Online Encyclopedia

OILS (adopted from the Fr. oile, mod....

Online Encyclopedia
Originally appearing in Volume V20, Page 46 of the 1911 Encyclopedia Britannica.
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OILS (adopted from the Fr. oile, mod. huile, Lat. oleum, olive oil), the generic expression for substances belonging to extensive series of bodies of diverse chemical character, all of which have the common physical property of being fluid either at the ordinary temperature cr at temperatures below the boiling-point of water. Formerly, when substances were principally classified by obvious characteristics, the word included such a body as " oil of vitriol " (sulphuric acid), which has of course nothing in common with what is now understood under the term oils. In its most comprehensive ordinary acceptation the word embraces at present the fluid fixed oils or fatty oils (e.g. olive oil), the soft fats which may be fluid in their country of origin (e.g. coco-nut oil, palm oil), the hard fats (e.g. tallow), the still harder vegetable and animal waxes (e.g. carnauba wax, beeswax), the odoriferous ethereal (essential) oils, and the fluid and solid volatile hydrocarbons—mineral hydrocarbons—found in nature or obtained from natural products by destructive distillation. The common characteristic of all these substances is that they consist principally, in some cases exclusively, of carbon and hydrogen. They are all readily inflammable and are practically insoluble in water. The mineral hydrocarbons found in nature or obtained by destructive distillation do not come within . the range of this article (see NAPHTHA, PARAFFIN, PETROLEUM), which is restricted to the following two large groups of bodies, formed naturally within the vegetable and animal organisms, viz. (I) Fixed oils, fats and waxes, and (2) Essential, ethereal or volatile oils. 1. Fixed Oils, Fats and Waxes. The substances to be considered under this head divide themselves naturally into two large classes, viz. fatty (fixed) oils and fats on the one hand, and waxes on the other, the distinction between the two classes being based on a most important chemical difference. The fixed oils and fats consist essentially of glycerides, i.e. esters formed by the union of three molecules of fatty acids with one molecule of the trihydric alcohol glycerin (q.v.), whereas the waxes consist of esters formed by the union of one molecule of fatty acid with one molecule of a monohydric alcohol, such as cetyl alcohol, cholesterol, &c. Only in the case of the wax coccerin two molecules of fatty acids are combined with one molecule of a dihydric (bivalent) alcohol. It must be pointed out that in common parlance this distinction does not find its ready expression. Thus Japan wax is a glyceride and should be more correctly termed Japan tallow, whereas sperm oil is, chemically speaking, a wax. Although these two classes of substances have a number of physical properties in common, they must be considered under separate heads. The true chemical constitution of oils and fats was first expounded by the classical researches of Chevreul, embodied in his work, Recherches sur les corps gras d'origine animale (1823, reprinted 188g). (a) Fatty (fixed) Oils and Fats.--The fatty (fixed) oils and fats form a well-defined and homogeneous group of substances, passing through all gradations of consistency, from oils which are fluid even below the freezing-point of water, up to the hardest fats which melt at about 5o° C. Therefore, no sharp distinction can be made between fatty oils and fats. Nevertheless, it is convenient to apply the term " oil " to those glycerides which are fluid below about 2o° C., and the term " fat " to those which are solid above this temperature. Chemical Composition.—No oil or fat is found in nature consisting of a single chemical individual, i.e. a fat consisting of the glyceride. of one fatty acid only, such as stearin or tristearin, C3H8(O.C18H350)3, the glycerin ester of stearic acid, C17H35,CO2H. The natural oils and fats are mixtures of at least two or three different triglycerides, the most important of which are tristearin, tripalmitin, C3H5(O•C,6H31O)3 and triolein, 'C3H5 (O•C1811330)3. These three glycerides have been usually considered the chief constituents of most oils and fats, but latterly there have been recognized as widely distributed trilinolin, the glyceride of linolic acid, and trilinolenin, the glyceride of linolenic acid. The two last-named glycerides are characteristic of the semi-drying and drying oils respectively. In addition to the fatty acids mentioned already there occur also, although in much smaller quantities, other fatty acids combined with glycerin, as natural glycerides, such as the glyceride of butyric acid in butter-fat, of caproic, caprylic and capric acids in butter-fat and in coco-nut oil, lauric acid in coco-nut and palm-nut oils, and myristic acid in mace butter. These glycerides are, therefore, characteristic of the oils and fats named. In the classified list below the most important fatty acids occurring in oils and fats are enumerated (cf. Waxes, below). Oils and fats must, therefore, not be looked upon as definite chemical individuals, but as representatives of natural species which vary, although within certain narrow limits, according to the climate and soil in which the plants which produce them are grown, or, in the case of animal fats, according to the climate, the race, the age of the animal, and especially the food, and also the idiosyncrasy of the individual animal. The oils and fats are distributed throughout the animal and vegetable kingdom from the lowest organism up to the most highly organized forms of animal and vegetable life, and are found in almost all tissues and organs. The vegetable oils and fats occur chiefly in the seeds, where they are stored to nourish the embryo, whereas in animals the oils and fats are enclosed mainly in the cellular tissues of the intestines and of the back. Boiling-point. mm. oC Melting-point. Characteristic of Pressure. C. I. Acids of the Acetic series CnH2nO2 C2H402 76o 119 17 Spindle-tree oil, Macassar oil Acetic acid . . Butyric acid . C4H802 76o 162.3 -6.5 Butter fat, Macassar oil Isovaleric acid C5H1002 76o 173.7 -51 Porpoise and dolphin oils Caproic acid . . 06H1202 770 202-203 -8 Butter fat, coco-nut oil, Caprylic acid . C8H1602 761 236-237 16.5 palm nut oil Capric acid . C1oH2oOz 760 268-270 31.3 pa Lauric acid C12H2402 100 225 43.6 Laurel oil, coco-nut oil Myristic acid C14H2802 100 250.5 53.8 Mace butter, nutmeg butter Isocetic acid (?) C15H3002 •• •• 55 Purging nut Palmitic acid C16Ha20z 100 271.5 62.62 Palm oil, Japan wax, myrtle Stearic acid . C18H3602 loo 291 69.32 wax, lard, tallow, &c. Tallow, cacao butter, &c. Arachidic acid . C20H4002 •• .• 77.0 Arachis oil Behenic acid 022H4402 •• .. 83-84 Ben oil Lignoceric acid , . . C24H48O2 .. .. 80.5 Arachis oil II. Acids of the Acrylic or Oleic series C,,H2,,-202 C611802 76o 198.5 64.5 Croton oil Tiglic acid Hypogaeic acid . C16Hao02 15 236 33-34 Arachis oil Physetoleic acid . C16H3oO2 .. .. 30 Caspian seal oil Oleic acid . . C18H3402 100 285.5-286 14 Most oils and fats Rapic acid C,,H3402 •• .. Rape oils Erucic acid C22H42O2 3o 281 33-34 Rape oils, fish oils Linolic acid Tariric acid . C18Ha202 .• 50.5 Oil of Picramnia Camboita Telfairic acid . 018H3202 13 220-225 Koeme oil Elaeomargaric acid . C18H3202 •• .. 48 Tung oil IV. Acids of the cyclic Chaulmoogric series CnH2n—402 Hydnocarpic acid . . C16H2802 59-60 Hydnocarpus, Lukrabo and Chaulmoogric acid . C18H8202 20 247-248 68 Chaulmoogra oils V. Acids of the Linolenic series CH2n-305 C18Hao02 Linseed oil Linolenic acid . Isolinolenic acid . . . C18H3002 VI. Acids of the series C,H2n-802 C18H2802 .. .. (liquid) Fish, liver and blubber oils Clupanodonic acid . . . Ricinoleic acid . Quince oil acid . C18H3403 .. .. .. Quince oil Dihydroxystearic acid . . . IX. Acids of the series C„H2n-z04 C22H42O4 .. .. 117.7-117.9 Japan wax Japanic acid Up to recently the oils and fats were looked upon as consisting in the main of a mixture of triglycerides, in which the three combined fatty acids are identical, as is the case in the above-named glycerides. Such glycerides are termed " simple glycerides." Recently, however, glycerides have been found in which the glycerin is combined with two and even three different acid radicals; examples of such glycerides are distearo-olein, C3H5(O.C18H35O)2, (0.C18H330), and stearo-palmito-olein, C3H5(O•C18H35O) (O•C16H31O) (O•C18H330). Such glycerides are termed " mixed glycerides." The glycerides occurring in natural oils and fats differ, therefore, in the first instance by the different fatty acids contained in them, and secondly, even if they do contain the same fatty acids, by different proportions of the several simple and mixed glycerides. Since the methods of preparing the vegetable and animal fats are comparatively crude ones, they usually contain certain impurities of one kind or another, such as colouring and mucilaginous matter, remnants of vegetable and animal tissues, &c. For the most part these foreign substances can be removed by processes of refining, but even after this purification they still retain small quantities of foreign substances, such as traces of colouring matters, albuminoid and (or) resinous substances, and other foreign substances, which remain dissolved in the oils and fats, and can only be isolated after saponification of the fat. These foreign substances are comprised in the term "unsaponifiable matter." The most important constituents of the " unsaponifiable matter " are phytosterol C26H430 or C27H440(?), and the isomeric cholesterol. The former occurs in all oils and fats of vegetable origin; the latter is characteristic of all oils and fats of animal origin. This important difference furnishes a method of distinguishing by chemical means vegetable oils and fats from animal oils and fats. This distinction will be made use of in the classification of the oils and fats. A second guiding principle is afforded by the different amounts of iodine (see Oil Testing below) the various oils and fats are capable of absorbing. Since this capacity runs parallel with one of the best-known properties of oils and fats, viz. the power of absorbing larger or smaller quantities of oxygen on exposure to the air, we arrive at the following classification: I. FATTY OILS OR LIQUID FATS A. Vegetable oils. B. Animal oils. 1. Drying oils. 1. Marine animal oils. 2. Semi-drying oils. (a) Fish oils. 3. Non-drying oils. (b) Liver oils (c) Blubber oils. 2. Terrestrial animal oils. II. SOLID FATS B. Animal fats. 1. Drying fats. 2. Semi-drying fats. 3. Non-drying fats. Physical Properties.—The specific gravities of oils and fats vary between the limits of 0.910 and 0.975. The lowest specific gravity is owned by the oils belonging to the rape oil group--from 0.913 to 0.916. The specific gravities of most non-drying oils lie between 0.916 and 0.920, and of most semi-drying oils between 0.920 and 0.925. whereas the drying oils have specific gravities of about 0.930. The animal and vegetable fats possess somewhat higher specific gravities, up to 0.930. The high specific gravity, 0.970, is owned by castor oil and cacao butter, and the highest specific gravity observed hitherto, 0.975, by Japan wax and myrtle wax. In their liquid state oils and fats easily penetrate into the pores of dry substances; on paper Lhey leave a translucent spot- -"grease spot "—which cannot be removed by washing with water and subsequeLt drying. A curious fact, which may be used for the detection of the minutest quantity of oils and fats, is that camphor crushed between layers of paper without having been torched with the fingers rotates when thrown on clean water, the rotation ceasing immediately when a trace of oil or fat is added such as introduced by touching the water with a needle which has been passed previously through the hair. The oils and fats are practically insoluble in water. With the exception of castor oil they are insoluble in cold alcohol; in boiling alcohol somewhat larger quantities dissolve. They are completely soluble in ether, carbon bisulphide, chloroform, carbon tetrachloride, petroleum ether, and benzene. Oils and fats have no distinct melting or solidifying point. This is not only due to the fact that they are mixtures of several glycerides, but also that even pure glycerides, such as tristearin, exhibit two melting-points, a so-called " double melting-point," the triglycerides melting at a certain temperature, then solidifying at a higher temperature to melt again on further heating. This curious behaviour was looked upon by Duffy as being due to the existence of two isomeric modifications, the actual occurrence of which has been proved (1907) in the case of several mixed glycerides. The freezing-points of those oils which are fluid at the ordinary temperature range from a few degrees above zero down to -28° C. (linseed oil). At low temperatures solid portions—usually termed ` stearine "—separate out from many oils; in the case of cotton-seed oil the separation takes place at 12° C. These solid portions can be filtered off, and thus are obtained the commercial " demargarinated oils " or " winter oils." Oils and fats can be heated to a temperature of 200° to 250° C. without undergoing any material change, provided prolonged contact with air is avoided. On being heated above 250° up to 300° some oils, like linseed oil, safflower oil, tung oil (Chinese or Japanese wood oil) and even castor oil, undergo a change which is most likely due to polymerization. In the case of castor oil solid products are formed. Above 300° C. all oils and fats are decomposed; this is evidenced by the evolution of acrolein, which possesses the well-known pungent odour of burning fat. At the same time hydro-carbons are formed (see PETROLEUM). On exposure to the atmosphere, oils and fats gradually undergo certain changes. The drying oils absorb oxygen somewhat rapidly and dry to a film or skin, especially if exposed in a thin layer. Extensive use of this property is made in the paint and varnish trades. The semi-drying oils absorb oxygen more slowly than the drying oils, and are, therefore, useless as paint oils. Still, in course of time, they absorb oxygen distinctly enough to become thickened. The property of the semi-dryingoils to absorb oxygen is accelerated by spreading such oils over a large surface, notably over woollen or cotton fibres, when absorption proceeds so rapidly that frequently spontaneous combustion will ensue. Many fires in cotton and woollen mills have been caused thereby. The non-drying oils, the type of which is olive oil, do not become oxidized readily on exposure to the air, although gradually a change takes place, the oils thickening slightly and acquiring that peculiar disagreeable smell and acrid taste, which are defined by the term " rancid." The changes conditioning rancidity, although not yet fully understood in all details, must be ascribed in the first instance to slow hydrolysis (" saponification ") of the oils and fats by the moisture of the air, especially if favoured by insolation, when water is taken up by the oils and fats, and free fatty acids are formed. The fatty acids so set free are then more readily attacked by the oxygen of the air, and oxygenated products are formed, which impart to the oils and fats the rancid smell and taste. The products of oxidation are not yet fully known; most likely they consist of lower fatty acids, such as formic and acetic acids, and perhaps also of aldehydes and ketones. If the fats and oils are well protected from air and light, they can be kept indefinitely. In fact C. Friedel has found unchanged triglycerides in the fat which had been buried several thousand years ago in the tombs of Abydos. If the action of air and moisture is allowed free play, the hydrolysis of the oils and fats may become so complete that only the insoluble fatty acids remain behind, the glycerin being washed away. This is exemplified by adipocere, and also by Irish bog butter, which consist chiefly of free fatty acids. The property of oils and fats of being readily hydrolysed is a most important one, and very extensive use of it is made in the arts (soap-making, candle-making and recovery of their by-products). If oils and fats are treated with water alone under high pressure (corresponding to a temperature of about 220° C.), or in the presence of water with caustic alkalis or alkaline earths or basic metallic oxides (which bodies act as " catalysers ") at lower pressures, they are converted in the first instance into free fatty acids and glycerin. If an amount of the bases sufficient to combine subsequently with the fatty acids be present, then the corresponding salts of these fatty acids are formed, such as sodium salts of fatty acids (hard soap) or potassium salts of the fatty acids (soft soap), soaps of the alkaline earth (lime soap), or soaps of the metallic oxides (zinc soap, &c.). The conversion of the glycerides (triglycerides) into fatty acids and glycerin must be looked upon as a reaction which takes place in stages, one molecule of a triglyceride being converted first into diglyceride and one molecule of fatty acid, the diglyceride then being changed into monoglyceride, and a second molecule of fatty acid, and finally the monoglyceride being converted into one molecule of fatty acid and glycerin. All these reactions take place concurrently, so that one molecule of a diglyceride may still retain its ephemeral existence, whilst another molecule is already broken up completely into free fatty acids and glycerin. The oils and fats used in the industries are not drawn from any very great number of sources. The tables on the following pages contain chiefly the most important oils and fats together with their sources, yields and principal uses, arranged according to the above classification, and according to the magnitude of the iodine value. It should be added that many other oils and fats are only waiting improved conditions of transport to enter into successful competition with some of those that are already on the market. Extraction.—Since the oils and fats have always served the human race as one of the most important articles of food, the oil and fat industry may well be considered to be as old as the human race itself. The methods of preparing oils and fats range themselves under three heads: (r) Extraction of oil by " rendering," i.e. boiling out with water; (2) Extraction of oil by expression; (3) Extraction of oil by means of solvents. Rendering.—The crudest method of rendering oils from seeds, still practised in Central Africa, in Indo-China and on some of the South Sea Islands, consists in heaping up oleaginous fruits and allowing them to melt by the heat of the sun, when the exuding oil runs off and is collected. In a somewhat improved form this process of rendering is practised in the preparation of palm oil, and the rendering the best (Cochin) coco-nut oil by boiling the fresh kernels with water. Since hardly any machinery, or only the simplest machinery, is required for these processes, this method has some fascination for A. Vegetable fats. inventors, and even at the present day processes are being patented, having for their object the boiling out of fruits with water or salt solutions, so as to facilitate the separation of the oil from the pulp by gravitation. Naturally these processes can only be applied tb those seeds which contain large quantities of fatty matter, such as coconuts and olives. The rendering process is, however, applied on a very large scale to the production of animal oils and fats, Formerly the animal oils and fats were obtained by heating the tissues containing the oils or fats over a free fire, when the cell membranes burst and the liquid fat flowed out. The cave-dweller who first collected the fat dripping off the deer on the roasting spit may well be looked upon as the first manufacturer of tallow. This crude process is now classed amongst the noxious trades, owing to the offensive stench given off, and must be considered as almost extinct in this country. Even on whaling vessels, where up to recently
End of Article: OILS (adopted from the Fr. oile, mod. huile, Lat. oleum, olive oil)
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