“The ideal method of analysis of food materials would involve quantitative determinations of the amounts of each of the several kinds or groups of nitrogenous compounds. This, however, is seldom attempted. The common practice is to multiply the percentage of nitrogen by the factor 6.25 and take the product as representing the total nitrogenous substance. For many materials, animal and vegetable, this factor would be nearly correct for the proteids, which contain, on the average, not far from 16 per cent. of nitrogen, although the nitrogen content of the individual proteids is quite varied. The variations in the nitrogen of the non-proteids are wider and they contain, on the average, more than 16 per cent. of nitrogen. It is evident, therefore, that the computation of the total nitrogenous substance in this way is by no means correct. In the flesh of meats and fish, which contain very little of carbohydrates, the nitrogenous substance is frequently estimated by difference—i.e. by subtracting the ether extract and ash from the total water-free substance. While this method is not always correct, it is oftentimes more nearly so than the determination by use of the usual factor.
“The distinction between protein and proteids is thus very sharp. The latter are definite chemical compounds while the former is an entirely arbitrary term used to designate a group which is commonly assumed to include all of the nitrogenous matter of the food except the nitrogenous fats.
“In the tables herewith the common usage is followed, by which the protein is given as estimated by factor, i.e., total nitrogen multiplied by 6.25. In the analyses of meats and fish, however, the figures for protein ‘by difference’ are also given. Where the proteid and non-proteid nitrogenous matter have been estimated in a food material the proportions are indicated in a footnote.
“Fats.—Under fats is included the total ether extract. Familiar examples of fat are fat of meat, fat of milk (butter), oil of corn, olive oil, etc. The ingredients of the ‘ether extract’ of animal and vegetable foods and feeding stuffs, which it is customary to group roughly as fats, include with the true fats various other substances, as fatty acids, lecithins (nitrogenous fats), and chlorophylls.
“Carbohydrates.—Carbohydrates are usually determined by difference. They include sugars, starches, cellulose, gums, woody fibre, etc. In many instances separate determinations of one or more of these groups have been made. The determinations of ‘fibre’ in vegetable foods, i.e., substances allied to carbohydrates but insoluble in dilute acid and alkali, and somewhat similar to woody fibre, are given in a separate column. The figures in parenthesis in the crude-fibre column show the number of analyses in which the fibre was determined. The figures for ‘total carbohydrates’ include the fibre, as well as sugars, starches, etc. Where the sugars or starches have been determined separately footnotes are added giving the average results.
“Ash or Mineral Matters.—Under this head are included phosphates, sulphates, chlorides, and other salts of potassium, sodium, magnesium, and other metallic elements. Where analyses of the mineral matters have been found they are added in the form of footnotes. These results usually give the percentage composition of the ash as produced by incineration rather than the proportions in which the different mineral ingredients occur in the food material.
“Fuel Value.—By fuel value is meant the number of calories of heat equivalent to the energy which it is assumed the body would be able to obtain from one pound of a given food material, provided the nutrients of the latter were completely digested. The fuel values of the different food materials are calculated by use of the factors of Rubner, which allow 4.1 calories for a gram of protein, the same for a gram of carbohydrates, and 9.3 calories per gram of fats. These amounts correspond to 18.6 calories of energy for each hundredth of a pound of protein and of carbohydrates, and 42.2 calories for each hundredth of a pound of fat in the given food material. In the following table the fuel value per pound has been calculated by use of these factors. In these calculations the values of protein by factor have been used in all cases with the exception of salt cod and hens’ eggs, in which the value of protein by difference was used.”
I now present a few extracts from these lengthy tables of the chemical composition of food materials—mentioning, first, some typical meats, then fishes, vegetables, grains, flours, etc., dairy products, fruits, nuts, and various sundries. I take but a few of each, in order to show the typical proteid value of the various foods, without making these tables too long; and the reader can readily see, by referring to the column of proteid percentage, that many articles of diet contain a far larger percentage of proteid than the best meats! I present the tables, however, before discussing this question at greater length.
See the [Displaced Table].