Nutritive Value of Some Plant Proteins

When we saw that the animal proteins would be scarce, we set about getting more information on the nutritional value of plant proteins. Before the war, people in the United States got most of their protein from animal sources; in a land of plenty, with an abundance of meat, milk, and eggs, little thought had been given to the use of plant proteins.

There are several farm products that offer fine possibilities for supplying plant proteins for human use but that have been used chiefly as feed for farm animals. These include soybeans, peanuts, cottonseed, and wheat and corn germs, the potential production of which is very great. Heat-processed soybean flour and soybean grits are being produced for human consumption in large quantities. Peanut and cottonseed flours. in limited amounts are also available. Most of the oil is first removed from the seeds by expression or by solvent extraction. The flours prepared from the residues contain from 45 to 50 percent of high-quality protein, a content exceeded by only very few other foods.

Soybeans have been used for centuries in the Orient as a source of protein in human diets. Until recently the small amount grown in the United States was used chiefly for oil. The byproduct, the press cake, has been highly regarded as feed for farm animals. But the war stimulated the growing of soybeans, and commercial methods were developed to produce soybean foods for human use. Large quantities of soybean flour and grits were shipped overseas for our armed forces, and for lend-lease supplies. Larger amounts were also consumed in the United States.

Dry, mature soybeans are one of the richest known sources of protein among naturally occurring foods, containing from 30 to 45 percent of protein, depending on variety, soil, and climatic conditions.

Three types of soybean flour are manufactured, low-fat, medium-fat, and full-fat. The low-fat flour is produced by solvent extraction, which removes practically all of the oil. It contains about 45 percent of protein. The medium-fat flour is made from the residue remaining after most of the oil has been removed by expression. It retains from 5 to 6 percent Of oil and contains about 43 percent protein. The full-fat flour has all the oil of the seed and contains about 40 percent protein. All of the flours are subjected to heat processing.

It is characteristic of soybean protein that it requires heat treatment in the presence of water or steam in order to develop its maximum nutritional value. In the raw state, its protein has low value. Excessive heating, on the other hand, has an injurious effect. This may develop when the oil is removed by expeller processes. Discrepancies often observed in the protein value of oilseed products may be attributable to the effect of excessive or insufficient heat treatment.

As sources of nutritionally efficient protein, wheat germ and corn germ, particularly the former, stand foremost among plant foods. In a form suitable for human food they offer a relatively cheap source of high-quality protein potentially available in large quantities. The-nutritive value of wheat-germ protein in some respects is nearly that of meat and milk.

The potential annual production of wheat germ in the United States is about 500 million pounds. The recent production has been between 30 million and 50 million pounds. Defatted wheat germ, containing 34 percent protein, prepared for human use and having excellent keeping qualities, is available in large quantities.

When fed at a 10-percent-protein level, wheat germ was found as efficient as skim-milk powder. But at higher levels the latter proved superior.

Defatted corn germ is also commercially prepared for human consumption. It contains about 20 percent protein. When fed at 10- and 15-percent-protein levels it gave values equal to, or greater than, that of peanut or soybean protein, but not so high as that of wheat germ.

Peanuts and soybeans, both legumes, have several characteristics in common. They have a high content of protein and oil, but contain insignificant quantities of starch. The protein content of peanut kernels ranges from 28 to 35 percent, but, unlike soybeans, peanuts do not require heat treatment to develop the maximum nutritional value of their proteins. Raw peanuts have. essentially the same protein value as when heat processed.

Arachin and conarachin represent almost entirely the total protein content of the peanut. About 75 percent of it consists of arachin. This protein contains only a small amount of methionine, but conarachin has enough of this amino acid to compensate for its deficiency in arachin.

Some peanut flour can be bought in stores in limited amounts, but much more of it might be made and eaten. The flour contains more than 50 percent protein, comparable in nutritive value to that of soybean and cottonseed.

Cottonseed is another source of plant protein that merits particular attention. Cottonseed meal, the ground residue remaining after removal of most of the oil, has long been valued as a protein feed for farm animals. It contains all of the nutritionally essential amino acids and is particularly suitable for supplementing the proteins of cereal grains.

A partially defatted cottonseed flour produced for human consumption is available. Its high protein content, 55 percent, is exceeded by only a few other foods. Large quantities of the flour were shipped overseas during the war; in the United States it is used to a considerable extent in commercial bakery products. In digestibility, cottonseed protein compares favorably with animal products, and its growth-promoting value is about four and a half times that of wheat flour. But in order to be safe and suitable for use as food, cottonseed flour must be thoroughly processed.

THE AUTHOR

D. Breese Jones, as principal chemist in the Bureau of Human Nutrition and Home Economics, is in charge of the Bureau's investigations on proteins and amino acids. He has conducted research in the fields of proteins and nutrition for the Department since 1915 and, together with his associates, has published more than 200 articles. In 1938 he represented the Department at the Sixteenth International Physiological Congress in Zurich, Switzerland.

Known Nutrients in Milk

by C. A. CARY and A. M. HARTMAN

AS CONSUMERS, we are interested in three practical problems in connection with a food. If, say, each of us consumes a quart of milk a day or 18 pounds of butter a year, as we did before the war, it is essential to know what nutrients and how much of each we actually obtain from these foods. It is essential also to know whether there are any precautions that we might take to preserve the nutrients in those foods or anything that can be done practically in their production to improve their nutritive value. And, thirdly, since there is certainly no reason to believe that we know all the nutrients in our foods and feeds or that we can live and thrive on a diet containing all the nutrients now known, it is essential to appraise the value of these foods as sources of still undiscovered or still unidentified nutrients that may play an important role in determining our welfare. Also, numerous observations indicate that it is hard to appraise fully the nutritive value of milk from information now available of the known nutrients reported to be present in it.

Milk is a rich source of protein; the nutritive value of milk proteins is unexcelled. S. K. Kon, of the National Institute for Research in Dairying, England, says: "The great value of the proteins of milk lies in their ability to enhance the biological value of the proteins of such staple vegetable foods as cereals and potatoes. Thus, for example, a combination of bread and cheese had, when tested on rats, the same biological value, 75.5, as cheese, though bread alone had a value of only 52.0; similarly a combination of milk and potato had a biological value of 86, almost as high as that of milk, 87, though potato alone had a value of only 71."

Similar results have been obtained with a mixture of corn and milk proteins, and results have been published that indicate a good utilization of the mixture of proteins in bread-and-milk. This is because milk proteins are rich in some of the essential amino acids that are relatively not so plentiful in many plant proteins.

Vitamin A Potency of Butter and Milk

Vitamin A itself is a product made only in animals, but animals can make it only from certain precursors, the carotenes, which are produced by plants. The green and leafy portions of plants are good sources of these precursors, as are also carrots and sweetpotatoes. We may eat some of these plant materials and make our own vitamin A, or we may eat animal products that contain ready-made vitamin A milk, butter, cheese, egg yolk, fish oils, and others. In milk and milk products some still unchanged carotene and ready-made vitamin A occur together, about 85 to 95 percent of the natural yellow color of butter being due to the unchanged carotene, which is itself yellow and which we can use to make vitamin A. Each natural food or feed that we depend upon as a source of vitamin A varies greatly sometimes severalfold in its vitamin A value, or potency, depending in the case of plant foods or feeds on how they have been produced, harvested, or preserved, and in the case of animal products on how the animal was fed. Fortunately, animals, to a certain extent, have the capacity to accommodate themselves to variations in their intake of this vitamin or its precursor; they store it when it is plentiful in the diet, and use these reserves when the vitamin A value of the food is below current needs.

Because of the importance of vitamin A in nutrition, a knowledge of its potency and the conditions that influence this potency is exceedingly important in the case of any food that is widely depended upon as a source of this nutrient.

Before the war, the people of the United States consumed on an average about 18 pounds of butter per capita a year, and the domestic consumption of milk in all forms was about 1.05 quarts per person a day. In a period of food shortages, when one food might be substituted for another or when we might be asked to reduce consumption as far as possible without sacrifice of health, precise information regarding food values is imperative. The Committee on Food and Nutrition (now the Food and Nutrition Board) of the National Research Council recognized this condition in 1941. At their suggestion, and in cooperation with the Department and the Association of Land-Grant Colleges and Universities, 21 State agricultural experiment stations and one Federal laboratory undertook to determine the average vitamin A potency of the butter consumed in the United States, and the variation in that potency from month to month or from season to season during the year in different parts of the country.

Sixteen State experiment stations (Minnesota, Wisconsin, Ohio, Iowa, Nebraska, Kansas, North Carolina, Louisiana, Mississippi, Washington, Oregon, California, Idaho, Montana, Wyoming, and Arizona) did a thorough job of determining the vitamin A potency of the butter produced in their creameries. These States produce about 64 percent of the creamery butter produced in the United States.

This butter is representative of that produced in the North Central, South Atlantic and Gulf, and Rocky Mountain and Pacific Coast States, or of about 97 percent of the national output. Seven State stations (Wisconsin, Minnesota, Ohio, California, Montana, North Carolina, and Mississippi) have studied the effect on the vitamin A potency of various methods of commercial storage and handling of market butters and four States Texas (independent of this survey), New York (Cornell), Pennsylvania, and Alabama have determined the vitamin A potency of the butter sold on their retail markets, including both locally produced and nationally known brands.

The results of the survey (more than 4,000 samples of butter) may be summed up thus :

Except in Arizona and Montana, there was a distinct difference between the vitamin A potency of the creamery butter produced during the winter months and that produced during the summer. Of all the butter produced in the United States, close to 36 percent may be considered "winter" butter with an average vitamin A potency close to 11,000 I. U. per pound; and about 64 percent is summer butter, produced during summer months when cows are on pasture, and has an average vitamin A potency of about 18,000 I. U. a pound. The weighted annual average obtained by using the production in all States (average of 1942 and 1943 productions) in each area and the seasonal distribution of production in the participating States, was found to be 15,700 I. U. a pound, which is as close as it is now possible to approximate the average vitamin A potency of the total output of creamery butter in this country.

In the study of the effect of the storage and handling of butter on its vitamin A potency, samples were handled and stored under conditions identical with those used in practice. The results indicate that both carotene and vitamin A are very stable in butter under the conditions tested; that little if any loss of vitamin A potency occurs during the periods that commercial butter is ordinarily stored; and that, insofar as the effect of storage is concerned, one would expect the average vitamin A potency of the butter sold on the retail markets in this country not to be significantly different from the average of the creamery butter produced in the country as a whole. The results of the study of the vitamin A potency of market butters confirmed this conclusion.

On the basis of the above results, one can calculate the average vitamin A potency of the milk produced in the United States. Winter milk i. e., milk produced by cows on barn rations would accordingly have an average vitamin A potency of 1,120 I. U. per quart; and summer milk or milk produced by cows on pasture an average potency of about 1,820 I. U. per quart. These calculations are based on milk with a fat content of 4 percent. It is estimated that about 60 percent of the milk consumed as milk or milk products by the people of the United States is summer milk. Thus the average vitamin A potency of milk, containing 4 percent of fat, consumed annually in this country is about 1,540 I. U. per quart; approximately one-third of the 5,000 units of vitamin A recommended by the National Research Council as the daily allowance for a normal adult is supplied by milk and milk products.