For the sake of brevity and convenience, certain nitrogen compounds of human foods, both vegetable and animal, are designated as a class by the single term protein. This term includes such compounds as albumins, globulins, and similar or related organic nitrogen bodies found in human foods. When, therefore, it is stated that a food stuff contains a certain percentage of protein, reference is made to the total mass of nitrogen compounds present, which may be many in number and of greatly differing characteristics.

39. Determination Of Protein

It should be stated, by way of preliminary explanation, that, in the past, the proportion of protein (total nitrogen compounds) in foods has been ascertained by determining the total amount of nitrogen and then multiplying its percentage number by the factor 6.25. This method is based on the assumption that the average percentage of nitrogen in the proteins is 16, which is not true to so close a degree of approximation as was formerly believed to be the case. It may happen in some instances that a determination made in this way is sufficiently accurate, while in other cases the margin of error is large. Recent investigations with perfected methods show percentages of nitrogen in the numerous single proteins found in the grains ranging from 15.25 to 18.78. These are largest in certain oil seeds and lupines and smallest in some of the winter grains. Prominent authorities concede that the factor 6.25 should be discarded, and suggest the use of 5.7 for the majority of cereal grains and leguminous seeds, 5.5 for the oil and lupine seeds, and 6 for barley, maize, buckwheat, soja bean, and white bean (Phaseolus), rape, and other brassicas. Nothing short of inability to secure greater accuracy justifies the longer continuance of a method of calculation which is apparently so greatly erroneous.

40. Various Proteins Unlike

As previously stated, protein is the accepted name for a class of compounds. Just how there came about such a grouping of a large number of substances under a single head it is not necessary to consider in this connection, but it should be made clear that the individual compounds which are included under this term are in part so unlike in chemical and physical properties as to warrant the assertion that they have but little in common except that they contain nitrogen; and we may believe that their unlikeness in composition is no greater than the differences in their nutritive functions. Moreover, the total protein of any particular foodstuff may be a mixture of several individual proteins. These mixtures differ greatly in the individual cereal grains.

It is very evident that it is not only convenient, but necessary, to classify such a heterogeneous group of bodies into subdivisions more nearly alike in their characteristics. When we come to consider doing this, we find there has existed a most unfortunate confusion of terms.

41. Classification Of Proteins

Some years ago a system of classification was reported by a committee on nomenclature, representing the Association of Agricultural Colleges and Experiment Stations.1

The first classification given is essentially this one, although there are included in it certain distinctions very clearly set forth by Professor Atwater in a paper associated with the above-mentioned report.

1 Report 1898, pp. 117-123.

In the arrangement adopted it was recognized that certain nitrogen bodies included under protein are so unlike the main and important members of this group as to be properly styled non-protein. It is also conceded that there are simple or native proteins which seem to stand in the relation of "mother" substances to a large number of protein bodies that have been modified either by various external agencies, or are the result of a union of proteins with compounds of another class.

Other nitrogen compounds are included with the protein by the present methods of chemical analysis, such as alkaloids and nitrates, but these are so uncommon in foods, or are present in such small quantities, that they may be safely ignored.