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. They are salted to lower the water activity (which inhibits microbial growth) and preserve the casing. Natural casings have been used in the production of meat specialties for centuries and have remained virtually unchanged in function, appearance, and composition. US and EU organic food regulations only allow natural casings, which can be derived from non-organically raised animals as there are no large scale slaughter plants which handle and process only organic animals and sell their casings as certified organic casings. As a result all large scale natural casing companies buy casings from around the world and send them to their selection facilities to be graded and packaged. There are four primary animal genera which are used to manufacture natural casings (although all mammals raised for meat could potentially be used to produce natural casing): cows, pigs, and lambs and sheep. Appearance Natural casing sausages are distinguishable from collagen or cellulose casings because of their irregularity, although high quality sheep and lamb casings can be almost indistinguishable from a collagen casing as they have no noticeable imperfections. The main difference is in the way they are twisted and linked which impacts the appearance of the final product and makes it easy to differentiate a hot dog using natural sheep or lamb casing vs collagen casing. The size and usage of natural casings are impacted by the animal they are derived