


AVAILABILITY AND FUEL VALUE OF FOOD MATERIALS. 89 
more common non-proteids. The heat of combustion of the 
protein of any given food material will depend to some extent 
upon the kinds of proteids and of non-proteids which furnish 
the nitrogen from which the protein is estimated, but more 
especially upon the proportion in which these two classes of 
compounds occur, and unless we have direct data concerning 
the heat of combustion of the protein this value must be com- 
puted after much the same method as was adopted in estimat- 
ing the nitrogen factor for protein (see pages 76-80). 
The heat of combustion of fat-free muscular tissue, from 
which the extractives, z. e., the non-proteids, have been re- 
moved, gives us a tolerably good idea of the potential energy 
of the protein of meats. The average value obtained for such 
protein is about 5.65 calories per gram. ‘The heat of combus- 
tion of meat proteids appears to be noticeably higher, averag- 
ing not far from 5.75 calories per gram. Creatin may be taken 
as a type of the non-proteids of meats with a heat of combustion 
of 4.27 calories per gram. If we had satisfactory data concern- 
ing the proportions of creatin and other extractives in muscular 
tissues we could calculate the energy per gram of meat protein, 
but at present the heat of combustion of fat-free muscular 
tissue from which the extractives have not been removed gives 
us the most satisfactory value for this quantity. Concerning 
the heats of combustion of the proteids or protein of fish we 
have almost no data. It is assumed in this article that their 
heat of combustion is practically the same as that of the corre- 
sponding compounds in meats. Eggs are commonly assumed 
to have less of non-proteids than meats, and the heat of com- 
_bustion would naturally be larger. The average heat of com- 
bustion of egg albumin appears to be about 5.70, and of protein 
of the nitrogenous matter of the yolk 5.80 calories per gram. 
It is probable, therefore, that the heat of combustion of the 
protein of egg is not far from 5.75 calories per gram. Con- 
cerning the heat of combustion of the protein of milk the data 
are somewhat conflicting. Casein as prepared in different ways 
has a heat of combustion of from 5.6 to 5.9 calories per gram. 
It has been assumed in the present article that the heat of com- 
bustion of milk protein is the same as that of muscular tissue, 
namely, 5.65 calories per gram. 
