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Casing, sausage casing, or sausage skin is the material that encloses the filling of a sausage. Casings are divided into two categories, natural and artificial. Artificial casings, such as collagen, cellulose, plastic, and extruded casings, are relatively new to the field, mainly borne of market demand during the technological advances of the early 20th century.
Natural casings 
It is often assumed that sausages were invented by the Sumerians in the region that is Iraq today, around 4000 BC. Reference to a cooked meat product stuffed in a goat stomach like a sausage was known in Babylon and described as a recipe in the world’s oldest cooking book 3,750 years ago (Yale Babylonian collection, New Haven Connecticut, USA).
The Chinese sausage làcháng, which consists of goat and lamb meat, was first mentioned in 589 BC. The Greek poet Homer mentioned a kind of blood sausage in his Odyssey (book 20, poem 25); Epicharmus (ca. 550 BC — ca. 460 BC) wrote a comedy entitled The Sausage. Numerous books report that sausages were already popular among the ancient Greeks and Romans.
Natural sausage casings (“casings”) are made from the sub-mucosa, a layer of the intestine that consists mainly of naturally occurring collagen. This should not be confused with collagen casings, which are artificially processed from collagen derived from the skins of cattle. Natural casings are derived from the intestinal tract of farmed animals, are edible and bear a close resemblance to the original intestine after processing. The outer fat and the inner mucosa lining are removed during processing.
Natural casings are traditional products that have been used in the production of meat specialties for centuries and have remained virtually unchanged in function, appearance, and composition. Salt and water are all that is used for cleaning and preservation. Natural casings are the only casings that can be used in organic sausage production.
A large variety of sausage is produced world-wide using intestines of pigs, sheep, goats, cattle and sometimes horses. Although the intestines were previously flushed, scraped and cleaned by hand, more recently, machinery has been used for large scale production.
Natural casings breathe, allowing smoking and cooking flavors to permeate the casing and infuse the meat, giving the sausage a rich, even flavor throughout. Natural casings have unique natural curves and sheen, with rounded ends where the sausage is linked giving the sausage visual appeal.
Due to their non-uniform appearance, sausages stuffed in natural casings are clearly distinguishable from mass-produced products and are therefore acceptable as a higher quality, premium product. However, newer machinery has enabled sausage producers to develop mass production scales of efficiencies in their processing plants. Processors of natural casings have developed long-stranded casings with uniformity and strength to support this new technology, as well as new tubing (Shirring) systems that speed up the stuffing process. This has revolutionized the sausage manufacturing world and kept natural casings preferred by some manufacturers and consumers.
Artificial casings 
Artificial casings are made of collagen, cellulose, or even plastic and may not be edible. Artificial casings from animal collagen can be edible, depending on the origin of the raw material.
Collagen casings are mainly produced from the collagen in beef or pig hides, and the bones and tendons. It can also be derived from poultry and fish. They have been made for more than 50 years and their share of the market has been increasing. 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 and, as a raw material, it is similar to bread dough prior to final production. It is then extruded through a die to the desired diameter, dried and shirred into short sticks up to 41 cm long that contain as much as 50m 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 but do not exhibit the “snap” or “bite” of natural casing sausages. The biggest volume of 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 permeable to smoke and moisture, are less expensive to use, give better weight and size control, and are easier to run when compared to natural casings.
Cellulose, usually from cotton linters or wood pulp, is processed to make viscose [a caustic-soluble xanthan salt] which is then 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. Cellulosic viscose solutions 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 
Plastic casings are extruded like 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 non-smoked products where high yields are expected. The inner surface can be laminated or co-extruded 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. Plastic casings are not commonly used any more due to health hazards.
- The International Natural Sausage Casing Association (INSCA) , INSCA, About Casings