In general, natural sausage casings are made from the submucosa of the intestines of meat animals such as beef, sheep and pork. This material, also known as sausage skin or simply belly, is used to enclose the filling of a sausage. Natural guts are made from the intestines or skin of animals, while artificial guts, introduced in the early 20th century, are composed of collagen and cellulose. The material is then molded by a continuous extrusion process, which produces a single sausage casing of indefinite length that is then cut to the desired lengths while the extrusion process continues.
The most popular sausage casings are made with the clean intestines of animals, particularly pigs, sheep and cows. These are referred to as “natural guts” and the most common type is the 32-34 mm pig's belly. Commercially used collagen wraps are more affordable than natural wraps and are made from pig and cow hides. In addition to that, animal bones and tendons are also used to make these guts.
The type of gut used for a sausage will influence its final flavor, how well it ferments, how well it is protected from light, whether it is airtight or not, etc. The use of natural casing dates back centuries and is one of the oldest ways of making sausages. It is a classic in the sausage tradition and sausages should never be pricked as this releases their juiciness and can cause them to split. Fresh collagen casings are specifically used for fresh sausages such as bratwurst sausage and links for breakfast.
Sheep, lamb and pork guts are available loose in brine in a vacuum-sealed plastic package; commonly known as vacuum packaging, they are mainly used by small to medium sized sausage manufacturers who use less than 100 skeins per month of gut. Sheep, lamb and pig guts are available in tubes where each strand is placed in a plastic tube allowing the sausage manufacturer to easily place the casing in the filling horn; in recent years these tubes have been modified so that they break along a seam and the operator can remove them on the side back of the horn for greater efficiency; in the industry they are known as zipper tubes due to their zipper-like nature. Lamb guts have a small diameter (19-24 mm) and are used for breakfast bar-type sausages and snack stick-type sausages. Made with the tip of a cow's large intestine, veal bungs are large diameter sausage guts that are generally used for large mortadella, head cheese, souse, capicola and mortadella.
This type of natural gut is also commonly used to make smoked Polish sausages, annular mortadella with a small diameter and landjäeger. But sausage casings are also important to ensure that the final product has an excellent flavor, texture and is evenly processed. To prepare flat sausage wrappers for use, submerge them completely in warm water with their ends tied lower down so that trapped air can escape. Once these guts have been filled it is important that they be left in the fridge overnight to rest.