UNITED STATES DEPARTMENT OF AGRICULTURE BULLETIN No. 717 Contribution from the States Relations Service Washington, D. C. A. C. TRUE, Director PROFESSIONAL PAPER September 25, 1918 DIGESTIBILITY OF PROTEIN SUPPLIED BY SOY- By ARTHUR D. HOLMES, Specialist in Charge of Digestion Experiments, Office of Home Economics. It has long been recognized that in order to maintain the wellbeing of an individual, or in fact to maintain the normal body processes, proteins of sufficient quantity and quality are absolutely essential. The proteins normally occurring in the human dietary are of a quite varied nature and are derived from both the animal and vegetable kingdoms. Those foods which contain the largest proportion of protein and are ordinarily referred to as protein-rich foods are obtained largely from the animal kingdom and include such food materials as meats, eggs, and milk. 1 Prepared under the direction of C. F. Langworthy, Chief, Office of Home Economics. NOTE.-This bulletin records studies of the digestibility of protein supplied by soy-bean and peanut press-cake flours. It is primarily of interest to students and investigators of food problems. 64347°-18-Bull. 717—1 The many studies which have been made to determine the actual importance for dietary purposes of the proteins supplied by meats, eggs, and milk show quite conclusively that these proteins are of high biologic value. Unfortunately the supply of these food materials is becoming constantly less adequate to meet the needs of the very rapidly increasing population of this country. Furthermore, these foods are relatively expensive, and consequently can not be eaten as freely as formerly by those of small incomes. As a result of these conditions, students of nutrition have given considerable attention to the possibility of replacing animal proteins with vegetable proteins, and as a consequence the demand for vegetable foods which supply proteins that are of value for human food is steadily increasing. The cereals which contain relatively small amounts of protein are already so extensively used that as a group they now supply nearly one-half of the total protein of the average American diet. While practically all of the common vegetable foods contain protein, the amount present in foods of vegetable origin is small except in the case of the legumes, which may be classed as protein-rich food. Accordingly it appears that a more extensive use of legumes as a source of protein is to be desired. Those legumes most commonly used in this country are the well-known navy beans, red kidney, and lima beans, the garden pea, frijoles, pinto beans, and several varieties of cowpeas. Many studies have been made of the nutritive value and possible uses of these legumes, and several investigators have conducted experiments to determine their digestibility. INVESTIGATIONS OF DIGESTIBILITY OF COMMON LEGUMES. The digestibility of hulled peas cooked thoroughly until soft and passed through a sieve was determined by Rubner,' who found that the protein was 72 per cent digested. In a second experiment in which the subject ate 600 grams of peas per day the digestibility was found to be 83 per cent. Malfatti determined the digestibility of the protein supplied by a porridge made of split peas and found it to be 86 per cent digested. Richter reported that in an experiment in which the subject consumed 600 grams of purée of pea the protein was 90 per cent digested. 4 Snyder conducted three experiments to determine the digestibility of pea protein and served a porridge (containing 82 per cent of water) made from dried peas. The porridge was eaten in conjunction with a basal ration consisting of rice, milk, and sugar; onethird of the protein of the diet was supplied by the peas. He found that the average digestibility of the pea protein was 80 per cent. 13tschr. Biol., 16 (1880), No. 1, pp. 119-128, 3 Arch. Hyg., 46 (1903), No. 3, pp. 264-273. Prausnitz1 in a study of the digestibility of white beans cooked in salted water until soft found that the protein was approximately 70 per cent digested. Snyder' studied the digestibility of navy beans which were prepared by cooking for 20 minutes in boiling water containing bicarbonate of soda, removing the skins, and baking the hulled beans in the usual way. The results of three experiments indicated that bean protein was 80 per cent utilized. Woods and Mansfield,3 in a study of the digestibility of rations for lumbermen, included three experiments in which the ration contained beans, and found that the digestibility of the protein of the total diet was 85 per cent, and estimated that the digestibility of the bean protein was 78 per cent. 4 Wait conducted a series of 72 experiments to determine the digestibility of kidney beans, white beans, and three varieties of cowpeas eaten in conjunction with a basal ration consisting of bread, milk, butter, pork, bananas, and sugar, and found that the coefficients of digestibility of the proteins were as follows: Kidney beans 77 per cent, white beans 78 per cent, Whippoorwill cowpeas 70 per cent, Clay cowpeas 74 per cent, and Lady cowpeas 83 per cent. The general conclusion to be drawn from the experimental data cited above is that while the proteins of the more widely used legumes are quite well utilized by the human body, comparing very favorably in this respect with the cereal proteins, they are not as completely utilized as the proteins supplied by such animal foods as meats, eggs, and milk. SOURCE AND AVAILABLE SUPPLY OF SOY-BEAN AND PEANUT PRESS CAKE. Considerable interest has recently been aroused regarding the nutritive value of two legumes, soy beans, which as yet have not been extensively used in this country for food purposes, and peanuts, which, though well known and commonly eaten, have been used as a casual rather than a staple article of diet. It is of course true that soy beans have found limited use in some localities and that peanuts have been quite extensively used as a constituent of confectionery, as "salted peanuts," and more recently as "peanut butter," but neither has been very extensively used in the manner in which peas and beans appear in the dietary. During very recent years the production of both these legumes has grown many fold. The increase in the culture of peanuts is due very likely to the fact that the boll weevil has made the growing of cotton unprofitable in many sections of the South and peanuts are now grown as a supplementary crop. The recent increase in the production of soy beans has probably resulted primarily from the present great need of additional food materials for both human and animal consumption. The present imperative demand for oils in the manufacture of explosives, coupled with the increased demand for oils for industrial and edible purposes, makes it highly probable that large quantities of soy beans and peanuts will be pressed in cottonseed-oil mills where the machinery is adapted for the expression of oil from these oil-bearing seeds as well as cotton seed. 2 It has been estimated1 that 150,000 bushels, or 9,000,000 pounds of soy beans was pressed during the season (1917-18) in North Carolina alone, which produced more than 3,500 tons of press cake. In addition to the supply of domestic soy beans there are large quantities of beans which have been imported from other countries. It is also reported that the greater portion of the present unusually large crop of peanuts will be pressed. The residue, which remains after most of the oil has been removed from soy beans and peanuts, commercially known as press cake, contains a high percentage of protein. Such press cake has been very largely used as stock feed and, because of its high nitrogen content, for fertilizing purposes. It is at once apparent, however, that if care were exercised in grading and cleaning sound soy beans and in shelling and sorting sound peanuts, and if they were pressed under sanitary conditions, the resulting press cakes should be of considerable value for use as human food, and especially as a source of protein. Furthermore, since the oil is not fully extracted by pressing, the "cakes" are also an important source of fat. Thus it would seem that the extensive studies which have already been made of the soy-bean and peanut proteins are of a very definite value and that further studies of the possible uses and value of high-grade soy-bean and peanut press cakes for human food are to be desired. FACTORS CONSIDERED IN DETERMINING FOOD VALUE OF A PROTEIN. In considering the value of any material as a source of protein for food purposes, at least three factors are ordinarily considered, namely, the amount, the quality, and the digestibility of the protein supplied by the material in question. The amount of the protein present in the material under consideration is determined by chemical analysis. The quality of the protein ordinarily is determined by either, or preferably both, of two methods: By ascertaining the number and nature of the amino acids making up the protein molecule, 1 Personal communication. Forage Crop Investigations, Bureau Plant Industry. 2 Personal communication, Bureau of Chemistry. or by ascertaining the biologic value of the protein by means of carefully controlled feeding experiments with laboratory animals. Such experiments give data regarding the ability of a protein to support normal life processes, such as growth, maintenance, and reproduction. The amount of protein retained for body uses from the food eaten is generally determined by digestion experiments in which the protein under consideration appears as the sole or major portion of the protein in the diet. The amount of protein supplied by soy-bean and peanut press-cake flours would obviously vary somewhat with the varieties of the legumes from which the flours were made and with the process of manufacture. Proximate analysis of the dried soy beans and peanuts shows them to have the following composition: Soy beans,1 water 9.9 per cent, protein 36.5 per cent, fat 17.5 per cent, carbohydrates 30.8 per cent, and ash 5.3 per cent, the fuel value being approximately 1,950 calories per pound; peanuts, water 9.2 per cent, protein 25.8 per cent, fat 38.6 per cent, carbohydrates 24.4 per cent, and ash 2 per cent, the fuel value being approximately 2,480 calories per pound. When the oil is expressed from these legumes the percentage of protein is correspondingly increased and the resulting commercial press cakes ordinarily contain from 40 to 50 per cent of protein. The press cakes used in the study here reported, which were obtained by expressing oil from soy beans and peanuts in a small-sized expeller type of oil press, contained a larger amount of protein. But in any case the press cake should be classed as protein-rich material and on the basis of chemical analysis alone may well be considered as valuable food material. In order to ascertain the true value of these proteins for dietary purposes it is necessary to consider the factors referred to above, first, the quality of the protein as shown by the nature of the amino acids supplied and by its biologic value, and second, the digestibility of the protein. THE AMINO ACIDS SUPPLIED BY SOY-BEAN AND PEANUT PROTEINS AS COMPARED WITH THOSE SUPPLIED BY COMMON CEREAL PROTEINS. Since in the commercial utilization of soy-bean and peanut flours as human food these flours would, in all probability, be grouped with the cereal flours and meals, it is natural to compare the amino acids obtained from glycinin, the principal protein of soy beans, and arachin, the principal protein of peanuts, with gliadin, zein, rye-prolamin, oryzinin, and hordein, the principal proteins of wheat, corn, rye, rice, and barley, respectively. 1 Personal communication, average of 121 analyses reported by Forage Crop Investigations, Bureau Plant Industry. U. S. Dept. Agr., Office Expt. Stas. Bul. 28 (1899), pp. 75. |