificial. Natural casings are made from the submucosa, a layer of the intestine that consists mainly of collagen. The fat and the inner mucosa lining are removed. Natural casings tend to be brittle once cooked and tend to "snap" when the sausage is bitten. They may also rupture during the cooking process; often, this indicates that the cooking was done too rapidly. Natural casings may be hardened and rendered less permeable through drying and smoking processes. Natural casings are generally made from porcine or ovine intestine, though beef intestine may also be used. Artificial casings are made of collagen, cellulose or even plastic and may not be edible. For collagen casings, shavings from the inside of beef hides are chopped and processed into a mass similar to bread dough. They can be extruded through a die to the desired diameter, then dried and shirred into short sticks about 18 in long that contain 20 to 30 ft of casing. In a newer process, the dough is coextruded with the meat blend, and the casing is formed by treating the outside with an acid (either liquid smoke or vinegar) to set the casing. Though they may be tougher than natural casings, collagen casings are edible. Collagen casings are smoke and moisture permeable, and are less expensive, give better weight and size control, and are easier to run when compared to natural casings. Similarly, cellulose, usually from cotton linters, is processed into a paste and extruded into clear, tough casings for making wieners and franks. They also are shirred for easier use, and can be treated with dye to make "red hots". The casing is peeled off after cooking, resulting in "skinless" franks. Cellulose fibers are combined with wood pulp to make large diameter "fibrous" casings for bologna, cotto salami, smoked ham and other products sliced for sandwiches. This type is also permeable to smoke and water vapor. They can be flat or shirred, depending on application, and can be pretreated with smoke, caramel color, or other surface treatments. Plastic casings are extruded as most other plastic products. They also can be flat or shirred. Generally, smoke and water do not pass through the casing, so plastic is used for unsmoked products where high yields are expected. The inner surface can be laminated or coextruded with a polymer with an affinity for meat protein, causing the meat to stick to the film, resulting in some loss when the casing is peeled, but higher overall yield due to better moisture control.