Pork Skin and Rind
Also known as rind, pork rind, couenne, cotenna, corteza, Schwarte, skórki
Almost pure collagen, and therefore the traditional source of gelatin in brawns, cooked sausages and jellied products. On a hanging cure it is a barrier that passes almost no moisture, which is sometimes the point and sometimes the problem.
Pork skin is roughly a third protein, and almost all of that protein is collagen. That single fact explains both of the jobs it does.
The first job is gelatin. Collagen is a triple helix that is tough and indigestible while it stays intact, and that unwinds into gelatin when it is held in moist heat for long enough. That conversion is the whole basis of the brawns, head cheeses, jellied hams, zampone, cotechino and the cooked and pressed products of every European tradition. A pot of simmered skin gives a stock that sets firm on cooling, and that set is what holds a brawn together and gives a cooked sausage its succulent, slightly sticky mouthfeel. Where a modern recipe adds powdered gelatin, a traditional one added skin.
The conversion needs both time and moisture, and it needs them together. Skin cooked fast and dry goes hard and glassy; skin simmered gently for hours softens and dissolves. This is why the cooked-sausage traditions poach rather than roast, and why a rushed brawn is rubbery rather than yielding. Ground raw skin, sometimes sold as rind emulsion or skin paste, is used industrially as a binder for exactly the same reason, and it will only do its work if the product is then cooked properly.
The second job is as a barrier, and here skin is passive rather than active. Skin passes almost no water vapour. On a dry-cured product this cuts both ways. A bone-in ham keeps its skin over much of its surface deliberately, so that drying happens through the trimmed face at a controlled rate rather than from everywhere at once. A pancetta left skin-on cures effectively from one side only. But skin left where it is not wanted, or an accidental patch of it, creates a slow spot that stays wetter than everything around it, and that is where spoilage begins.
Skin also carries the surface flora of the animal and is the part most handled during dressing, so it wants scraping, singeing or scalding and thorough cleaning before it goes into a pot. Bristle roots are a quality problem rather than a safety one, but a brawn full of bristle is a brawn nobody eats.
In the sausage traditions skin appears as a deliberate ingredient in the coarse and offal products, and as an unwanted contaminant in a fine one, where a strip through the grinder becomes an elastic thread that will not chew.