I recently simmered a pot of chicken feet (including skin) for the first time and made an astonishingly gelatinous gelatin. It's clearly suffused with chicken fat, but I have no idea how much -- I don't think I could guess the fat % by weight or the fat:protein ratio to within a factor of 10.
I tried melting some in a tall, narrow container and letting the fat settle on top, but that was unsuccessful because 1) the gelatin fraction solidified at room temperature, and 2) some of the fat appears to be emulsified and won't separate out on its own. So, consider this a food chemistry thought experiment: Given a sample of gelatin with fat in it, how would you estimate the fat content?
Responses to some likely questions:
1) If I wanted a homogeneous, clean-looking and -tasting product I definitely should have removed the skin. This was more of a nose-to-tail-eating experiment.
2) I know a little chicken fat won't kill me. But I'm doing nutritional tracking to investigate some health issues, so a rough estimate would be useful.