III. Fats and Oils Technology
Decolorizing or Bleaching
Most crude vegetable oils are deeply colored. They must be bleached. There are many methods, but the one in most general use in America is to agitate them, after they have been refined, with some solid material which absorbs the color. The usual material is fuller's earth (see II. Properties of Fats and Oils -- Other useful tests). Since the war various forms of carbon and charcoals have also come into use, usually in combination with fuller's earth. This is the result of the stimulus to the production during the war of good absorbent carbons for use in gas-masks. To the dry oil dry fuller's earth is added, usually only a few per cent, the quantity depending upon the character of the oil and the temperature; the mixture is then warmed, usually to less than 80 degrees Centigrade, and agitated for one-half to one hour. It is then pumped through a filter press which retains the fuller's earth and permits the oil to run out clear. (A filter press is an assemblage of square screens of canvas all of the same size. They are placed on a horizontal rack like books on a shelf. Thus a series of thin, flat compartments is formed very much as though one were to stack upright along a shelf a series of ordinary square window-screens in which the wire netting had been replaced by canvas. The canvas is usually backed by a corrugated metal plate so that it will not burst under pressure. In the filter press the frames are pressed so tightly together by means of a powerful screw that liquid cannot escape between them. The frames of the screens are perforated with a set of holes so placed as to form a continuous tube reaching from one end of the stack to the other when the screens are assembled. This tube opens into every other compartment. Through this tube the liquid is pumped into these compartments and forced through their canvas walls into the adjacent compartments which do not communicate with the tube. The canvas holds back the fuller's earth and only permits clear oil to ooze through into the adjacent compartments. The latter are provided with channels which permit the clear oil to escape to the outside where it runs into storage tanks.) An appreciable amount of the oil remains in the earth retained in the press at the end of the operation. A considerable proportion of this is sometimes recovered by forcing dry steam through the press which carries out with it much of this residual oil. Nevertheless appreciable amounts of oil remain behind. It has been proposed to recover these by the solvent extraction process but this does not seem to be done at the present time in America.
Since the necessity of using fuller's earth involves not merely expense but also loss of oil and since, therefore, the costs of decolorizing rise with the amount of earth it is necessary to use, the price paid for an oil, and especially for cottonseed oil in the United States, depends among other factors upon the ease with which it may be decolorized or bleached. Furthermore, if too much fuller's earth has to be used, the oil acquires an earthy flavor. The American trade has, therefore, established two classes of cottonseed oil -- viz., bleachable and unbleachable. A bleachable oil is one that may be reduced to a very definite color standard when treated in a specifically prescribed way. (Rules Regulating Transactions in Cottonseed Products among Members of the New York Produce Exchange, Rule 22, Sec. 4, p. 20.) All other oil is unbleachable or off oil.
Many oils even after they have been refined and decolorized to transparent whiteness retain a disagreeable odor and flavor. This is especially true of cottonseed oil. Such oils may be deodorized by blowing steam through them, since the substances responsible for odors and flavors are usually volatile. A still more effective way is to blow the steam through the oil after it has been heated to a high temperature, say 34O deg Fahrenheit. The most effective method is to carry on this treatment in a vacuum.
This is the method now widely used to prepare cottonseed oil for use as a salad oil.
As stated in an earlier section (see II. Properties of Fats and Oils -- Triglycerids and fatty acids), fats and oils are mechanical mixtures of a number of triglycerids. Now, different triglycerids solidify and melt at different temperatures. Therefore if an oil consists of a number of triglycerids, some of which remain liquid at low temperatures while others become solid, and such an oil is exposed to a low temperature, more or less sediment forms consisting of the triglycerids that separate out at that temperature. Indeed, the oil may be completely converted into a solid cake if it is cooled down far enough. Every housewife who has permitted a bottle of olive oil to stand outdoors in winter has made this observation. This solidification was especially objectionable when fatty oils were used in lamps. It is objectionable in salad oils today because the housewife, being ignorant of the nature of the phenomenon, is apt to believe the oil spoiled. Manufacturers, therefore, subject such oils to a process which prevents the separation of solids in all but the most extremely cold weather. An oil so treated is known, naturally, as a winter oil, and the process is known as winterizing. Conversely, an oil that has not been winterized may be known as a summer oil. Winterizing is a very simple procedure. The oil is very slowly chilled in large tanks to the temperature at which it is to remain clear. It is allowed to stand quietly at that temperature for a considerable length of time to permit the separation of the solid crystalline materials from the liquid to become complete. The oil with the suspended solid matter is then pumped through filter presses which retain the solid and allow the liquid to run out. The solid remaining in the press is known as stearin because largely composed of the glycerids of stearic acid. It is used to stiffen lard compounds, and in soap and candle making.
Production of Stearin
The amount of stearin obtained by winterizing oils is small and would not supply the demand for it. Therefore, not merely oils but also solid fats are treated to separate them into a more solid and a more liquid fraction. The fat is melted and allowed to cool slowly in large tanks to a given temperature which is so chosen that the solid portion which gradually separates has the desired melting point. The fat is held at this temperature for some time in order to permit the solid portion to form completely. Cooling slowly and prolonged holding have another effect. The solid portion is caused to separate in coarse granular masses rather than in fine particles which could not so easily be freed from the liquid portion. Hence this process is known as graining.
When it is judged that the mixture has the desired granular texture, it is placed in a powerful press and the liquid portion squeezed out and thus separated from the solid portion, the stearin. In this manner are prepared lard stearin and lard oil (usually from grease, less nowadays from lard), and oleostearin and oleo oil from edible tallow. Some tallow stearin and tallow oil is also produced from inedible tallow. Lard stearin, if produced from lard, is used especially to mix with other lard that is destined for a warm climate to stiffen it. Such lard is known in America as Cuba lard. Lard stearin from grease is used for soaps, candles, and lubricants. Lard oil was formerly widely used as an illuminant instead of sperm oil. Though it is still so used to a very limited extent, it has been displaced nearly entirely by petroleum products. Lard oil is used in compounding lubricants and especially as a so-called cutting oil, i.e., an oil used to lubricate the cutting edges of steel tools in metal working. Oleostearin is used principally in lard compounds, in shortening agents, while oleo oil is used in oleomargarin and for shortening. Tallow stearin is used in lubrication, candle and soap making, while tallow oil is used principally in lubrication and soap making.
It has already been stated (see II. Properties of Fats and Oils -- Triglycerids and fatty acids) that many of the fats are not saturated with respect to hydrogen and that they may be made to combine with this gas by chemical treatment, a process known as hydrogenation. As it happens, practically all the important unsaturated triglycerids, e.g., triolein, trilinolin, and trilinolenin, are liquids at ordinary temperatures; hence fats which contain them in considerable proportions are oils or, if the proportion is smaller, they are soft solids. On the other hand, the important saturated triglycerids, e.g., tristearin and tripalmitin, are solid, and fats which contain them in preponderating proportion are firm at ordinary temperatures. Now, since hydrogenation converts unsaturated triglycerids into saturated ones, it changes oils into solid fats. This process is of the greatest commercial and economic importance, since it permits of the conversion of oils into fats and thereby widely extends the substitutibility of oils for solid fats and even of oils for one another. In practice, hydrogenation is widespread, but it is used far more for edible than for industrial products, since the cost is of the order of magnitude of one-fourth to one-half a cent a pound, which may be prohibitive for many industrial uses. Moreover, in the inedible field the possibilities of substitution of one natural fat or oil for another are greater than among edible products. Nevertheless, the introduction of hydrogenation has had a profound effect upon the fat and oil industries and trade of the world, for not merely has this discovery widened the uses of oils but the hydrogenated product is usually improved in keeping quality and in color, odor, and flavor as well.
The commercial process of hydrogenation is based upon the purely scientific researches of the French chemist Sabatier and of his students. Though they discovered the scientific principles, commercial application of them was made by others who have taken out a host of patents. The principle itself is simple. To the perfectly dry oil is added a small amount of very finely divided nickel or compound of nickel, called a catalyst. (A catalyst or catalyzer or catalytic agent -- the terms are synonymous -- is a substance which affects the velocity of a chemical reaction without itself appearing in the final product. In the present case nickel is the catalyst. It speeds up the rate at which the oil absorbs hydrogen and may be recovered in undiminished amount at the end of the reaction. A catalyzer cannot start a reaction; it merely modifies the velocity of the reaction. A large quantity of the reacting substances can be transformed by a very small quantity of the catalyzing agent.) Other metals may be used, but nickel is the one most widely employed. The oil with the nickel suspended in it is placed in a tight, strong metal vessel and heated. At the same time pure hydrogen gas is forced into it until a definite pressure is reached. The vessel contains, commonly, some mechanical device to churn up the oil as the gas passes in so that all parts of the oil may be mixed intimately with the hydrogen. The process is interrupted when a sample of oil withdrawn from the vessel is found to have the desired properties. The oil is then withdrawn and cooled sufficiently to permit of its being filtered to remove the suspended nickel.
Substitutability as a Technological Objective
Hydrogenation, refining, deodorizing, decolorizing, stearin pressing, winterizing, and a large number of other technological processes which have been evolved in the course of the last 150 years have all had one object: to make one fat substitutible for another. These substitutions have at different periods had different purposes.
When candles were important and hard fats were in demand, the preparation of a hard fat from a soft one by separating the stearin was discovered. When with the development of lamps burning oils became more important, the same method of pressing stearin made fluid oils derived from harder fats available. When with the development of the production of vegetable oils these became more abundant than solid fats, lard compounds were developed, facilitating the use of oils as cooking fats in countries where hard cooking fats are preferred to oils. This was followed by the introduction of hydrogenation which widened the substitutibility of oils for hard fats. It still remains for chemists to discover a commercial method of converting a saturated fat into an unsaturated drying oil; but even without it the most striking characteristics of the evolution of fat and oil technology have been to increase greatly the possibilities of substitution of one fat for another. If substitution is not more widely practiced, the deterring factors are price, cost of the treatment, and finally the fact that despite the great progress in the treatment of fat it is as yet not possible to modify all fats so as to give each and every one of them the peculiar properties of every other. Some of these peculiarities will be treated in some detail in a later section on the utilization of fats and oils.
Having sketched in outline the principal technological processes, we may now consider the major types of uses of the fats and oils and the way in which the various products are manufactured.
The principles of soap boiling have already been indicated (see II. Properties of Fats and Oils -- Chemical composition). They consist in splitting (hydrolyzing, saponifying) the fat or mixture of fat into glycerin and fatty acids and the conversion of the fatty acids into the salt usually of sodium (hard soap) or of potassium (soft soap). There are many methods, but they all may be divided into two groups. Soap is formed either in one operation or in two. If it is done in one operation, the fat is simply treated, usually hot but sometimes cold, with the appropriate amount of a solution of caustic soda or potash. This forms soap and glycerin. There are a number of methods of separating the soap from the water, glycerin, the excess of alkali, and impurities, but the commonest is simply to add a considerable amount of ordinary salt. This dissolves in the water present and forms brine. Now, soap is but slightly soluble in strong brine. Therefore, the mixture separates into three layers: an upper layer consisting of the purer portion of the soap; a middle layer, dark in color, consisting of impure soap and known as nigre (from the French nègre, black); and a bottom layer of brine containing glycerin. The upper layer is run into molds or otherwise formed into the well-known commercial soap units. The nigre is worked over and purified in various ways and finally worked into soap. The brine may be run into the sewer ultimately or the glycerin may first be recovered from it.
If soap making is carried on in two steps, the first is to split the fat into glycerin and free fatty acid. This may be done in many ways, but the end result is that a mass of fairly pure fatty acids is obtained. The second step is to treat them with the proper amount of caustic soda or sodium carbonate to convert them into the corresponding soap, which is then worked in the customary way.
Up to the early part of the nineteenth century candles were made from beeswax, spermaceti, or tallow. Those made from the first two were the firmest and gave the best light; but they were also expensive and became more and more so with the decline of whaling, for spermaceti is a solid wax which separates from the oil obtained from cavities in the head of the sperm whale.
To meet the scarcity of beeswax and spermaceti, candles began to be made from stearin. By the 1830's, however, they began to be made from the solid fatty acids obtained in the saponification of fat, and this process has remained important to the present day. It consists in splitting tallow, grease, stearin, or any mixture of them into glycerin and free fatty acids by any suitable method. Thus a mixture of free fatty acids is obtained which, like the fats from which they were produced, consists of both more fluid and more solid acids. The solid portion is separated from the fluid portion by cooling, graining, and pressing (see above, Production of stearin), much as lard stearin is separated from lard. This solid portion is known in the trade as stearic acid and the candles made from it as stearic or stearin candles, although it is by no means pure stearic acid. It is nearly always a mixture of palmitic and stearic acid, as well as of any other solid acids that happen to be contained in the raw materials from which it was produced. The oil which is obtained in expressing the stearic acid is known as red oil. It consists mainly of impure oleic acid and is used either in soap making or as soap for use in woollen and other textile mills. From the stearic acid candles are made by melting and casting in suitable molds. To overcome their brittleness they are usually mixed with some other material such as paraffin. Indeed, paraffin has to some extent displaced stearic acid in candles.
The candle maker in purchasing his raw materials is primarily interested in the amount of solid acids they contain, for it is these that he wishes to use in his candles. For this reason he prefers fats with a high content of solid fatty acids. As the titre test (II. Properties of Fats and Oils -- Other useful tests) is an index of this, it is of especial value to him. Since the content of solid fatty acids is usually comparatively high in tallows and stearins, he prefers these fats to greases and oils. In buying raw materials he is also interested in their color, for from dark greases are obtained dark acids from which white candles cannot be made directly. The acids must either be bleached or distilled. The latter is the preferred practice. The common fatty acids are not readily volatile at the pressure of the atmosphere. When distilled under ordinary conditions they char and a great proportion is converted into a sort of pitch. If, however, the distillation be done in a high vacuum, there is but little decomposition and but little pitch is formed. The acids that distill over are pure and white even though the grease from which they are produced may have been very dark. The candle maker is thus enabled to utilize raw materials that would otherwise be of no use to him. The soap maker also uses the process to utilize similar materials. It enables him to make white soaps even from dark materials.
Until the rise of the petroleum industry the most important lubricants were tallows, greases, and vegetable oils. For about seventy-five years now petroleum products have gradually been displacing them, but this displacement has by no means been complete. Material quantities of fats and oils are still used in lubrication, usually mixed with greater or lesser proportions of petroleum derivatives. It is therefore worth while to consider the manner in which fats and oils are used in lubrication. Only non-drying oils are suitable, for drying and semi-drying oils absorb oxygen and become sticky and gummy.
Fats are still used in so-called cylinder oils, used to lubricate the pistons within the cylinders of steam engines. They are no longer used alone but mixed in relatively large and increasing proportions with mineral oils. Tallows, greases, and lard oil are used in all types of engines except marine engines, in which compounds containing rapeseed oil are the only ones acceptable. However, not only is the percentage used on the decline but the total amount of cylinder oils consumed is diminishing relatively because of the displacement of the steam engine by the steam turbine, the internal combustion engine, and the electric motor.
A very large use of lubricants is in automobile engines. Motor oils are for the most part purely mineral products. In a few brands, however, some lard oil is mixed with mineral oils.
Lubricating greases, also extensively used for automobiles as well as for other machinery, consist of mixtures of mineral oil, animal grease or tallow, vegetable oil, and soap made from such grease, tallow, or oil. The soap apparently serves not merely as lubricant but also to emulsify the mixture so as to give it the desired consistency. Such greases represent one of the more important lubrication uses of fats.
Another considerable use of fats and oils for lubrication is in metal working, to lubricate the cutting edge of tools. Lard oil is perhaps the preferred oil for this purpose. In recent years, with the development of high-speed tools, so-called soluble oils are being used. These consist of mixtures of animal or vegetable oil, mineral oil, and alcohol with some other minor ingredients. Soluble oils are not used directly but are first mixed with ten to twenty times their volume of water. They at once disperse in the water to form a milky emulsion -- hence the term soluble oils. Their increasing use tends to cut down the amount of fat of various kinds used in cutting oils.
Margarin is designed to furnish a substitute for butter. It was invented in 1869 and was originally a purely animal product. It is usually made today either from a mixture of animal and vegetable fat or from vegetable products alone. The most important materials are neutral lard, oleo stock (premier jus), oleo oil, coconut oil, palm kernel oil, peanut oil, cottonseed oil, oleostearin, lard stearin, and sesame oil. Any two or more of these are mixed together so that the mix has a melting point from 26 to 27.5 deg Centigrade. The different formulas used vary greatly and sometimes depend upon the price relations of the different fats. The mixture chosen is melted. It is then cooled and mixed with skim milk ripened or soured with a pure culture of bacteria. The mixture is agitated to emulsify the fat and then cooled by causing it to fall against a spray of ice-cold water which carries it into a tank of ice water from the surface of which it is skimmed off. It is thereafter handled exactly like butter. In Europe, in some factories, it is cooled on a cooling drum instead of with ice water, but apparently this machine is not used in the United States.
Margarins, then, are mixtures of fat emulsified in skim milk and made into the semblance of butter. In the United States there are two main types: one consists usually of neutral lard or oleo oil and cottonseed oil; the other of coconut oil and peanut or cottonseed oil. Hydrogenated fats may also be used, usually so-called hydrogenated coconut oil, which is usually a hydrogenated mixture of coconut oil with a small proportion of peanut or cottonseed oil.
The production of lard compounds began in the late 1870's as adulteration of lard with tallow or beef stearin. Soon thereafter, cottonseed oil was also used as an adulterant. The proportions of tallow or stearin and of cottonseed oil were gradually increased until they formed the major constituents and lard the minor ingredient. By 1890 brands were on the market which contained only enough lard to give the characteristic flavor. Up to that time these products were sold as refined lard, pure family lard, etc. About this time, owing to a Congressional investigation, manufacturers began to brand products of this kind as lard compounds, the designation by which they have been known ever since.
About 1908, shortly after hydrogenation began to be practiced in the United States, a new type of cooking fat began to be introduced which consisted of cottonseed oil hydrogenated to the desired consistency. Products of this kind have not usually been marketed as lard compounds but under their own distinctive brand names.
Today there are three principal types of lard compound on the market:
- The original type of lard compound consisting of beef tallow or beef stearin and a vegetable oil, preponderatingly cottonseed oil. It may also contain some lard or lard stearin.
- A mixture of cottonseed oil more or less completely hardened by hydrogenation and a vegetable oil, preponderatingly cottonseed oil.
- Cottonseed oil partially hydrogenated to the desired consistency.
The method of manufacture is relatively simple: The component fats, so proportioned that the mixture has the desired melting point, are melted together. The resulting liquid is run onto hollow revolving cylinders chilled from within, known as lard rolls (see above, Hog fats or lard). The mixture is thus rapidly chilled, thereby acquiring the texture and appearance of lard. The chilled fat is automatically scraped off and drops into a trough in which a worm conveyor beats it up and transports it to storage tanks from which it is packed into the shipping containers. Sometimes the conveyor is so arranged as to beat air into the compound, thereby increasing its volume and lightening its color.
Next: IV. Conditions and Trends of Production
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