Peanut Protein for Industrial Uses

Jett C. Arthur, Jr.

The peanut is a pea, not a nut. It belongs to the bean family. It matures its fruit, or pod, underground. Small yellow flowers form at the joints where the leaves are attached to the stems; after pollination the flower fades and a shoot is formed. The shoot elongates and enters the ground, where a pod develops.

By the time it reaches maturity, the cellulosic pod, or shell, contains 1 to 3 nuts or kernels. The shell is 20 to 30 percent of the weight of the mature peanut. The kernels contain 43 to 50 percent oil, 25 to 30 percent protein, 5 to 12 percent carbohydrates, about 3 percent crude fiber, and 2.5 percent ash.

Peanuts grow best where a loose surface of the soil can be maintained. Our principal peanut-producing regions are the Virginia-North Carolina area (Virginia, North Carolina, and Tennessee), the southeastern area (South Carolina, Georgia, Florida, Alabama, and Mississippi) , and the southwestern area (Arkansas, Louisiana, Oklahoma, and Texas).

Peanuts have become one of the South's leading cash crops. In 1863, we produced about 10,000 tons of peanuts. During the 1920's, production averaged 395,000 tons annually, with a farm value of about 39 million dollars. During the 1930's, and again during the war years, production increased sharply; beginning with the1942 crop, the average annual production has been more than 1 million tons, with a farm value of almost 200 million dollars. The 194 crop was marketed as follows: 200,000 tons were kept on the farm for eating and seeding; 500,000 tons entered trade as peanut butter, candy, and salted peanuts; and 426,000 tons were crushed for oil or exported. Of the peanuts in the edible trade, 50 to 60 percent were used in peanut butter, 25 to 30 percent in salted nuts, about 10 percent in candy, and about 1.5 percent in bakery products.

Peanuts are nutritious. One pound of peanut butter contains more calories, protein, minerals, and vitamins than a pound of beefsteak.

Peanuts also are crushed to produce oil, which is used for food in the form of margarine, shortening, and cooking oils. After the oil has been separated from the rest of the peanut kernel, the residue is used for feeding livestock.

Since 1940, about 15 to 30 percent of the peanut crop has been crushed for oil and meal, more than half of it from peanuts grown in the southeastern area. Large amounts were exported under the European aid program in 1948. Except for two brief periods, the value of peanut oil and meal since 1935 has not been high enough to permit crushers to pay as much as the Government support price for peanuts; except for small quantities discarded at shelling plants as unfit for food, all the peanuts sold for crushing were subsidized by the Government.

Production for nonfood uses can be made economically feasible by reducing the cost of production, by increasing the value of peanut oil and meal, or by combining the two factors in some way.

State and Federal research is being directed toward the development of improved production, harvesting, and handling methods for peanuts and the development of higher-quality, higher-yielding varieties. At the Southern Regional Research Laboratory research has been directed toward increasing the value of the oil and meal from peanuts. One of the main aims has been to develop industrial uses for peanut meal and protein, such as glues, adhesives, paper sizes, and textile fibers.

Two GENERAL methods are used to separate the oil from the kernels mechanical pressing and solvent extraction. The only large volume of commercial peanut meal produced in the United States in 1947 was made by mechanically pressing the oil from the kernels. To increase the efficiency of the oil extraction, the peanut flakes are first cooked to temperatures from 175 to 250 F. The cooked flakes thereafter are subjected to high pressure to remove the oil. The high temperature and pressures employed in the expression processes adversely affect the quality of the protein, especially its solubility in water and in salt solutions. The solubility in water of the protein in peanut meal made by expression methods ranges from 15 to 50 percent, depending on the conditions of cooking and pressing. In a weak solution of caustic soda, however, 75 to 95 percent of the protein in the meal can be dissolved.

When the oil has been removed from the flaked peanut kernels by means of low-boiling hydrocarbons (particularly petroleum hydrocarbons, such as hexane), the solubility of the protein in the oil-free meal is about 80 percent in water and more than 95 percent in dilute caustic soda solutions.

In sum, the conditions of processing the peanut kernels for oil and meal markedly affect the solubility of the protein of the residual meal or cake in different solvents. Short periods of processing, low temperatures in preparing the flaked kernels, and low temperatures and humidity in removing the solvent from the extracted meal generally are essential to the production of peanut meals that have a high percentage of soluble proteins.

THE STEPS followed in separating protein from solvent-extracted peanut meal are : (1) Preparation of a water-meal mixture and addition of protein-dissolving chemicals; (2) clarification of the mixture by screening, filtering., and (sometimes) centrifuging; (3) precipitation of the protein as a cheese-like curd from the water-protein solution by the addition of coagulating chemicals; (4) separation of the protein curd from the solution; and (5) drying of the protein curd.

Let us examine a typical process for producing peanut protein. Solvent-extracted peanut meal is mixed with water in a ratio of 10 pounds of water to 1 pound of meal. A solution of caustic soda (30 percent) is added until the water-meal mixture is slightly alkaline, and most of the peanut meal protein has been dissolved. A half pound or so of caustic soda is required to dissolve the protein in 100 pounds of peanut meal. The solution containing the dissolved protein is separated from the insoluble portion of the meal by screening and centrifuging.

The dissolved peanut protein is recovered from the clarified solution by adding gaseous sulfur dioxide until the solution is sufficiently acid to precipitate the greatest possible amount of peanut protein as a dense cheeselike curd. The protein curd is separated from the solution and dried in a forced-air draft at 120 F.

About two-thirds of the total weight of protein in the meal (or about 40 pounds of protein from each 100 pounds of meal) can be recovered in this way. A part of the protein is not dissolved during the process, however, and the insoluble material remaining after extraction of the protein still contains about 20 to 25 percent protein. This residue has a relatively high nutritive value for livestock and is satisfactory for feeding in mixture with other high-protein concentrates.

Upon addition of salts, the water solutions remaining after separation of the protein may be utilized for the propagation of yeast. It is possible thereby to produce food and feed yeasts and to dispose of the waste solutions without polluting streams.