How efficient is stock for protein extraction?

Question

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.

Answer 1

From Harlod McGee's "On Food and Cooking: The Science and Lore of the Kitchen":

The muscles that make up meat are mainly water and the protein fibers that do the work of contraction, which are not dispersable in water. The soluble and dispersable materials in muscle include about 1% by weight of collagen, 5% other cell proteins, 2% amino acids and other savory molecules, 1% sugars and other carbohydates, and 1% minerals, mainly phosphorus and potassium. Bones are around 20% collagen, pig skin around 30%, and cartilaginous veal knuckles up to 40%. Bones and skin are thus much better sources of gelatin and thickening power than meat. However, they carry only a small fraction of the other soluble molecules that provide flavor. (pg 598)

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:

The cold start and slow heating allow the soluble proteins to escape the solids and coagulate slowly, forming large aggregates that either rise to the surface and are easily skimmed off, or settle onto the sides and bottom. A hot start produces many separate and tiny protein particles that reamin suspended and cloud the stock; and a boil churns particles and fat droplets into a cloudy suspension and emulsion. (pg 599)

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.)

Answer 2

Soup, stocks etc. do contain surprising large amounts of fat trapped within the collagen and other dissolved products

Boned chicken at best is 25% protein (15% for chicken with bone). At the very best case, by the time you have added water etc. to make a stock, you will be down to about 5% protein. For pure stocks this will unlikely to be even 1%

Good quality milk is 5% protein