Fat Chickens, or Questions on Soup

[GilaB]

When I make chicken soup, I try to skim off two substances that rise to the top - a beige foam (which rises earlier), and the layer of clear yellow liquid fat that rises somewhat later. I’m pretty sure that the clear yellow stuff is rendered fat, but what’s the foam? It rises up, implying that it’s something lipid-based, but why doesn’t it render out into a liquid like the rest of the fats, and why does it foam up? Beef stock does the something similar, but the foam is dark brown and there’s not nearly as much of it as with chickens.

[Kimstu]

Apparently it’s from the albumen in the meat. The 1876 Scholars’ Handbook of Household Management and Cookery says this:

11. When meat is thoroughly washed to obtain the fibrin, a soluble substance, similar to the white of egg, passes away in the water; this is termed albumen. There are from three to five pounds of albumen in every hundred of meat; it also forms a very large proportion of the brain and of the blood. In cold or warm water it is easily dissolved, but if heated to near the boiling point of water it becomes solid. […]

Carême, one of the most celebrated French cooks, gives the following directions:-
"The good housewife puts her meat into an earthen pot, and pours cold water on it, in the proportion of two quarts to three pounds of the beef. She sets it at the side of the fire.
"The pot grows gradually hot, and as the water heats it dilates the muscular fibres of the flesh by dissolving the gelatinous matter which covers them, and allows the albumen to detach itself easily, and rise to the surface of the water in light foam or scum, while the savoury juice of the meat, dissolving little by little, adds flavour to the broth.
“By this simple proceeding of slow cooking, the housewife obtains a savoury and nourishing broth, and tender boiled meat, and with a good flavour. But by placing the pot-au-feu on too hot a fire, it boils too soon; the albumen coagulates and the fibre hardens; the sad result is that you have only a hard piece of boiled meat, and a broth without flavour or goodness. A little fresh water poured into the pot at intervals helps the scum to rise more abundantly.”