These kinds 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 kinds of collagen, specially the only characteristic of variable permeability.
Natural casings become hardened and watertight through the processes of drying and smoking, while heat and humidity make them softer and more porous, those are the reasons why smoking, cooking, drying and humidity must be controlled so carefully.
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 and juicy appearance. For consumers, osmotic property allows flavors and aromas interaction even more while cooking, increasing that tasty sensation they deliver at first sight.