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I am really curious about the behavior of chicken fat in soup. Why does the top layer of fat (in contact with the air) on hot broth form a thin skin that can be lifted off the surface of the soup? This is not the same phenomenon as when the broth cools and the chicken fat becomes a solid layer. As far as I can tell from my own experience it only happens with chicken fat...not with beef fat, butter, olive oil, etc. If you drop the lifted-off "skin" of fat back into the soup it will slowly melt. It also melts in the mouth if you eat it, and tastes oily and like chicken fat. It is as if the top layer of fat is becoming dehydrated and forming a skin, similar to what happens when you heat milk. But how can fat become dehydrated?!

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The skin on milk has nothing to do with fat, it is curdled casein (a milk protein). I suspect that the skin on chicken broth is again a kind of protein, probably gelatine, because there is lots of it in chicken soup, because it has this smooth feeling you describe as oily, and because its set form is thermoreversible. –  rumtscho Jul 5 '13 at 11:58
    
@rumtscho that sounds a good answer. The fact you can pick up that skin means it's not likely to be fat and more likely gelatin (entangled protein, cooled and curled). I don't know many lipid structures that you can 'pick up' with fingers. Fat molecules usually slip past each other. –  MandoMando Jul 5 '13 at 14:10
    
@MandoMando it is just speculation on my side, so I am not comfortable enough to put it as an answer. And while I have never seen an edible fat which a) is solid at just-cooked temperatures (I can pick up a piece of lard), and b) creates solid structures with the physical properties of a "skin" in terms of flexibility and elasticity, I don't have a good source telling me that these don't exist at all. So I treat the gelatine hypothesis as a very good assumption, but not a proven one. –  rumtscho Jul 5 '13 at 14:54

1 Answer 1

The skin isn't fat, it's protein (or at least mostly protein). Hot liquids that are rich in protein will form a skin as the surface loses liquid and begins to dry, pushing the proteins together. A related example would be "tofu skin" which is formed by skimming the protein that coagulates on the surface of heated soymilk.

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