Fractionating Fats for New Products

R. O. Feuge.

Salt and sugar and many other household staples are single compounds or mixtures of a few single compounds. Anyone who casually examines a fatty material like shortening might think that it, too, is a single compound or, at most, a mixture of only a few compounds. To all appearances it is quite simple. Shortening is usually a creamy white, homogeneous mass, nearly odorless and tasteless. It melts into a clear, almost colorless, oily liquid when it is warmed slightly.

But, actually, shortening is an extremely complex material. Like most other fatty compounds, it contains some ingredients that, when pure, remain hard and glasslike even in hot water, but it also contains ingredients that are liquid at temperatures as low as those of ice. Shortening melts completely when it is warmed only because the high-melting ingredients are dissolved by the lower-melting ingredients. Some of the fatty parts are so resistant to oxidation by air that they become rancid only after long periods; other parts can become rancid in a few hours if they are heated in air.

Shortening is a highly processed fat: It is a natural fat that has been treated to give it useful properties. Salad or cooking oils need much less processing to be ready for household use, but they are chemically just as complex as shortening. Both shortening and salad oils are obtained from fats or oils,which are in turn extracted from seeds, fruits, or animal tissue. Each such fat or oil is a complex mixture of simple fats, which the chemist calls triglycerides because they contain three fatty acid molecules attached to a single molecule of glycerin.

Naturally occurring fats and oils contain, in addition to a variety of triglycerides, up to 5 percent of such other substances as antioxidants, vitamins, pigments, free fatty acids, and resins. Some of these nonfat components have no special significance, but others have physiological or biological activity that makes them especially valuable particularly after separation from the fat or oil. In still other instances, the minor components unfavorably affect the fat or oil for certain uses.

Crude fats and oils are generally not very useful in the form in which they are obtained from seeds, fruits, or other materials. They have to be subjected to many forms of processing to increase their utility. One is known as fractionation, which is any process whereby a complex natural product is separated into simpler products with properties and uses different from those of the original product. Among the methods of fractionation are distillation, which separates the low-boiling from the high-boiling portions; chilling, which freezes out the most easily solidified portion; and extraction with a solvent, which separates the most readily soluble portion, or fraction.

Some of the processes have long been used in industry. When candles were the principal source of artificial illumination, for example, the art of separating solid stearic acid from liquid oleic acid was developed. It was accomplished by chilling the mixture and pressing it to separate the liquid acid, or red oil, from the solid stearic acid. The solid acid was used for making candles. The red oil was sulfonated for use in the textile industry.

After the cottonseed-oil industry developed in this country and the demand grew for oils that would remain bright and clear when stored in the refrigerator, the process of winterization was developed.

The process involves slow chilling for 4 or 5 days, followed by filtering the chilled oil to remove a part of the high-melting fat, or glycerides, which it contains. Thereby the oil is made usable in salad dressings.

As the importance of vitamins became generally known, the recovery of the fat-soluble vitamins from fish, fish-liver, soybean, wheat-germ, and other oils assumed commercial importance. The oldest method of separating the vitamins was to saponify the fat or oil with caustic soda, extract the crude unsaponifiable material with an organic solvent, and then re-extract or distill the extracted material to concentrate the vitamins.

These older processes generally did not effect complete separation of one type of fatty product from another. Often the methods were tedious and required constant attention. Since about 1920, specialists have realized that the separation could probably be achieved more readily and efficiently by dissolving the fats and oils in organic solvents, from which relatively pure components would separate when the temperature of the solutions was changed. Such processes seemed attractive and technically intriguing, but before 1944 no very large plants were built to put them in operation. The obstacles were technical and economic not to mention the age-old reluctance to adopt revolutionary processing methods. Research on the separation of fats and oils by means of solvents, that is, by so-called solvent fractionation, however, had advanced to a point where change seemed inevitable and several large solvent-separation plants were built. The impetus of the new processes and plants, which are now in daily operation, and the improved products that thus became available established the desirability of other similar plants and processes.

THE VITAMINS and antioxidants in fats and oils are by far the most valuable nonfat constituents, weight for weight. The antioxidants complex substances present in minute amounts are important because they can prolong the shelf life and increase the resistance of fats and oils to rancidification. We do not know their exact chemical structure or the way they act to prevent rancidification.

In many vegetable oils, the tocopherols, which are considered to be vitamin E, are the most abundant of the fat-soluble vitamins. All four of the common tocopherols exhibit both vitamin E and antioxidant activity, though not in the same degree. Apparently the types that have the greatest antioxidant activity have the least vitamin E activity, and vice versa. Tocopherols occur to the extent of about 0.1 percent in cottonseed, soybean, corn, and rice-bran oils. Wheat-germ oil contains about 0.4 percent.

Palm oil contains a great deal of carotene, or provitamin A, which is changed into vitamin A in the liver of animals. Fish oils are rich in vitamins A and D. Some, indeed, contain as much as 7.5 percent of vitamin A.

When the amount of a vitamin in an oil is large enough, the oil can be sold directly as a vitamin preparation. Frequently, however, the vitamins are concentrated, either by the process I mentioned or by one known as molecular distillation.

EXPERIMENTS BY W. S. Singleton and A. E. Bailey at the Southern Regional Research Laboratory proved that tocopherols can be concentrated by fractional crystallization from solvents. The oil is simply mixed with a solvent and the solution cooled until most of the glycerides, or true fats, have solidified in the form of fine, white crystals. The separated crystals are then removed from the solution by filtration. The remaining solution, which contains the vitamins and the antioxidants, together with some fat, is warmed to evaporate the solvent.

When refined and bleached cottonseed oil that had a rather low tocopherol content, 0.05 percent, was mixed with 8 parts of acetone and chilled to a temperature of 76 F., the chemists got a concentrate that contained 5.4 percent of tocopherols. The separation was practically perfect, because all tocopherols were removed from the fat, or glyceride, portion by a single crystallization.

The separation, or fractionation, was even more efficient after the cottonseed oil was converted into a shortening-like product by partial hydrogenation or hardening. By using the same ratio of acetone to oil and reducing the temperature to 98 F., the chemists prepared a concentrate that contained 29.6 percent of tocopherols, or 94 percent of all the tocopherols that were present in the original oil. When they reduced the temperature still further, they got a more potent concentrate, but the percentage of total tocopherols in the concentrate decreased. At 130 F., the concentrate contained 37.8 percent, but only 76 percent of the tocopherols originally present in the oil were separated. Similar results were obtained by research workers in the laboratories of the National Oil Products Company when they investigated the concentration of vitamins and antioxidants in wheat-germ, soybean, and fish-liver oils.

The technical feasibility of the process has been proved beyond any doubt.

With slight variations, it can be adapted to remove other minor nonfat Constituents, such as waxes, pigments, and free fatty acids. Such processes can also be incorporated with other fractional-crystallization procedures (to be described later), thereby reducing the cost of separating the minor constituents from the natural fats and oils.

As for commercial applications, the fractional separation of fatty acids is the most successful of the several solvent-crystallization processes yet proposed. We can attribute this advance partly to the ease with which fatty acids solidify into large, well-formed, rigid crystals, and partly to the simplicity of the fatty acid mixtures as compared to the natural fats from which the acids are obtained by saponification.

Whenever a domestic fat is converted into fatty acids by one of the several processes of saponification, about 96 pounds of a mixture of acids are produced from each 100 pounds of fat. The physical and chemical characteristics of the resulting mixtures of fatty acids vary with the type of fat from which they were produced. Most domestic fats, however, consist principally of combinations of several, or all, of just five acids palmitic, stearic, oleic, linoleic, and linolenic acids.

Interest in the separation of fatty acids into normally solid and normally liquid fractions was first evinced more than a century ago by candle makers who were trying to improve the quality of their product. A fractional-crystallization process that does not employ solvents was developed and is still in common use, although it has never been improved much beyond its original efficiency. Until about 1947, practically all commercial stearic acid and red oil were manufactured by this method. It consists essentially of molding the mixed acids into slabs, cooling them, and forcing out the liquid fraction by mechanical pressing.

Effective separation of the solid and liquid portions requires that about 40 percent of the total acids in process be recycled; that is, the molding and pressing must be repeated two or three times. The large amount of hand labor required is another disadvantage. Also, the utilizable raw materials are limited because efficient operation demands that the ratio of palmitic to stearic acid in the original fatty acid mixture (feed stock) be in the neighborhood of 55 to 45.

Distillation was the only other fractionation method of recognized commercial utility before 1946. Distillation permits the separation of fatty acids according to their boiling points and usually works well when the acids to be separated are of appreciably different molecular weights. Unfortunately, several of the most common acids (stearic, oleic, linoleic, and linolenic) have molecules of approximately the same weight, and therefore cannot be fractionated readily by distillation.