thumb|250px|Casing from beef (in bucket) and sheep (on rear edge of bucket)
Sausage casing, also known as sausage skin or simply casing, is the material that encloses the filling of a sausage. Natural casings are made from animal intestines or skin; artificial casings, introduced in the early 20th century, are made of collagen and cellulose. The material is then shaped via a continuous extrusion process—producing a single sausage casing of indefinite length—which is then cut into desired lengths, usually while the extrusion process continues.
Natural casings
Origin
Natural sausage casings are made from the sub-mucosa of the small intestine of meat animals, a layer of the intestine that consists mainly of naturally occurring collagen. In Western European cuisine and Chinese cuisine, most casings come from pigs, but elsewhere the intestines of sheep, goats, cattle and sometimes horses are also used. To prepare the intestines as casings, they are flushed, scraped and cleaned with water and salt by hand or with machinery; today they are primarily machine cleaned. The outer fat and the inner mucosa lining are removed during processing. Usually the cost to produce sausages in collagen is significantly lower than making sausages in gut because of higher production speeds and lower labor requirements.
The collagen for artificial casings is processed extensively. It is formed by extrusion through a die to the desired diameter, dried and shirred into short sticks up to long that contain as much as of casing. In a newer process, a form of dough is coextruded with the meat blend, and a coating is formed by treating the outside with a calcium solution to set the coating.
The latest generation of collagen casings are usually more tender than natural casings and do not exhibit the "snap" or "bite" of natural casing sausages. Most collagen casings are edible, but a special form of thicker collagen casings is used for salamis and large caliber sausages where the casing is usually peeled off the sausage by the consumer. Collagen casings are less expensive to use, give better weight and size control, and are easier to run when compared to natural casings.
Vegetarian casings
Some innovative coextrusion processes have been developed in recent years, allowing 100% plant-based vegetarian casings to be created. Some special alginate coextrusion equipment is required to make casings that can be used in halal or kosher food making.
Inedible casings
Casings made from cellulose and plastics are peeled off food products and are not eaten. are shrinkable casings and will shrink during the cooking process thereby reducing the water loss. Non-oriented polyamide casings remain the same diameter during the cooking process and thereby allow for expansion of the meat during cooking. The use of polyamide casings has expanded recently with the advent of various varieties and structures of casings such as multilayer casings.