Proteins from collagen and tissues thicken stock. Fat is rendered too. Since fat settles to the surface after refrigeration, I assume that you can extract most or all of the fat through skimming chilled stock or with a gravy separator. That means that stock can be mostly a liquid protein, like a low calorie protein shake substitute.
My question is how efficient this extraction should be. If I made stock from 2kg of chicken thighs which have Xg protein and Xg fat, what fraction of that protein actually winds up in the stock if cooked in an optimal fashion? (and what is such an optimal fashion?) Are certain fats emulsified in the stock which cannot be skimmed? I have consulted online guides to stock nutrition facts and looked at supermarket varieties, but as we all know, those are thin soupy broths that don't congeal.
Best Answer
From Harlod McGee's "On Food and Cooking: The Science and Lore of the Kitchen":
Accordingly, that 7% (5% + 2%) may be considered the upper bound for protein extraction, which will obviously depend on the specifics of the meat and cooking process. McGee then goes on to discuss the proper process for extraction, starting with cold water gradually heated:
This also indicates that the the amount of emulsified fat will depend greatly on on the cooking process in addition to the fat content of the original ingredients. (I haven't been able to find much on differences in the extraction process for different animal fats.)