Vegetable Oils and Fats for Edible Use

R. O. Feuge.

Crude edible vegetable oils are subjected to various types of processing for several reasons. One is consumer preference. Most people prefer to eat light-colored and bland oils. The preference stems partly from the apparently reasonable but incorrect assumption that only colorless, odorless, and tasteless oils and fats are pure. It is true that such products can be used without masking or detracting from the characteristic flavor and palatability of the foods to which they have been added. In the United States the only notable exception is olive oil, which is prized primarily because of its own odor, flavor, and color.

From the oil processor's standpoint, purification of crude oils is advantageous. Some consumers could be persuaded to accept a few oils having characteristic flavors and odors, but marketing a wide variety of oils of different origins is greatly facilitated by purifying practically all of them to the point where their origin is not evident to the senses.

Two factors besides preference and convenience are responsible for the existence of vegetable-oil refineries. A number of oils, including some of our most common and plentiful oils, are practically inedible in the crude state and must be processed to become edible. The supply of naturally liquid oils is greater than the supply of naturally semisolid, or plastic, fats, but the demand is just the reverse. Therefore, in order to supply the big demand for the scarce plastic fats, the abundant liquid oils have to be converted to semisolid fats of good quality.

Because of those requirements, a large and widely dispersed industry now converts crude oils from many sources into a variety of refined edible products. The industry may appear complex, but actually it consists of a number of more or less standardized operations, or unit processes refining, bleaching, hydrogenating, winterizing, and deodorizing, which are combined and modified to meet special conditions. Nearly all edible vegetable oils are first subjected to refining and bleaching. Then they may be deodorized and go directly to consumers, or they may be processed further to yield either winterized oils or so-called hardened fats. Regardless of the kind and number of intermediate steps, deodorization is nearly always the final step in processing any edible fat of vegetable origin.

PURE FATS OR OILS comprise a mixture of fatty acids chemically combined to form what the chemist calls triglycerides. Refining is therefore concerned with the removal of undesirable nonglyceride components. In vegetable oils, the impurities normally consist of oil-soluble substances, which are extracted from the seed along with the oil, particles of meal, gummy substances, and degradation products of the oil itself. The amounts and kinds of the impurities depend mainly on the type and origin of the seed processed and the method of extraction. The undesirable constituents generally amount to less than 10 percent by weight of the crude oil more often to only 3 to 5 percent.

The oldest and most important method of refining vegetable oils for edible use consists in treating the crude oil with a solution of caustic soda. It looks like a simple process, but it involves complicated chemical and physical phenomena. The caustic soda reacts with the free fatty acids that are present in the crude oil to form soaps, which are insoluble in the oil and therefore tend to pass into the water phase where hydration ( absorption of water) begins. However, the concentration of the remaining caustic soda in the water is so high that the soap is kept out or is thrown out of the water solution by a phenomenon called salting out.

THE RESULT of the different reactions is the transformation of the water, the caustic soda, and the soap into numerous minute, soft, gelatinous particles. Simultaneously the other nonfat substances phosphatides, protein, protein-degradation products, and other gummy substances are forced out of solution in the form of coarse particles suspended in the oil. Some of the caustic soda reacts with a portion of the fat or oil to form more soap and free glycerin. Almost as soon as the various types of particles are formed they begin to clump together, or coalesce. As the particles grow in size and gather in the color bodies and other collodial impurities, they also adsorb (occlude) some of the refined oil. The oil remaining after completion of the operation is much lighter in color and retains only traces of nonfat compounds.

In some of the older and smaller refineries, this treatment of crude oil with caustic soda is carried out in large open kettles. Generally, the kettles are cylindrical, have cone-shaped bottoms, and hold 60,000 pounds of oil apiece. Each kettle has a steam coil for heating the charge and a stirrer for mixing. Oils like cottonseed, soybean, and peanut are charged into the kettle at a temperature of about 70 F. After the bubbles of air have escaped from the oil, the stirrer is set for rapid mixing, and the caustic soda is added. A milk-like emulsion forms immediately; as the mixing continues small particles of soap appear and start to grow. After a mixing period of 10 to 45 minutes, the surface of the mixture of oil and caustic solution gradually assumes a slightly granular texture because of the clumping together of soap particles. This is known as the pin break. Stirring is slowed down, and steam is turned into the heating coils to raise the temperature of the oil to about 140 F., at which point relatively large flakes of soft soap form. Heating and agitation are discontinued, and the mixture is allowed to separate into an upper layer of clear, refined oil and a lower layer of semisolid soap stock, called foots. The two layers are separated after cooling and hardening of the soap stock.

The refiner who employs the batch method generally uses enough caustic soda to neutralize the free fatty acids in the oil and provide an excess of 0.2 to 0.6 percent, based on the weight of the crude oil. The exact excess of caustic needed is the amount that will produce the desired color in the oil with the lowest refining loss. An excess of 0.2 percent is often sufficient for peanut oil, but cottonseed oil often requires 0.5 percent or more. The concentration of caustic soda ordinarily used for refining is 8.0 to 16.4 percent, which is determined and expressed on an arbitrary specific-gravity scale as degrees Baume in this case, 12 to 20 Baume. The lower concentrations are used for the purer and more easily refined oils.

CONTINUOUS methods of refining vegetable oils with caustic soda have largely replaced the batch method in the United States. Continuous refining was first advocated in Europe in the 1890's. Only in the early 1930's, however, did the development in continuous centrifuges, proportioning pumps, and general mechanical equipment reach a point where the process became practical and economically feasible.

Continuous operation has the advantage of eliminating large and intermittently unused equipment in favor of smaller and continuously working apparatus. Also the amount of material actually being processed at any moment is relatively small, so that the process is flexible and responsive to the will of the operator. In the batch method, an error in judgment might necessitate re-refining a batch of 60,000 pounds of oil, but in the continuous method a similar error can be detected and corrected before an appreciable amount of oil is damaged. Another important advantage of continuous operation is that the over-all time of contact between oil and caustic soda is only 2 or 3 minutes, and the attack on the oil itself by the caustic soda consequently is kept to a minimum. The centrifugal force employed in separating the soap stock and oil is tremendously greater than the force of gravity, which acts during settling in the kettle process. Because the rate of separating the refined oil and soap stock is higher in centrifuging than in settling by gravity, the excess caustic soda has less time in which to attack the refined oil and thereby increase the refining loss.

In the continuous process, the oil and caustic soda solution (both at 75 to 85 F.) are delivered to separate measuring devices, which usually consist of a positive displacement-type pump for the oil and a synchronized adjustable metering pump for the caustic soda solution. After the proper proportions of the oil and caustic soda are measured by the respective pumps, the two streams are combined and discharged into a mixing chamber, where an emulsion is formed. After about a minute in the mixing chamber, the water-oil emulsion is forced into a tube-type heat exchanger, where hot water, or low-pressure steam, quickly heats the emulsion to 140 F. and causes it to break, or separate, into soap stock suspended in the oil. The oil and soap-stock mixture is now conveyed to a special, high-speed centrifuge, which separates the mixture and discharges two continuous streams, one consisting of refined oil that contains a small amount of soap stock and water, and the other consisting of a small amount of the semisolid soap stock. Normally, less than 3 minutes elapse from the time the crude oil enters the proportioning device until it emerges as refined oil.

The oil from the refining centrifuge can be settled, filtered, and stored. It is customary, however, to wash it almost immediately once or twice with 5 to 10 percent of hot water (180 F.). Washing is also carried out continuously by mixing the refined oil with hot water and centrifuging again to separate the soapy wash water and oil. The washed oil, containing about 0.5 percent of moisture, flows to an automatic vacuum-drier system, which is a vertical cylindrical tank in which a low pressure ( 1 to 2 inches of mercury) is maintained by steam-ejector pumps. The heated, wet oil is sprayed into this tank, where the water is evaporated, and the oil, which contains less than 0.1 percent of moisture, is removed at the bottom.

A modification of the continuous refining process was introduced about 1942. It is said to reduce the refining loss of an oil to nearly the theoretical minimum. The new process, known as the continuous soda ash process, subjects an oil to two separate refinings. In the first, a solution of soda ash ( which is a much weaker alkali than caustic soda and, therefore, cannot attack the oil) removes most of the free fatty acids with which it can combine to form soaps. The second refining, which is carried out with caustic soda solution, removes the pigments and the remainder (about 0.1 percent) of the free fatty acids.

While most vegetable oils processed in the United States for edible use are refined by one of these three processes, or modifications of them, several other processes are used, too. The simplest of these is a water washing or degumming, which is referred to as refining by hydration. Warm, dilute solutions of weak alkalies or salts may be used, but as a rule only warm water is employed. Warm water reacts with some oil-soluble substances, such as phosphatides, proteins, and other gummy materials, to form relatively insoluble products, which can be separated from the oil by continuous centrifuging.

Water washing before alkali refining reduces the total refining loss of an oil, but that advantage probably is appreciable in only a few instances. Some oil is always lost during washing, and the washed oil may be more difficult to refine because of a greater tendency to form stable emulsions or emulsions that are hard to break. Crude soybean and corn oils are washed primarily to separate and recover the phosphatides. About 8 million pounds of soybean phosphatides, called lecithin in the trade, and a half million pounds of corn phosphatides are produced annually in the United States by this process.