Sources and Methods of Computing Food Analysis: Historical Aspect. Cuts of Meat: Beef; Lamb; Pork; Veal, etc. AVERAGE CHEMICAL COMPOSITION OF AMERICAN FOOD MATERIALS Meaning of Protein.-Nitrogenous matter is usually designated "protein." Nominally the total nitrogenous substance of animal and vegetable food materials, exclusive of the so-called nitrogenous fats, is included in this term. Actually it is employed, in common usage, to designate the product of the total nitrogen in a substance by an empirical factor generally expressed as 6.25. The common practice of European and American chemists and physiologists is to multiply the percentage of nitrogen by 6.25 and take the product as representing the total nitrogenous substance. For many food materials, animal and vegetable, this factor, 6.25, would be nearly correct for proteins, which contain on the average not far from 16 per cent of nitrogen, although the nitrogen content of the individual protein is quite varied. The variations in the nitrogen of nonproteins are wider, and they contain on an 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. Throughout most of the tables for meats and other animal foods the protein is calculated by subtracting the other ingredients, including water and mineral matter, from 100 per cent. The difference in the two estimates of "protein" is usually less than 1 per cent of the total meat, and the Nx 6.25 estimate is usually, but not always, in excess of the estimate by difference. While this latter method of calculation is by no means absolutely correct, it is nevertheless oftentimes more nearly so than the determination by use of the usual formula, N × 6.25. Sources and Methods of Computing Food Analysis Tables. In the compilation of these tables only edible portions are retained, unless otherwise stated, especially in animal foods. By edible foodstuff is meant the material as ordinarily prepared for cooking, after removal of bones, skin, entrails, peelings, cores, pods, rinds, etc. Thus the edible foodstuff still contains material ordinarily removed by dainty, finicky or fastidious diners, as fat, gristle, etc., of meats, fine skins and other parts of vegetable foods, the under crust of pie, and other materials in foods. This unused material is a physiological waste. The material for this chapter has been taken largely from the revised work of Atwater and Bryant (1), which represents by far the best compilation extant of analyses of American foods down to 1906. In abstracting these tables, the minimum, maximum and averages only of edible foods are given, which suffice for the purpose of this work. For the complete tables the reader is referred to the original report (2). In the preparation of these tables much care has been exercised to keep the protein, fat and carbohydrate percentages corrected to show true protein and digestible carbohydrate. For meats the N x 6.25 method of calculation has been followed. Many of the computations as to the protein content of vegetables are slightly excessive, since no actual estimate of protein proper was made. This is of no practical importance, however, because if the protein content is large, as in the legumes, the nonprotein nitrogenous substances are comparatively insignificant; while, if the total nitrogenous constituents are small, the error for any ordinary use of the food is slight in the aggregate. In amplifying the fuel value of the foods great care has been exercised by the author to estimate the caloric value per hundred grams of the various foods. Realizing the fallacy of exactitude in determining positive fuel values, some license has been taken to state fractions of a per cent in the nearest per cent. It will be realized, therefore, that in using these tables to ascertain the organic nutrients of foodstuffs ingested, only approximate results can be hoped for, hence it will serve every purpose to use average figures and to ignore fractions of a per cent or even the nearest aliquot fraction of the whole. HISTORICAL ASPECT.-The earliest quantitative analyses of food materials with which we have come in contact are those of potatoes, reported by George Pearson in England in 1795 (3). Einhoff (4), in 1805, made analyses of potatoes and rye. He attempted to separate the albumin in the latter and in the former, he also determined several of the constituents of the ash. Other workers in this field of research, chiefly in the period from 1840 to 1865, have reported their findings. Nearly all our definite knowledge of the chemical composition of food materials and their nutritive value, however, has accumulated within comparatively the past few years. The great advance in the analytical chemistry of foodstuffs was made in 1864, when Henneberg and his associates elaborated the so-called "Weende method" for proximate analysis. This method with slight variations is used to-day wherever food analyses are made. "The methods followed in different countries agree so closely that for the last twenty years it has been possible to accept analyses by chemists in different parts of the world and compare them with one another without hesitation" (Atwater and Bryant). The first analyses by these methods in the United States of which record has been found were a series of analyses of corn in 1869 (5). With thanks to the research workers in the Experiment Stations of the United States Department of Agriculture, who have perhaps led the world in scientific work of this character, we now have a definite knowledge of the chemical composition of foodstuffs, their fuel value, and nutritive properties. Explanation of Terms (6).-The terms used in reporting analyses of foods and feeding stuffs need some explanation. Some of these terms have a technical meaning which is well recognized and understood by scientists, although the dictionaries and similar books of reference have not yet included these terms in their definitions. In other cases the same word has been used by scientists in different ways. The more usual terms are defined and explained below in the sense in which they are employed in this bulletin and other publications of this office: COMPOSITION OF FOOD MATERIALS Ordinary food materials, such as meat, fish, eggs, potatoes, wheat, etc., consist of: Refuse. As the bones of meat and fish, shells of shellfish, skin of potatoes, bran of wheat, etc. Edible Portion.-As the flesh of meat and fish, the white and yolk of eggs, wheat flour, etc. This edible portion consists of water (usully incorporated in the tissue and not visible as such), and nutritive ingredients or nutrients. The principal kinds of nutritive ingredients are protein, fats, carbohydrates, and ash or mineral matters. The water and refuse of various foods and the salt of salted meat and fish are called nonnutrients. In comparing the values of different food materials for nourishment they are left out of account. Protein. This term is used to include nominally the total nitrogenous substance of animal and vegetable food materials, exclusive of the so-called nitrogenous fats. Actually it is employed, in common usage, to designate the product of the total nitrogen by an empirical factor, generally 6.25. This total nitrogenous substance consists of a great variety of chemical compounds, conveniently divided into two principal classes, proteids and nonproteids. The following is taken from Bulletin 28, U. S. Department of Agriculture: The term proteid, as here employed, includes the simple proteids, e.g., (a) albuminoids, globulins, and their derivatives, such as acid and alkali albumins, coagulated proteids, proteoses, and peptones; (b) the so-called combined or compound proteids; and (c) the so-called gelatinoids (sometimes called “glutinoids”) which are characteristic of animal connective tissue. The term albuminoids has long been used by European and American chemists and physiologists as a collective designation for the substances of the first two groups though many apply it to all three of these groups. Of late a number of investigators and writers have employed it as a special designation for compounds of the third class. The term nonproteid is here used synonymously with nonalbuminoid, and includes nitrogenous animal and vegetable compounds of simpler constitution than the proteids. The most important animal compounds of this class are the socalled "nitrogenous extractives" of muscular and connective tissue, such as creatin, creatinin, xanthin, hypoxanthin, and allied cleavage products of the proteids, To some of these the term "meat bases" has been applied. The latter, with certain mineral salts (potassium phosphates, etc.), are the most important constituents of beef tea and many commercial "meat extracts." The nonproteid nitrogenous compounds in vegetable foods consist of amids and amido-acids, of which asparagin and aspartic acid are familiar examples. 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 nonproteids 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. |