Recovering Oil and Meal

E. L. D'Aquin, E. A. Gastrock, O. L. Brekke.

The three methods now in common use to recover oil and meal from oilseeds are hydraulic pressing, screw pressing, and solvent extraction. The first two use pressure to remove the oil from the material. The third, as its name implies, employs a solvent to leach out or dissolve the oil, after which the mixture is separated. All the methods, alone or in combination, are used commercially on the many oil-bearing types of seeds, nuts, and beans that are grown throughout the world.

HYDRAULIC PRESSING, the earliest of the processes, is said to have originated in Europe in 1795. The hydraulic press, so named because it works on the principle of the hydraulic ram, operates like an ordinary machine-shop press. It was first used extensively in this country for processing linseed and cottonseed. Later it was used for soybeans; it is still the principal process for crushing cottonseed. Because it is a batch-type method that requires much hand labor, its use is declining in favor of the other methods, which are continuous, require much less labor, and recover the oil more completely. Different types of hydraulic presses are known as box presses, plate presses, cage presses, and pot presses.

THE CONTINUOUS SCREW PRESS is much like a household meat grinder in that it has a rotating screw to force the material under a very high pressure through a cagelike cylinder. The oil is squeezed out through narrow openings in the wall of the cylinder. The first successful continuous screw press was probably of American origin and was introduced about 50 years ago. Today the presses are used the world over to process every known type of oil-bearing material. In the United States they are used mainly for soybeans and flaxseed but also for cottonseed, copra, corn germ, tung nuts, and other oilseeds. American presses are unlike those in Europe, where the practice is to remove only part of the oil in order to procure a meal suitable for re-extraction with solvents or presses. In this country, continuous presses are designed to remove nearly all the oil in a single pass through the press by the use of a much higher pressure.

SOLVENT EXTRACTION originated as a batch process in Europe in 1870, and its use soon spread to the United States, where it is still employed for such materials as meat scraps, pine chips, garbage, and castor pomace.

Technological advances in Europe shortly after the First World War led to the development of continuous solvent-extraction systems, which proved excellent for processing soybeans to a very low oil content. As a result, the process was introduced in the United States for soybeans in the 1930's. Since then its use has increased tremendously to keep pace with our expansion in soybean production.

Despite its outstanding success with soybeans, solvent extraction has almost never proved fully satisfactory for directly extracting oil from other oilseeds. It is, however, satisfactory for indirect extraction of the other oils; that is, for application to the press residue obtained after mechanically pressing out the bulk of the oil. This is the general practice in Europe today.

American engineers, on the other hand, are experimenting with both methods, and already a number of plants have been erected for the solvent extraction of the oil from cottonseed, flaxseed, castor beans, corn germ, and rice bran.

Several mechanical operations are needed to prepare oilseeds before the oil can be removed. Cleaning to remove foreign matter comes first. Next the meats are separated from the hulls or shells. The meats are then reduced in size. The last step is usually a cooking operation, which conditions the material for extraction of the oil.

Actually, the procedure is not that simple. It varies for each oilseed and for each process. Moreover, every step in the sequence depends on how carefully the previous operation was carried out. In general, proper size reduction, moisture content, and cooking are requisites. All those factors must be integrated by experience and controlled if the best yields and quality of oil and meal are to be obtained.

First come storage and cleaning. Deliveries to the oil mill of soybeans and cottonseed are usually in bulk and by trucks or rail. Peanuts are handled in sacks. Unloading is done mainly by power shovels, pneumatic systems, and hydraulic lifts. Arrivals in excess of crushing requirements are placed in storage. Cottonseed is stored in seed houses of the Muskogee type. Silos and grain elevators are used for soybeans. Soybeans containing excess moisture are dried before storing. Cottonseed is not dried, but the houses are generally equipped with facilities for circulating air through the seeds to cool them if they heat up in storage.

Oilseeds contain miscellaneous foreign matter. Cottonseed, for example, contains bolls, twigs, stones, leaves, sand, and tramp metal matter that must be removed by cleaning to prevent damage to the machines or costly shutdowns and to improve the quality of the oil and meal products.

Cleaning equipment varies from mill to mill. Shaker screens and rotary reels generally are used, along with air aspiration to remove the very light and the very heavy impurities. The most effective and versatile cleaners are the pneumatic-mechanical type, which employ both reciprocating shakers and air currents for making the various separations. Magnets are also used at one or more points to remove iron and steel.

Hulling and separating are next. Hulling follows cleaning for all but cottonseed, which must first be passed through delinting machines to remove most of the short fibers, or linters, left on after ginning. Because the oil content of hulls is very low, the hulls are broken off and separated before extraction except tiny seeds like sesame and flaxseed, which are usually processed without this decortication. Hulls of medium-size seeds, like cottonseed, soybeans, and peanuts, are cracked or cut in disc or bar hullers or in attrition mills. Larger oilseeds, including palm kernels, tung nuts, and copra, are first broken up in hammer mills.

After cracking in the huller, the mixture of hulls and kernels is separated. The standard machines for the purpose are reciprocating and rotary screens equipped with air aspiration. Usually the hulls are removed as completely as possible, because their presence reduces the capacity of the extraction equipment and lowers the total yield of oil in pressing. Cottonseed is more difficult to separate than peanuts. Tung nuts are even harder to separate completely. Soybeans have only a thin skin coat, which sometimes is removed prior to solvent extraction.

After the hulls are removed, oilseed meats are reduced in size to facilitate flaking, cooking, and extraction of the oil. The general practice is to flatten them into thin flakes. The machine almost universally used is a vertical stand of five superimposed cylindrical rolls, between which the material is given four successive crushings as it passes from the top to the bottom. The flaking, cooking, and pressing are most effective when the meats are of proper moisture content and are rolled to uniform flakes 0.005 to 0.010 inch thick. The moisture content is a critical factor in flaking; the rolls will pulverize the meats if they are too dry. They will gum up on the roll surfaces if they are too wet.

When the oilseed is to be hydraulic-pressed, flaking is considered necessary. Cottonseed meats and corn germ may be flaked also for the continuous pressing, but soybeans, flaxseed, and peanuts are merely reduced in cracking rolls to grit-size particles. Flaking rolls are essential in preparing an oilseed for solvent extraction, because no other form of mill gives particles thin enough to permit ready extraction and allow percolation of the solvent without crumbling.

NEXT, THE OILSEED MEATS are subjected to some form of cooking or heat treatment. Not all the changes that take place in cooking are well understood, but cooking seems to coagulate the protein and to rupture the oil cells, a development that makes the oil more readily extractable. Careful control of cooking is essential in mechanical pressing, because both quality and yield of oil are affected by the conditions of moisture and temperature and duration of cooking.

For hydraulic pressing, the practice is to cook the flaked meats to a moisture content of 5 to 8 percent. For screw pressing, the material must be cooked until it is very dry (1 to 3 percent moisture). If the oil is to be solvent-extracted, the procedure is somewhat different, in that the meats are first given a moderate heat tempering. Flaking is the last step.

The cooked oilseed is ready for oil extraction by hydraulic pressing, continuous screw pressing, or solvent extraction.

A stack cooker is used for cooking flakes for hydraulic pressing. It is a vertical bank of three to six steam-jacketed kettles, stacked one above the other so that the meats are progressively cooked as they pass continuously through the kettles. Stirring blades facilitate heating. The typical modern procedure in cooking cottonseed meats is as follows. The flakes, preferably of 11 to 12 percent moisture content and 0.005 to 0.010 inch thick, are fed into the top kettle, where their temperature is brought up quickly to 180 to 190 F. Then the temperature is gradually raised as the material progresses for 80 to 120 minutes through the other kettles. In the last kettle the temperature is brought up to 225 to 235 F. and the moisture content down to 5 to 8 percent. If the final moisture content is too high or too low, the cooked material does not handle properly in the presses.

As they are discharged from the cooker, the hot, cooked meats are formed into uniform cakes in press cloths made of human hair and are charged by hand into the boxes of the press. The typical hydraulic press used for cottonseed consists of a heavy steel frame made up of four vertical steel columns fastened at the top and bottom to heavy iron blocks. The space within the columns is closely filled with a series of horizontal plates, which are hung from the top block. Each plate is suspended from the plate above it by linkages, which permit the entire assembly to be compressed by the upward motion of a hydraulic ram built into the bottom block. Most presses contain 15 openings or boxes, and the cooked material in the thick press cloths is inserted between the plates until the press is filled.

When all the boxes are filled, hydraulic pressure generated by a pump is applied to the ram. This forces the ram upward; that, in turn, slowly compresses the cake and causes the oil to flow through the cloths and down into the drip pan of the press. Effective operation requires that the pressure be applied at a carefully controlled rate. That is accomplished through the use of a change valve. The highest pressure to which the cake is subjected is about 1,800 pounds to the square inch.