These kind of casings are made of sub-mucosa, a high content collagen material. Inner fat and mucosa are removed during the process of manufacturing. Since small intestine is made of collagen, it shares all the properties of all kind of collagen, specially the unique characteristic of variable permeability.
Natural casings offer to sausage makers the endothermic property advantage, which protects the product from exterior heat, keeping it under the environmental temperature, providing a higher protection and a fresh appearance.
For consumers, osmotic property allows flavors and aromas interaction even more while cooking, increasing that tasty sensation they deliver at first sight.