II. Properties of Fats and Oils
FOR an understanding of the place of fats and oils in the diet and in the arts, some elementary knowledge of their chemical and physical properties is essential. It is the object of this section to present the minimum of such necessary information in the simplest way practicable. For complete treatment of the chemistry of fats and oils see J. Lewkowltsch, Chemical Technology and Analysis of Oils, Fats, and Waxes (London, Macmillan, 1922, 3 vols., 6th edition).
As already stated, fats may be decomposed into glycerin and fatty acids. This manner of decomposition takes place only in the presence of moisture. For each molecule (a molecule is the smallest particle of a substance that can exist and still exhibit the properties of that substance) of glycerin set free there are set free three molecules of fatty acid. In the process three molecules of water are taken up, partly to help re-form the glycerin and partly to help re-form the fatty acids. Conversely (in the laboratory) the fat may be reconstituted from glycerin and fatty acid, in which event three molecules of water are set free for each molecule of fat synthesized.
The process of splitting a substance whereby water is taken up is known to chemists as hydrolysis, a word which is merely Greek for cleavage by water. The process is often termed saponification, since it was first observed to take place in the manufacture of soap. The term saponification (instead of the more exact term hydrolysis) is, however, applied indiscriminately and inappropriately to any chemical change of this nature, whether or not soap is formed. Nowadays in industry fats are very often converted into glycerin and fatty acids -- that is, hydrolyzed -- without the formation of any soap whatever. A soap is merely the combination of a fatty acid with a metal, i.e., it is a salt. The commonest soaps are the fatty-acid salts of sodium (sodium is a soft, white metal obtained from common salt, sodium chloride) and potassium. (Potassium is also a soft, white metal obtained from wood ashes or from certain minerals found in Germany, Alsace, and elsewhere. Both sodium and potassium oxidize with great rapidity when exposed to the air, and hence are never found in nature except in the form of their compounds.) Hard soaps are sodium salts; soft soaps, potassium salts. The fatty-acid salts of ammonium are also sometimes used for cleansing. Only a few other soaps are of practical importance, for example lead soaps which are used in medicinal plasters, zinc soaps which are used in ointments, and aluminum soaps which are used in waterproofing. Very few of the salts of fatty acids have the properties of common soap. Most of them are but slightly soluble in water, and therefore do not yield suds and have little or no detergent (i.e., cleansing) action. All are nevertheless termed soaps by chemists.
Triglycerids and Fatty Acids
As above stated, fats may be split into glycerin and fatty acids, the resulting mixture containing three molecules of fatty acid for each molecule of glycerin. Because of this proportion of acid to glycerin, the chemical compounds found in the fat before it was split are known to chemists as triglycerids. Since there are a number of different fatty acids that occur in natural fats, a great many different triglycerids are encountered in nature. These are named according to the fatty acid or acids they contain. Thus triolein is the triglycerid of oleic acid, tripalmitin that of palmitic acid, tristearin that of stearic acid, while monopalmitin-distearin contains, as the name indicates, one molecule of palmitic and two of stearic acid. While a large variety of fatty acids is found in natural fats and oils, only a few of them are of outstanding commercial importance. These are myristic acid, lauric acid, palmitic acid, stearic acid, oleic acid, linolic acid, and linolenic acid. Though the number of triglycerids encountered in nature is great, the triglycerids of these seven acids (see table of formulas below) make up the great bulk of the natural fats and oils. Fats and oils are practically always mixtures of triglycerids in varying proportions. In some fats one triglycerid predominates, in others another, and in still others several are present in material amounts. Apparently no natural fat or oil consists solely of a single triglycerid. The properties of different fats and oils depend upon the characteristics of the triglycerids of which they are mixtures and upon the proportions of these triglycerids to one another. (See fatty acids table below.) The fats of different species of animals and plants vary widely. Indeed, the fat from a given natural source, say a given species of animal or plant, may contain the same triglycerids in slightly different proportions, depending upon the conditions of the environment prevailing while the fat was being formed. It was pointed out in the preceding section that the properties of the fat of an animal vary somewhat with the diet and also with the tissue from which it is obtained. It was also pointed out that a fruit may yield two fats of different properties, one from the pulp and one from the kernel. In the case of plants the fat may also vary with the cultural variety of the plant and with the climatic and soil conditions under which the plant was grown. Thus the linseed oils from Argentina, India, Russia, and the United States have slightly different chemical and physical properties.
The formulas of these acids (disregarding isomers) are as follows:
Acid Elementary Formula Constitutional Formula Lauric C12H24O2 CH3(CH2)10COOH Myristic C14H28O2 CH3(CH2)12COOH Palmitic C16H32O2 CH3(CH2)14COOH Stearic C18H36O2 CH3(CH2)16COOH Oleic C18H34O2 CH3(CH2)14(CH)2COOH Linolic C18H32O2 CH3(CH2)12(CH)4COOH Linolenic C18H30O2 CH3(CH2)10(CH)6COOH
E. T. Webb, Oils and Fats in Soap Manufacture, Soap Gazette and Perfumer, October 1, 1926, xxviii, 302, gives the following percentages of the more important fatty acids in commonly used fats and oils. Other investigators may find somewhat different proportions, but in general these are representative:
Fat or oil Lauric Myristic Palmitic Stearic Oleic Linolic Linolenic Coconut 45 20 5 3 6 - - Palm kernel 55 12 6 4 10 - - Tallow (beef) - 2 29.0 24.5 44.5 - - Tallow (mutton) - 2 27.2 25.0 43.1 2.7 - Lard - - 24.6 15.0 50.4 10.0 - Olive - - 14.6 - 75.4 10.0 - Arachis (peanut) - - 8.5 6.00 51.6 26.0 - Cottonseed - - 23.4 - 31.6 45.0 - Maize - - 6.0 2.0 44.0 48.0 - Linseed - 3 6.0 - - 74.0 17.0 Soy bean - - 11.0 2.0 20.0 64.0 3.0
Fats and oils being mere mechanical mixtures of triglycerids, it is possible in many cases to separate them more or less completely into their component triglycerids by simple mechanical means, chilling and pressure. Such processes have considerable commercial importance, as, for example, the separation of lard into lard oil and lard stearin or of beef tallow into oleo oil and oleostearin (see III. Fats and Oils Technology -- Hydrogenation).
For the details of the chemical nature of the fatty acids, the reader is referred to the texts on organic chemistry. (A. Hollemann (trans. H. C. Cooper), A Textbook of Inorganic Chemistry, New York, Wiley, 1904.) Here it is sufficient to point out that they all possess the characteristic property of acids in general, viz., to combine with bases to form salts. These salts, as was pointed out above, are known as soaps whether or not they have detergent action. Moreover, all fatty acids contain carbon, hydrogen, and a small proportion of oxygen. They differ from one another in the number of carbon atoms in each molecule, in the proportion of carbon to oxygen their molecules contain, and also in the proportion of carbon to hydrogen. Upon these ratios the physical and chemical properties of the acids and of their triglycerids very largely depend.
Saturated and Unsaturated Fatty Acids
It would carry us too far to discuss these relationships in detail here. For present purposes it is sufficient to limit consideration to one of the aspects of the carbon-hydrogen ratio of the fatty acids. When the fatty-acid molecule contains the maximum of hydrogen possible, the acid is said to be a saturated fatty acid. It is saturated with respect to hydrogen. Myristic, lauric, palmitic, and stearic acids are such saturated acids. They are solids at ordinary temperatures. When, however, the fatty-acid molecule does not contain the maximum amount of hydrogen possible, the acid is said to be an unsaturated fatty acid. It is unsaturated with respect to hydrogen. Such unsaturated acids are oleic, linolic, and linolenic acids. They are liquids at ordinary temperatures. By chemical means these acids may be made to take up, i.e., combine with, hydrogen. This process is known as hydrogenation. It converts a more unsaturated fatty acid into a less unsaturated one, or, if the hydrogenation is carried to completion, into a saturated fatty acid. Thus by hydrogenation oleic acid is converted into stearic acid. Linolic acid when hydrogenated can be made to take up twice as much hydrogen as oleic acid, and linolenic acid three times as much. Linolic acid is, therefore, a more highly unsaturated acid than oleic, while linolenic acid is more highly unsaturated than linolic.
Unsaturated fatty acids can be made to combine with other substances instead of with hydrogen. For example, they may be made to take up iodin or oxygen. Acids of a low degree of unsaturation, such as oleic acid, do not combine with oxygen with any great degree of avidity, but acids of a greater degree of unsaturation, such as linolic or linolenic, combine with it very readily; they do so merely upon exposure to the air.
The properties of the fatty acids just described, which depend upon their degree of saturation or unsaturation with respect to hydrogen, are retained by them when they are in combination with glycerin as triglycerids. Hence the different fats are also more or less saturated, according as they contain greater or lesser proportions of the triglycerids of saturated or unsaturated fatty acids. When fats contain large amounts of trilinolin and trilinolenin, these absorb oxygen avidly. It is upon the presence, then, of these unsaturated triglycerids that the properties of drying oils described in the preceding chapter depend. Resin-like films are formed by them when oxygen is absorbed because the oxidation products of these unsaturated triglycerids are relatively insoluble solids. Upon this reaction, as already pointed out, is based the behavior of paints.
Measures of Unsaturation
It is obvious, then, that it is important for industrial users of fats to know the degree of unsaturation of a given parcel of fat. This might be ascertained by determining the amount of hydrogen required to convert it into a saturated fat. In practice this is a complicated procedure and so simpler methods are resorted to. The simplest of these is the determination of the amount of iodin that can be made to combine with the fat. The percentage by weight of iodin absorbed by the fat in the natural state is known as the iodin number. It is an index to the degree of unsaturation of the fat. The iodin number of the commoner fats are given in Table 1. (For details concerning this and other methods of testing fats, see Standard Methods for the Sampling and Analysis of Commercial Fats and Oils, Industrial and Engineering Chemistry, December 1926, xviii, 1346.) Examination of the table shows that the fats with the highest iodin numbers are the drying oils par excellence, linseed and tung oil, with which must also be classified menhaden fish oil.
Table 1. Iodin Numbers of Common Fats* Fat or Oil lodin number Linseed oil 173 - 201 Tung Oil 170.6 Menhaden oil 139 - 173 Whale oil 121 - 146.6 Soy bean oil 137 - 143 Sunflower oil 119 - 135 Corn oil 111 - 130 Cottonseed oil 108 - 110 Sesame oil 103 - 108 Rapeseed oil 94 - 102 Peanut oil (arachis) 83 - 100 Olive oil 79 - 88 Horse oil 71 - 86 Lard 46 - 70 Palm oil 51.5 - 57 Milk fat 26 - 50 Beef tallow 38 - 46 Mutton tallow 35 - 46 Cacao butter 32 - 41 Palm kernel oil 13 - 17 Coconut oil 8 - 10
Data from J. Lewkowitsch, Chemical Technology and Analysis of Oils, Fats, and Waxes, pp. 419-24.
Other Useful Tests
There are, of course, many other tests besides iodin absorption that are used in commercial practice. This is not the place to discuss them in detail. A few of them, however, deserve mention in passing.
The iodin number of a fat tells us the degree of unsaturation of a fat. It does not tell us whether the unsaturation is the result of the presence of triolein only, of trilinolin only, of trilinolenin only, or of a mixture of the three. As the drying qualities depend mainly upon trilinolin and trilinolenin, paint manufacturers are not always satisfied with the determination of the iodin number. In such cases they determine the amount of oxygen the oil tested will absorb under standard conditions. As the absorption of oxygen is mainly by the trilinolin and trilinolenin, this test is used to supplement the iodin number.
In the preceding section it was stated that fats often become decomposed and rancid and that they then contain free fatty acids -- i.e., acid uncombined with glycerin. It was also pointed out that it is important to the industrial user to know the amount of free fatty acid present, since this determines in large measure the refining loss. The amount of free fatty acid is estimated by determining the quantity of alkali that must be added to the fat to render it quite neutral. Sometimes, in addition to estimating the free fatty acid in this way, the actual loss in refining is also determined. This is done by warming a known amount of the fat with strong aqueous caustic soda solution, which converts the free fatty acid into soap. (Caustic soda is a compound of one atom each of sodium, oxygen, and hydrogen; its formula is therefore NaOH. Its proper scientific name is sodium hydroxid. It is also known as soda lye or simply as lye. It is very alkalin and corrosive.) This soap is then removed and the amount of fat remaining is then determined. The loss is estimated by subtracting this amount from the amount of fat originally taken for the test. The amount and strength of caustic soda solution, the temperature, and the length of treatment are so chosen that only the free fatty acid and other impurities present in the oil are removed and but little, if any, saponification of neutral fat takes place. (Cf. Rules Governing Transactions between Members of the Texas Cottonseed Crushers' Association (Dallas. Texas, 1927), pp. 71-76.)
As also pointed out in the first section, many crude fats as they come upon the market are either naturally deeply colored or have become so through decomposition. Since for many uses such fats must be decolorized, the ease with which this may be done is an important factor in determining their commercial value. Hence one of the commonest tests applied to fats is the test of bleachability. This is done by mixing a given weight of alkali-refined fat with a given weight of fuller's earth and then estimating the amount of color remaining in the fat or oil after this treatment. (Fuller's earth is a special kind of clay that has the property of absorbing coloring matters. It derives its name from the fact that it has been used in the fulling of cloth to remove grease.)
Many fats and oils contain substances that are not triglycerids. These may be natural constituents or they may be adulterants or contaminants. The presence of a considerable proportion of them of course reduces the commercial value of the fat. The commonest of these is moisture. It is estimated very simply by placing a weighed portion of the fat in an oven heated to a temperature slightly higher than that of boiling water. The moisture is thereby driven off. The fat is then again weighed; the loss is regarded as moisture.
The determination of non-fat materials other than water is done by saponifying the fat by heating with strong caustic soda or potash solution until all the triglycerids have been decomposed into glycerin and soap. (Caustic potash is the compound of potassium analogous to caustic soda.) These are soluble in water and may be washed away. What remains behind is the non-triglycerid part of the fat and may be weighed. It is known as the unsaponifiable matter. In practice the procedure is not as simple as this, but the basic principle is correctly stated above.
The determination of unsaponifiable matter must not be confused with the saponification number of a fat. The saponification number is the number of milligrams of potassium hydroxid required to convert one gram of the fat completely into glycerin and potassium soap. It gives information concerning the character of the fatty acids of the fat and in particular concerning the solubility of their soaps in water. The higher the saponification number of a fat free from moisture and unsaponifiable matter, the more soluble the soap that can be made from it. The information is of especial importance to soap makers. Table 2 gives the saponification numbers of the commoner commercial fats and oils.
Table 2. Saponification Numbers of Common Fats* Fat or Oil Saponification number Rapeseed oil 170 - 179 Menhaden oil 190.6 Corn oil 188 - 193 Olive oil 185 - 196 Soy bean oil 193 Cacao butter 193.55 Linseed oil 192 - 195 Cottonseed oil 193 - 195 Lard 195.4 Mutton tallow 192 - 195.5 Peanut oil (arachis) 190 - 196 Horse oil 195 - 197 Beef tallow 193.2 - 200 Palm oil 196 - 205 Butter 220 - 233 Palm kernel oil 242 - 250 Coconut oil 246 - 260
Data from J. Lewkowitsch, Chemical Technology and Analysis of Oils, Fats, and Waxes, pp. 395-400.
Examination of this table shows that butter ranks with palm kernel oil and coconut oil as having a very high saponification number. This is due to the fact that its triglycerids contain appreciable quantities of myristic acid and small quantities of lauric acid, both of which when they form soap combine with relatively more sodium than the more common acids of fats. These acids occur in undecomposed butter in chemical combination as triglycerids. Their sodium soaps are quite soluble in water. The high saponification number of coconut oil and palm kernel oil is due to the large proportion of lauric acid and myristic acid that they contain. These oils therefore yield quite soluble soaps.
Before leaving the subject of the commercial chemical testing of fats, the titre test deserves mention because it is of much importance in certain branches of industry. The titre of a fat or oil is the temperature at which the mixture of fatty acids derived from it solidifies after it has been melted. The test is performed in several steps. First, the fat is completely saponified, usually by heating with a solution of caustic soda. Then the mixture of soaps thus obtained is treated with a strong acid, usually sulfuric, which takes the sodium away from the soaps, thereby converting them into free fatty acids. After these have been washed and dried they are melted and the temperature at which the melted mass solidifies when cooled is noted. This temperature gives an index to the consistency of the original fat, a matter of great importance to manufacturers of candles and of products like margarin, in which consistency and texture are of the utmost importance.
Finally, the viscosity of a fat is a property of commercial significance, especially to manufacturers of lubricants. It is usually estimated by comparing the length of time it takes a given volume of oil (or melted fat) to flow through a tube of small bore, or through a small orifice, with the time it takes an identical volume of water. Castor oil has the highest viscosity of any fat that is fluid at ordinary temperatures. Olive oil has the highest viscosity of any of the common vegetable oils. The viscosities vary greatly with the temperature. When fats are cooled to the solidifying point they can no longer be said to be viscous. They have become plastic.
Next: III. Fats and Oils Technology
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