# 1911 Encyclopædia Britannica/Oils

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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 or 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. carnaüba 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. (1) 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 1889).
(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 50° 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 20° 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, C$_3$H$_5$(O·C$_{18}$H$_{35}$O)$_3$, the glycerin ester of stearic acid, C$_{17}$H$_{35}$·CO$_2$H. The natural oils and fats are mixtures of at least two or three different triglycerides, the most important of which are tristearin, tripalmitin, C$_3$H$_5$(O·C$_{16}$H$_{31}$O)$_3$ and triolein, C$_3$H$_5$ (O·C$_{18}$H$_{33}$O)$_3$. 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).
| | | | Boiling-point. | Boiling-point. | Melting-point. °C. | Characteristic of | Characteristic of |
| | | | mm.  Pressure | °C. | Melting-point. °C. | Characteristic of | Characteristic of |
| I. | Acids of the Acetic series C$_𝑛$H$_{2𝑛}$O$_2$— | | | | | |
| | Acetic acid | C$_2$H$_4$O$_2$ | 760 | 119 | 17 | Spindle-tree oil, Macassar oil | Spindle-tree oil, Macassar oil |
| | Butyric acid | C$_4$H$_8$O$_2$ | 760 | 162·3 | −6·5 | Butter fat, Macassar oil | Butter fat, Macassar oil |
| | Isovaleric acid | C$_5$H$_{10}$O$_2$ | 760 | 173·7 | −51 | Porpoise and dolphin oils | Porpoise and dolphin oils |
| | Caproic acid | C$_6$H$_{12}$O$_2$ | 770 | 202–203 | −8 | ${\Bigg \}$ | Butter fat, coco-nut oil, palm nut oil |
| | Caprylic acid | C$_8$H$_{16}$O$_2$ | 761 | 236–237 | 16·5 | ${\Bigg \}$ | Butter fat, coco-nut oil, palm nut oil |
| | Capric acid | C$_{10}$H$_{20}$O$_2$ | 760 | 268–270 | 31·3 | ${\Bigg \}$ | Butter fat, coco-nut oil, palm nut oil |
| | Lauric acid | C$_{12}$H$_{24}$O$_2$ | 100 | 225 | 43·6 | Laurel oil, coco-nut oil | Laurel oil, coco-nut oil |
| | Myristic acid | C$_{14}$H$_{28}$O$_2$ | 100 | 250·5 | 53·8 | Mace butter, nutmeg butter | Mace butter, nutmeg butter |
| | Isocetic acid (?) | C$_{15}$H$_{30}$O$_2$ | .. | .. | 55 | Purging nut | Purging nut |
| | Palmitic acid | C$_{16}$H$_{32}$O$_2$ | 100 | 271·5 | 62·62 | Palm oil, Japan wax, myrtle wax, lard, tallow, &c. | Palm oil, Japan wax, myrtle wax, lard, tallow, &c. |
| | Stearic acid | C$_{18}$H$_{36}$O$_2$ | 100 | 291 | 69·32 | Tallow, cacao butter, &c. | Tallow, cacao butter, &c. |
| | Arachidic acid | C$_{20}$H$_{40}$O$_2$ | .. | .. | 77·0 | Arachis oil | Arachis oil |
| | Behenic acid | C$_{22}$H$_{44}$O$_2$ | .. | .. | 83–84 | Ben oil | Ben oil |
| | Lignoceric acid | C$_{24}$H$_{48}$O$_2$ | .. | .. | 80·5 | Arachis oil | Arachis oil |
| II. | Acids of the Acrylic or Oleic series C$_𝑛$H$_{2𝑛−2}$O$_2$— | | | | | |
| | Tigic acid | C$_5$H$_8$O$_2$ | 760 | 198·5 | 64·5 | Croton oil | Croton oil |
| | Hypogaeic acid | C$_{16}$H$_{30}$O$_2$ | 15 | 236 | 33–34 | Arachis oil | Arachis oil |
| | Physetoleic acid | C$_{16}$H$_{30}$O$_2$ | .. | .. | 30 | Caspian seal oil | Caspian seal oil |
| | Oleic acid | C$_{18}$H$_{34}$O$_2$ | 100 | 285·5–286 | 14 | Most oils and fats | Most oils and fats |
| | Rapic acid | C$_{18}$H$_{34}$O$_2$ | .. | .. | .. | Rape oils | Rape oils |
| | Erucic acid | C$_{22}$H$_{42}$O$_2$ | 30 | 281 | 33–34 | Rape oils, fish oils | Rape oils, fish oils |
| III. | Acids of the Linolic series C$_𝑛$H$_{2𝑛−4}$O$_2$— | | | | | |
| | Linolic acid | C$_{18}$H$_{32}$O$_2$ | .. | .. | .. | Maize oil, cotton-seed oil | Maize oil, cotton-seed oil |
| | Tariric acid | C$_{18}$H$_{32}$O$_2$ | .. | .. | 50·5 | Oil of Picramnia Camboita | Oil of Picramnia Camboita |
| | Telfairic acid | C$_{18}$H$_{32}$O$_2$ | 13 | 220–225 | .. | Koëme oil | Koëme oil |
| | Elaeomargaric acid | C$_{18}$H$_{32}$O$_2$ | .. | .. | 48 | Tung oil | Tung oil |
| IV. | Acids of the cyclic Chaulmoogric series C$_𝑛$H$_{2𝑛−4}$O$_2$— | | | | | |
| | Hydnocarpic acid | C$_{16}$H$_{28}$O$_2$ | .. | .. | 59–60 | ${\Big \}$ | Hydnocarpus, Lukrapo and Chaulmoogra oils |
| | Chaulmoogric acid | C$_{18}$H$_{32}$O$_2$ | 20 | 247–248 | 68 | ${\Big \}$ | Hydnocarpus, Lukrapo and Chaulmoogra oils |
| V. | Acids of the Linolenic series C$_𝑛$H$_{2𝑛−6}$O$_2$— | | | | | |
| | Linolenic acid | C$_{18}$H$_{30}$O$_2$ | .. | .. | .. | ${\Big \}$ | Linseed oil |
| | Isolinolenic acid | C$_{18}$H$_{30}$O$_2$ | .. | .. | .. | ${\Big \}$ | Linseed oil |
| VI. | Acids of the series C$_𝑛$H$_{2𝑛−8}$O$_2$— | | | | | |
| | Clupanodonic acid | C$_{18}$H$_{28}$O$_2$ | .. | .. | (liquid) | Fish, liver and blubber oils | Fish, liver and blubber oils |
| VII. | Acids of the Ricinoleic series C$_𝑛$H$_{2𝑛–2}$O$_3$— | | | | | |
| | Ricinoleic acid | C$_{18}$H$_{34}$O$_3$ | 15 | 250 | 4–5 | Castor oil | Castor oil |
| | Quince oil acid | C$_{18}$H$_{32}$O$_3$ | .. | .. | .. | Quince oil | Quince oil |
| VIII. | Dihydroxylated acids of the series C$_𝑛$H$_{2𝑛}$O$_4$— | | | | | |
| | Dihydroxystearic acid | C$_{18}$H$_{36}$O$_4$ | .. | .. | 141–143 | Castor oil | Castor oil |
| IX. | Acids of the series C$_𝑛$H$_{2𝑛–2}$O$_4$— | | | | | |
| | Japanic acid | C$_{22}$H$_{42}$O$_4$ | .. | .. | 117·7–117·9 | Japan wax | Japan wax |
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, C$_3$H$_5$(O·C$_{18}$H$_{35}$O)$_2$, (O·C$_{18}$H$_{33}$O), and stearo-palmito-olein, C$_3$H$_5$(O·C$_{18}$H$_{35}$O) (O·C$_{16}$H$_{31}$O) (O·C$_{18}$H$_{33}$O). 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.
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.
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 C$_{26}$H$_{44}$O or C$_{27}$H$_{44}$O(?), 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. | B. Animal oils. |
| | 1. Drying oils. 2. Semi-drying oils. 3. Non-drying oils. | | 1. Marine animal oils. (a) Fish oils. (b) Liver oils. (c) Blubber oils. 2. Terrestrial animal oils. | 1. Marine animal oils. (a) Fish oils. (b) Liver oils. (c) Blubber oils. 2. Terrestrial animal oils. |
| II. Solid Fats ||||||
| A. Vegetable fats. | A. Vegetable fats. | A. Vegetable 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 they leave a translucent spot—“grease spot”—which cannot be removed by washing with water and subsequent 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 touched 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 hydrocarbons 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-drying oils 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: (1) 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 ​ 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 to 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 whale oil, seal oil and sperm oil (see Waxes, below) were obtained exclusively by “trying,” i.e. by melting the blubber over a free fire, the process of rendering is fast becoming obsolete, the modern practice being to deliver the blubber in as fresh a state as possible to the “whaling establishments,” where the oil is rendered by methods closely resembling those worked in the enormous rendering establishments (for tallow, lard, bone fat) in the United States and in South America. The method consists essentially in cutting up the fatty matter into small fragments, which are transferred into vessels containing water, wherein the comminuted mass is heated by steam, either under ordinary pressure in open vessels or under higher pressure in digestors. The fat gradually exudes and collects on the top of the water, whilst the membranous matter, “greaves,” falls to the bottom. The fat is then drawn off the aqueous (gluey) layer, and strained through sieves or filters.
The greaves are placed ​ in hair or woollen bags and submitted to hydraulic pressure, by which a further portion of oil or fat is obtained (cf. Pressing, below). In the case of those animal fats which are intended for edible purposes, such as lard, suet for margarine, the greatest cleanliness must, of course, be observed, and the temperature must be kept as low as possible in order to obtain a perfectly sweet and pure material.
Pressing.—The boiling out process cannot be applied to small seeds, such as linseed and rape seed. Whilst the original method of obtaining seed oils may perhaps have been the same which is still used in India, viz. trituration of (rape) seeds in a mortar so that the oil can exude, it may be safely assumed that the process of expressing has been applied in the first instance to the preparation of olive oil. The first woman who expressed olives packed in a sack by heaping stones on them may be considered as the forerunner of the inventors of all the presses that subsequently came into use. Pliny describes in detail the apparatus and processes for obtaining olive oil in vogue among his Roman contemporaries, who used already a simple screw press, a knowledge of which they had derived from the Greeks. In the East, where vegetable oils form an important article of food and serve also for other domestic purposes, various ingenious applications of lever presses and wedge presses, and even of combined lever and wedge presses, have been used from, the remotest time. At an early stage of history the Chinese employed the same series of operations which are followed in the most advanced oil mills of modern time, viz. bruising and reducing the seeds to meal under an edge-stone, heating the meal in an open pan, and pressing out the oil in a wedge press in which the wedges were driven home by hammers. This primitive process is still being carried out in Manchuria, in the production of soja bean cake and soja bean oil, one of the staple industries of that country. The olive press, which was also used in the vineyards for expressing the grape juice, found its way from the south of France to the north, and was employed there for expressing poppy seed and rape seed. The apparatus was then gradually improved, and thus were evolved the modern forms of the screw press, next the Dutch or stamper press, and finally the hydraulic press. With the screw press, even in its most improved form, the amount of pressure practically obtainable is limited from the failure of its parts under the severe inelastic strain. Hence this kind of press finds only limited application, as in the industry of olive oil for expressing the best and finest virgin oil, and in the production of animal fats for edible purposes, such as lard and oleomargarine. The Dutch or stamper press, invented in Holland in the 17th century, was up to the early years of the 19th century almost exclusively employed in Europe for pressing oil-seeds. It consists of two principal parts, an oblong rectangular box with an arrangement of plates, blocks and wedges, and over it a framework with heavy stampers which produce the pressure by their fall. The press box first consisted of strongly bound oaken planks, but later on cast-iron boxes were introduced. At each extremity of the box a bag of oil-meal was placed between two perforated iron plates, next to which were inserted filling-up pieces of wood, two of which were oblique, so that the wedges which exercised the pressure could be readily driven home. This press has had to yield place to the hydraulic press, although in some old-fashioned establishments in Holland the stamper press could still be seen at work in the ’eighties of the 19th century. The invention of the hydraulic press in 1795 by Joseph Bramah (Eng. pat., 30th April 1795) effected the greatest revolution in the oil industry, bringing a new, easily controlled and almost unlimited source of power into play; the limit of the power being solely reached by the limit of the strength of the material which the engineer is able to produce. Since then the hydraulic press has practically completely superseded all other appliances used for expression, and in consequence of this epoch-making invention, assisted as it was later on by the accumulator—invented by William George (later Lord) Armstrong in 1843—the seed-crushing industry reached a perfection of mechanical detail which soon secured its supremacy for England.
The sequence of operations in treating oil seeds, oil nuts, &c., for the separation of their contained oils is at the present time as follows: As a preliminary operation the oil seeds and nuts are freed from dust, sand and other impurities by sifting in an inclined revolving cylinder or sieving machine, covered with woven wire, having meshes varying according to the size and nature of the seed operated upon. This preliminary purification is of the greatest importance, especially for the preparation of edible oils and fats. In the case of those seeds amongst which are found pieces of iron (hammer heads amongst palm kernels, &c.), the seeds are passed over magnetic separators, which retain the pieces of iron. The seeds and nuts are then decorticated (where required), the shells removed, and the kernels (“meats”) converted into a pulpy mass or meal (in older establishments by crushing and grinding between stones in edge-runners) on passing through a hopper over rollers consisting of five chilled iron or steel cylinders mounted vertically like the bowls of a calendar. These rollers are finely grooved so that the seed is cut up whilst passing in succession between the first and second rollers in the series, then between the second and the third, and so ​ on to the last, when the grains are sufficiently bruised, crushed and ground. The distance between the rollers can be easily regulated so that the seed leaving the bottom roller has the desired fineness. The comminuted mass, forming a more or less coarse meal, is either expressed in this state or subjected to a preliminary heating, according to the quality of the product to be manufactured. For the preparation of edible oils and fats the meal is expressed in the cold, after having been packed into bags and placed in hydraulic presses under a pressure of three hundred atmospheres or even more. The cakes are allowed to remain under pressure for about seven minutes. The oil exuding in the cold dissolves the smallest amount of colouring matter, &c., and hence has suffered least in its quality. Oils so obtained are known in commerce as “cold drawn oils,” “cold pressed oils,” “salad oils,” “virgin oils.”
By pressing in the cold, obviously only part of the oil or fat is recovered. A further quantity is obtained by expressing the seed meal at a somewhat elevated temperature, reached by warming the comminuted seeds or fruits either immediately after they leave the five-roller mill, or after the “cold drawn oil” has been taken off. Of course the cold pressed cakes must be first disintegrated, which may be done under an edge-runner. The same operation may be repeated once more. Thus oils of the “second expression” and of the “third expression” are obtained.
In the case of oleaginous seeds of low value (cotton-seed, linseed) it is of importance to express in one operation the largest possible quantity of oil. Hence the bruised seed is, after leaving the five-roller mill, generally warmed at once in a steam-jacketed kettle fitted with a mixing gear, by passing steam into the jacket, and sending at the same time some steam through a rose, fixed inside the kettle, into the mass while it is being agitated. This practice is a survival of the older method of moistening the seed with a little water, while the seeds were bruised under edge-runners, so as to lower the temperature and facilitate the bursting of the cells. The warm meal is then delivered through measuring boxes into closed pressbags (“scourtins” of the “Marseilles” press), or through measuring boxes, combined with an automatic moulding machine, into cloths open at two sides (Anglo-American press), so that the preliminarily pressed cakes can be put at once into the hydraulic press. In the latest constructions of cage presses, the use of bags is entirely dispensed with, a measured-out quantity of seed falling direct into the circular press cage and being separated from the material forming the next cake by a circular plate of sheet iron. The essentials of proper oil pressing are a slowly accumulating pressure, so that the liberated oil may have time to flow out and escape, a pressure that increases in proportion as the resistance of the material increases, and that maintains itself as the volume of material decreases through the escape of oil.
Numerous forms of hydraulic presses have been devised. Horizontal presses have practically ceased to be used in this branch of industry. At present vertical presses are almost exclusively in vogue; the three chief types of these have been already mentioned. Continuously working presses (compression by a conical screw) have been patented, but hitherto they have not been found practicable. Of the vertical presses the Anglo-American type of press is most in use. It represents an open press fitted with a number (usually sixteen) of iron press plates, between which the cakes are inserted by hand. A hydraulic ram then forces the table carrying the cakes against a press-head, and the exuding oil flows down the sides into a tank below. The “Marseilles press” is largely used in the south of France. There the meal is packed by hand in “scourtins,” bags made of plaited coco-nut leaves—replacing the woollen cloths used in England. The packing of the press requires more manual labour than in the case of the Anglo-American press; moreover, the Marseilles press offers inconvenience in keeping the bags straight, and the pressure cannot be raised to the same height as in the more modern hydraulic presses. Oil obtained from heated meal is usually more highly coloured and harsher to the taste than cold drawn oil, more of the extractive substances being dissolved and intermixed with the oil. Such oils are hardly suitable for edible purposes, and they are chiefly used for manufacturing processes. According to the care exercised by the manufacturer in the range of temperature to which the seed is heated, various grades of oils are obtained.
In the case of those seeds which contain more than 40% of oil, such as arachis nuts and sesame seed, the first expression in pressbags leads to difficulty, as the meal causes “spueing,” i.e. the meal exudes and escapes from the press. Hence, in modern installations, the first expression of those seeds is carried out in so-called cage (clodding) presses, consisting of hydraulic presses provided with circular boxes or cages, into which the meal is filled. These cages or boxes are either constructed of metal staves held together by a number of steel rings, or consist of one cylinder having a large number of perforations. The presses having perforated cylinders, although presenting mechanically a more perfect arrangement, are not preferable to the press cages formed by staves, as the holes become easily clogged up by the meal, when the cylinder must be carefully cleaned out. Modern improvements, with a view to cheapening of cost, effect the transport of the cages from one press battery to another on rails. In order to dispense even with the charging of the presses by hand, in some systems the cages are first charged in a preliminary press, from which they are transferred mechanically by a swinging arrangement into the final press.
Whilst the meal is under pressure the oil works its way to the edge of the cake, whence it exudes. For this reason an oblong form is the most favourable one for the easy separation of the oil. The edges of the cakes invariably retain a considerable portion of oil; hence the soft edges are pared off, in the case of the oblong cake in a cake-paring machine, and the parings are returned to edge-runners, to be ground up and again pressed with fresh meal. Through the introduction of the cage (clodding) presses circular cakes have become fashionable, and as the material of these presses can be made much stronger and therefore higher pressure can be employed, more oil is expressed from the meal than in open presses. The oil flowing from the presses is caught in reservoirs placed under the level of the floor, from which it is pumped into storage tanks for settling and clarifying.
Extraction by Solvents.—The cakes obtained in the foregoing process still retain considerable proportions of oil, not less than 4 to 5%—usually, however, about 10%. If it be desired to obtain larger quantities than are yielded by the above-described methods, processes having for their object the extraction of the seeds by volatile solvents must be resorted to. Extraction by means of carbon bisulphide was first introduced in 1843 by Jesse Fisher of Birmingham. Thirteen years later E. Deiss of Brunswick again patented the extraction by means of carbon bisulphide (Eng. Pat. No. 390, 1856), and added “chloroform, ether, essences, or benzine or benzole” to the list of solvents. For several years afterwards the process made little advance, for the colour of the oils produced was higher and the taste much sharper. The oil retained traces of sulphur, which showed themselves disagreeably in the smell of soaps made from it, and in the blackening of substances with which it was used. Of course, the meal left by the process was so tainted with carbon bisulphide that it was absolutely out of the question to use the extracted meal as cattle food. With the improvement in the manufacture of carbon bisulphide, these drawbacks have been surmounted to a large extent, and the process of extracting with carbon bisulphide has specially gained much extension in the extraction of expressed olive marc in the south of France, in Italy and in Spain. Yet even now traces of carbon bisulphide are retained by the extracted meal, so that it is impossible to feed cattle with it. Carbon bisulphide is comparatively cheap, and it is heavier than water, hence there are certain advantages in storing so volatile and inflammable a liquid. But owing to the physiological effect carbon bisulphide has on the workmen, coupled with the chemical action of impure carbon bisulphide on iron which has frequently led to conflagrations, the employment of carbon bisulphide must remain restricted. In 1863 Richardson, Lundy and Irvine secured a patent (Eng. Pat. No. 2315) for obtaining oil from crushed seeds, or from refuse cake, by the solvent action of volatile hydrocarbons from “petroleum, earth oils, asphaltum oil, coal oil or shale oil, such hydrocarbons being required to be volatile under 212° F.” Since that time the development of the petroleum industry in all parts of the world and the large quantities of low boiling-point hydrocarbons—naphtha—obtained from the petroleum fields, and also the improvements in the apparatus employed, have raised this system of extraction to the rank of a competing practical method of oil production. Of the other proposed volatile solvents ordinary ether has found no practical application, as it is far too volatile and hence far too dangerous. Carbon tetrachloride, chloroform, acetone and benzene are far too expensive. Carbon tetrachloride would be an ideal solvent, as it is non-inflammable and shares with carbon bisulphide the advantage of being heavier than water. Efforts have been made during the last few years to introduce this solvent on a large scale, but its high price and its physiological effect on the workmen have hitherto militated against it. At the present time the choice lies practically only between the two solvents, carbon bisulphide and naphtha (petroleum ether). Naphtha is preferable for oil seeds, as it extracts neither resins nor gummy matters from the oil seeds, and takes up less colouring matter than carbon bisulphide. Yet even with naphtha traces of the solvents remain, so that the meal obtained cannot be used for cattle feeding, notwithstanding the many statements by interested parties to the contrary. It is true that on the continent extracted meal, especially rape meal from good Indian seed and palm kernel meal, are somewhat largely used as focd for cattle in admixture with press cakes, but in England no extracted meal is used for feeding cattle, but finds its proper use in manuring the land.
The apparatus employed on a large scale depends on the temperature at which the extraction is carried out. In the main two types of extracting apparatus are differentiated, viz. for extraction in the cold and for extraction in the hot. The seed is prepared in a similar manner as for pressing, except that it is not reduced to a fine meal, so as not to impede the percolation of the solvent through the mass. In the case of cold extraction the seed is placed in a series of closed vessels, through which the solvent percolates by displacement, on the “counter-current” system. A battery of vessels is so arranged that one vessel can always be made the last of the series to discharge finished meal and to be recharged with fresh meal, so that the process is practically a continuous one. The solution of the extracted oil or fat is then transferred to a steam-heated still, where the solvent ​ is driven off and recovered by condensing the vapours in a cooling coil, to be used again. The last remnant of volatile solvent in the oil is driven off by a current of open steam blown through the oil in the warm state. The extracting process in the hot is carried out in apparatus, the principle of which is exemplified by the well-known Soxhlet extractor. The comminuted seed is placed inside a vessel connected with an upright refrigerator on trays or baskets, and is surrounded there by the volatile solvent. On heating the solvent with steam through a coil or jacket, the vapours rise through and around the meal. They pass into the refrigerator, where they are condensed and fall back as a condensed liquid through the meal, percolating it as they pass downwards, and reaching to the bottom of the vessel as a more or less saturated solution of oil in the solvent. The solvent is again evaporated, leaving the oil at the bottom of the vessel until the extraction is deemed finished. The solution of fat is then run off into a still, as described already, and the last traces of solvent are driven out. The solvent is recovered and used again.
With regard to the merits and demerits of the last two mentioned processes—expression and extraction—the adoption of either will largely depend on local conditions and the objects for which the products are intended. Wherever the cake is the main product, expression will commend itself as the most advantageous process. Where, however, the fatty material forms the main product, as in the case of palm kernel oil, or sesame and coco-nut oils from damaged seeds (which would no longer yield proper cattle food), the process of extraction will be preferred, especially when the price of oils is high. In some cases the combination of the two processes commends itself, as in the case of the production of olive oil. The fruits are expressed, and after the edible qualities and best class of oils for technical purposes have been taken off by expression, the remaining pulp is extracted by means of solvents. This process is known under the name of mixed process (huilerie mixte).
Refining and Bleaching.—The oils and fats prepared by any of the methods detailed above are in their fresh state, and, if got from perfectly fresh (“sweet”) material, practically neutral. If care be exercised in the process of rendering animal oils and fats or expressing oils in the cold, the products are, as a rule, sufficiently pure to be delivered to the consumer, after a preliminary settling has allowed any mucilaginous matter, such as animal or vegetable fibres or other impurities, and also traces of moisture, to separate out. This spontaneous clarification was at one time the only method in vogue. This process is now shortened by filtering oils through filter presses, or otherwise brightening them, e.g. by blowing with air. In many cases these methods still suffice for the production of commercial oils and fats.
In special cases, such as the preparation of edible oils and fats, a further improvement in colour and greater purity is obtained by filtering the oils over charcoal, or over natural absorbent earths, such as fuller’s earth. Where this process does not suffice, as in the case of coco-nut oil or palm kernel oil, a preliminary purification in a current of steam must be resorted to before the final purification, described above, is carried out. Oils intended for use on the table which deposit “stearine” in winter must be freed from such solid fats. This is done by allowing the oil to cool down to a low temperature and pressing it through cloths in a press, when a limpid oil exudes, which remains proof against cold—“winter oil.” Most olive oils are naturally non-congealing oils, whereas the Tunisian and Algerian olive oils deposit so much “stearine” that they must be “demargarinated.” Similar methods are employed in the production of lard oil, edible cotton-seed oil, &c. For refining oils and fats intended for edible purposes only the foregoing methods, which may be summarized by the name of physical methods, can be used; the only chemicals permissible are alkalis or alkaline earths to remove free fatty acids present. Treatment with other chemicals renders the oils and fats unfit for consumption. Therefore all bleaching and refining processes involving other means than those enumerated can only be used for technical oils and fats, such as lubricating oils, burning oils, paint oils, soap-making oils, &c.
Bleaching by the aid of chemicals requires great circumspection. There is no universal method of oil-refining applicable to any and every oil or fat. Not only must each kind of oil or fat be considered as a special problem, but frequently even varieties of one and the same oil or fat are apt to cause the same difficulties as would a new individual. In many cases the purification by means of sulphuric acid, invented and patented by Charles Gower in 1792 (frequently ascribed to Thénard), is still usefully applied. It consists in treating the oil with a small percentage of a more or less concentrated sulphuric acid, according to the nature of the oil or fat. The acid not only takes up water, but it acts on the suspended impurities, carbonizing them to some extent, and thus causing them to coagulate and fall down in the form of a flocculent mass, which carries with it mechanically other impurities which have not been acted upon. This method is chiefly used in the refining of linseed and rape oils. Purification by means of strong caustic soda was first recommended as a general process by Louis C. Arthur Barreswil, his suggestion being to heat the oil and add 2% to 3% of caustic soda. In most cases the purification consisted in removing the free fatty acids from rancid oils and fats, the caustic soda forming a soap with the fatty acids, which would either rise as a scum and lift up with it impurities, or fall to the bottom and carry down impurities. This process is a useful one in the case of cotton-seed oil. As a rule, however, it is a very precarious one, since emulsions are formed which prevent in many cases the separation of oil altogether. After the treatment with sulphuric acid or caustic soda, the oils must be washed to remove the last traces of chemicals. The water is then allowed to settle out, and the oils are finally filtered. The number of chemicals which have been proposed from time to time for the purification of oils and fats is almost legion, and so long as the nature of oils and fats was little understood, a secret trade in oil-purifying chemicals flourished. With our present knowledge most of these chemicals may be removed into the limbo of useless things. The general methods of bleaching besides those mentioned already as physical methods, viz. filtration over charcoal or bleaching earth, are chiefly methods based on bleaching by means of oxygen or by chlorine. The methods of bleaching by oxygen include all those which aim at the bleaching by exposure to the air and to sunlight (as in the case of artists’ linseed-oil), or where oxygen or ozone is introduced in the form of gas or is evolved by chemicals, as manganese dioxide, potassium bichromate or potassium permanganate and sulphuric acid. In the process of bleaching by means of chlorine either bleaching powder or bichromates and hydrochloric acid are used. It must again be emphasized that no general rule can be laid down as to which process should be employed in each given case. There is still a wide field open for the application of proper processes for the removal of impurities and colouring matters without running the risk of attacking the oil or fat itself.
Oil Testing.—Reliable scientific methods for testing oils and fats date back only to the end of the ’seventies of the 19th century. Before that time it was believed that not only could individual oils and fats be distinguished from each other by colour reactions, but it was also maintained that falsification could be detected thereby. With one or two exceptions (detection of sesame oil and perhaps also of cotton-seed oil) all colour reactions are entirely useless. The modern methods of oil testing rest chiefly on so-called “quantitative” reactions, a number of characteristic “values” being determined which, being based on the special nature of the fatty acids contained in each individual oil or fat, assist in identifying them and also in revealing adulteration. These “values,” together with other useful methods, are enumerated in the order of their utility for the purposes of testing.
The saponification value (saponification number) denotes the number of milligrams which one gramme of an oil or fat requires for saponification, or, in other words, for the neutralization of the total fatty acids contained in an oil or fat. We thus measure the alkali absorption value of all fatty acids contained in an oil or fat. The saponification values of most oils and fats lie in the neighbourhood of 195. But the oils belonging to the rape oil group are characterized by considerably lower saponification values, viz. about 175 on account of their containing notable quantities of erucic acid, C$_{22}$H$_{42}$O$_2$. In the case of those oils which do not belong to the rape oils and yet show abnormally low saponification values, the suspicion is raised at once that a certain amount of mineral oils (which do not absorb ​ alkali and are therefore termed “unsaponifiable”) has been admixed fraudulently. Their amount can be determined in a direct manner by exhausting the saponified mass, after dilution with water, with ether, evaporating the latter and weighing the amount of mineral oil left behind. A few of the blubber oils, like dolphin jaw and porpoise jaw oils (used for lubricating typewriting machines), have exceedingly high saponification values owing to their containing volatile fatty acids with a small number of carbon atoms. Notable also are coco-nut and palm-nut oils, the saponification numbers of which vary from 240 to 260, and especially butter-fat, which has a saponification value of about 227. These high saponification values are due to the presence of (glycerides of) volatile fatty acids, and are of extreme usefulness to the analyst, especially in testing butter-fat for added margarine and other fats. These volatile acids are specially measured by the Reichert value (Reichert-Wollny value). To ascertain this value the volatile acids contained in 5 grammes of an oil or fat are distilled in a minutely prescribed manner, and the distilled-off acids are measured by titration with decinormal alkali. Whereas most of the oils and fats, viz. all those the saponification value of which lies at or below 195, contain practically no volatile acids, i.e. have extremely low Reichert-Wollny values, all those oils and fats having saponification values above 195 contain notable amounts of volatile fatty acids. Thus, the Reichert-Meissl value of butter-fat is 25–30, that of coco-nut oil 6–7, and of palm kernel oil about 5–6. This value is indispensable for judging the purity of a butter.
One of the most important values in oil testing is the iodine value. This indicates the percentage of iodine absorbed by an oil or fat when the latter is dissolved in chloroform or carbon tetrachloride, and treated with an accurately measured amount of free iodine supplied in the form of iodine chloride. By this means a measure is obtained of the unsaturated fatty acids contained in an oil or fat. On this value a scientific classification of all oils and fats can be based, as is shown by the above-given list of oils and fats. The unsaturated fatty acids which occur chiefly in oils and fats are oleic acid, iodine value 90·07; erucic acid, iodine value 75·15; linolic acid, iodine value 181·42; linolenic acid, iodine value 274·1; and clupanodonic acid, iodine value 367·7. Oleic acid occurs in all non-drying oils and fats, and to some extent in the semi-drying oils and fats. Linolic acid is a characteristic constituent of all semi-drying, and to some extent of all drying oils. Linolenic acid characterizes all vegetable drying oils; similarly clupanodonic acid characterizes all marine animal oils.
If one individual oil or fat is given, the iodine value alone furnishes the readiest means of finding its place in the above system, and in many cases of identifying it. Even if a mixture of several oils and fats be present, the iodine value assists greatly in the identification of the components of the mixture, and furnishes the most important key for the attacking and resolving of this not very simple problem. Thus it points the way to the application of a further method to resolve the isolated fatty acids of an oil or fat into saturated fatty acids, which do not absorb iodine, and into unsaturated fatty acids, which absorb iodine in various proportions as shown above. This separation is effected by converting the alkali soaps of the fatty acids into lead soaps and treating the latter with ether, in which the lead salts of the saturated acids are insoluble, whereas the salts of the above-named unsaturated acids are soluble. The saturated fatty acids can then be further examined, and valuable information is gained by the determination of the melting-points and by treatment with solvents. Thus some individual fatty acids, such as stearic acid and arachidic acid (which is characteristic of ground nut oil) can be identified. In the mixture of unsaturated fatty acids, by means of some more refined methods, clupanodonic acid, linolenic acid, linolic acid and oleic acid can be recognized. By combining the various methods which have been outlined here, and by the help of some further additional special methods, and by reasoning in a strictly logical manner, it is possible to resolve a mixture of two oils and fats, and even of three and four, into their components and determine approximately their quantities. The methods sketched here do not yet exhaust the armoury of the analytical chemist, but it can only be pointed out in passing that the detection of hydroxylated acids enables the analyst to ascertain the presence of castor oil, just as the isolation and determination of oxidized fatty acids enables him to differentiate blown oils from other oils.
Tests such as the Maumené test, the elaïdin test and others, which formerly were the only resource of the chemist, have been practically superseded by the foregoing methods. The viscosity test, although of considerable importance in the examination of lubricating oils, has been shown to have very little discriminative value as a general test.
Commerce.—It may be safely said of the United Kingdom that it takes the foremost position in the world as regards the extent of the oil and fat industries. An estimate made by the writer (Cantor Lectures, “Oils and Fats, their Uses and Applications,” Society of Arts, 1904, p. 795), and based on the most reliable information obtainable, led to the conclusion that the sums involved in the oil and fat trade exceeded £1,000,000 per week; in 1907 they approximated £1,250,000 per week. The great centres of the seed-oil trade (linseed, cotton-seed, rapeseed, castor-seed) are Hull, London, Liverpool, Bristol, Leith and Glasgow. Linseed is imported principally from the East Indies, Argentina, Canada, Russia and the United States; cotton-seed is chiefly supplied by Egypt and East India; rape-seed and castor-seed chiefly by East India. The importation of copra and palm kernels for the production of coco-nut oil and palm-nut oil is also considerable, but in these two cases Great Britain does not take the first place. Fish and blubber oils are principally produced in Dundee, London and Greenock. The manufacture of cod-liver oil for pharmaceutical purposes is naturally somewhat limited, as Norway, Newfoundland, and latterly also Japan, are more favourably situated as regards the supply of fresh cod, but the technical liver oils (cod oil, shark-liver oil) are produced in very large quantities in Grimsby, Hull, Aberdeen, and latterly also on the west coasts of the United Kingdom. The production of edible fats (margarine, lard compounds, and vegetable butters) has taken root in this country, and bids fair to extend largely. With regard to edible oils, edible cottonseed oil is the only table oil produced in Great Britain. The United Kingdom is also one of the largest importers of fatty materials.
Practically the whole trade in palm oil, which comes exclusively from West Africa, is confined to Liverpool, and the bulk of the tallow imported into Europe from Australasia, South America and the United States, is sold in the marts of London and Liverpool. Lard reaches Great Britain chiefly from the United States. Amongst the edible oils and fats which are largely imported, butter takes the first rank (to an amount of almost £25,000,000 per annum). This food-stuff reaches Great Britain not only from all butter-exporting countries of the continent of Europe, but in increasing quantities also from Australia, Canada, Argentine, Siberia and the United States of America. Next in importance is margarine, the British production of which does not suffice for the consumption, so that large quantities must be imported from Holland, edible olive oil from Italy, the south of France, Spain and the Mediterranean ports generally. Coco-nut oil and copra, both for edible and technical purposes, are largely shipped to Great Britain from the East Indies and Ceylon, Java and the West Indies. Of lesser importance are greases, which form the by-product of the large slaughter-houses in the United States and Argentina, and American (Canadian) and Japanese fish oils.
On the continent of Europe the largest oil-trading centres are on the Mediterranean (Marseilles and Triest), which are geographically more favourably placed than England for the production of such edible oils (in addition to the home-grown olive oil) as arachis oil, sesame oil and coco-nut oil. Moreover, the native population itself constitutes a large consumer of these oils. In the north of Europe, Hamburg, Rotterdam, Antwerp and Copenhagen are the largest centres of the oil and fat trade. Hamburg and its neighbourhood produces, curiously enough, at present the largest amount of palm-nut oil. The United States takes the foremost place in the world for the production of cottonseed and maize oils, lard, bone fat and fish oils. Canada is likely to outstrip the United States in the trade of fish and blubber oils, and in the near future Japan bids fair to become a very serious competitor in the supply of these oils. Vast stores of hard vegetable fats are still practically wasted in tropical countries, such as India, Indo-China and the Sunda Islands, tropical South America, Africa and China. With the improvement in transport these will no doubt reach European manufacturing centres in larger quantities than has been the case hitherto.
Waxes
The waxes consist chiefly of the fatty acid esters of the higher monohydric alcohols, with which are frequently associated free alcohols as also free fatty acids. In the following two tables the “acids” and “alcohols” hitherto identified in waxes are enumerated in a classified order:—
Spermaceti consists practically of cetyl palmitate, Chinese wax of ceryl palmitate. The other waxes are of more complex composition, especially so wool wax.
The waxes can be classified similarly to the oils and fats as follows:—
* I. Liquid waxes.
* II. Solid waxes.
* A. Vegetable waxes.
* B. Animal waxes.
The table enumerates the most important waxes:—
There are only two liquid waxes known, sperm oil and arctic sperm oil (bottlenose-whale oil), formerly always classed together with the animal oils. In their physical properties the natural waxes simulate the fatty oils and fats. They behave similarly to solvents; and in their liquid condition leave a grease spot on paper. An important property of waxes is that of easily forming emulsions with water, so that large quantities of water can be incorporated with them (lanolin).
The liquid waxes occur in the blubber of the sperm whale, and in the head cavities of those whales which yield spermaceti; this latter is obtained by cooling the crude oil obtained from the head cavities. Vegetable waxes appear to be very widely distributed throughout the vegetable kingdom, and occur mostly as a very thin film covering leaves and also fruits. A few only are found in sufficiently large quantities to be of commercial importance. So far carnaüba wax is practically the only vegetable wax which is of importance in the world’s markets. The animal waxes are widely distributed amongst the insects, the most important being beeswax, which is collected in almost all parts of the world. An exceptional position is occupied by wool wax, the main constituent of the natural wool fat which covers the hair of sheep, and is obtained as a by-product in scouring the raw wool. Wool fat is now being purified on a large scale and brought into commerce, under the name of lanolin, as an ​ ointment the beneficent properties of which were known to Dioscorides in the beginning of the present era. Its chemical composition is exceedingly complex, and specially remarkable on account of the considerable proportions of cholesterol and isocholesterol it contains.
Commerce.—The sperm oils are generally sold in the same markets as the fish and blubber oils (see above). For beeswax London is one of the chief marts of the world. In Yorkshire, the centre of the woollen industry, the largest amounts of wool-fat are produced, all attempts to recover the hitherto wasted material in Argentine and Australia having so far not been attended with any marked success. Spermaceti is a comparatively unimportant article of commerce; and of Chinese wax small quantities only are imported, as the home consumption takes up the bulk of the wax for the manufacture of candles, polishes and sizes.
2. Essential or Ethereal Oils.
The essential, ethereal, or “volatile” oils constitute a very extensive class of bodies, which possess, in a concentrated form, the odour characteristic of the plants or vegetable substances from which they are obtained. The oils are usually contained in special cells, glands, cavities, or canals within the plants either as such or intermixed with resinous substances; in the latter case the mixtures form oleo-resins, balsams or resins according as the product is viscid, or solid and hard. A few do not exist ready formed in the plants, but result from chemical change of inodorous substances; as for instance, bitter almonds and essential oil of mustard.
The essential oils are for the most part insoluble or only very sparingly soluble in water, but in alcohol, ether, fatty oils and mineral oils they dissolve freely. They ignite with great ease, emitting a smoke freely, owing to the large proportion of carbon they contain. Their chief physical distinction from the fatty oils is that they are as a rule not oleaginous to the touch and leave no permanent grease spot. They have an aromatic smell and a hot burning taste, and can be distilled unchanged. The crude oils are at the ordinary temperature mostly liquid, some are solid substances, others, again, deposit on standing a crystalline portion (“stearoptene” in contradistinction to the liquid portion (“elaeoptene”). The essential oils possess a high refractive power, and most of them rotate the plane of the polarized light. Even so nearly related oils as the oils of turpentine, if obtained from different sources, rotate the plane of the polarized light in opposite directions. In specific gravity the essential oils range from 0·850 to 1·142; the majority are, however, specifically lighter than water. In their chemical constitution the essential oils present no relationship to the fats and oils. They represent a large number of classes of substances of which the most important are: (1) Hydrocarbons, such as pinene in oil of turpentine, camphene in citronella oil, limonene in lemon and orange-peel oils, caryophyllene in clove oil and cumene in oil of thyme; (2) ketones, such as camphor from the camphor tree, and irone which occurs in orris root; (3) phenols, such as eugenol in clove oil, thymol in thyme oil, saffrol in sassafras oil, anethol in anise oil; (4) aldehydes, such as citral and citronellal, the most important constituents of lemon oil and lemon-grass oil, benzaldehyde in the oil of bitter almonds, cinnamic aldehyde in cassia oil, vanillin in gum benzoin and heliotropin in the spiraea oil, &c.; (5) alcohols and their esters, such as geraniol (rhodinol) in rose oil and geranium oil, linalool, occurring in bergamot and lavender oils, and as the acetic ester in rose oil, terpineol in cardamom oil, menthol in peppermint oil, eucalyptol in eucalyptus oil and borneol in rosemary oil and Borneo camphor; (6) acids and their anhydrides, such as cinnamic acid in Peru balsam and coumarin in woodruff; and (7) nitrogenous compounds, such as mustard oil, indol in jasmine oil and anthranilic methyl-ester in neroli and jasmine oils.
Preparation from Plants.—Before essential oils could be prepared synthetically they were obtained from plants by one of the following methods: (1) distillation, (2) expression, (3) extraction, (4) enfleurage, (5) maceration.
The most important of these processes is the first, as it is applicable to a large number of substances of the widest range, such as oil of peppermint and camphor. The process is based on the principle that whilst the odoriferous substances are insoluble in water, their vapour tension is reduced on being treated with steam so that they are carried over by a current of steam. The distillation is generally performed in a still with an inlet for steam and an outlet to carry the vapours laden with essential oils into a condenser, where the water and oil vapours are condensed. On standing, the distillate separates into two layers, an aqueous and an oily layer, the oil floating on or sinking through the water according to its specific gravity. The process of expression is applicable to the obtaining of essential oils which are contained in the rind or skin of the fruits belonging to the citron family, such as orange and lemon oils. The oranges, lemons, &c., are peeled, and the peel is pressed against a large number of fine needles, the exuding oil being absorbed by sponges. It is intended to introduce machinery to replace manual labour. The process of extraction with volatile solvents is similar to that used in the extraction of oils and fats, but as only the most highly purified solvents can be used, this process has not yet gained commercial importance. The process of enfleurage is used in those cases where the odoriferous substance is present to a very small extent, and is so tender and liable to deterioration that it cannot be separated by way of distillation. Thus in the case of neroli oil the petals of orange blossom are loosely spread on trays covered with purified lard or with fine olive oil. The fatty materials then take up and fix the essential oil. This process is principally employed for preparing pomades and perfumed oils. Less tender plants can be treated by the analogous method of maceration, which consists in extracting the odoriferous substances by macerating the flowers in hot oil or molten fat. The essential oil is then dissolved by the fatty substances. The essential oil itself can be recovered from the perfumed oils, prepared either by enfleurage or maceration, by agitating the perfumed fat in a shaking machine with pure concentrated alcohol. The essential oil passes into the alcoholic solution, which is used as such in perfumery.
Synthetic Preparation.—Since the chemistry of the essential oils has been investigated in a systematic fashion a large number of the chemical individuals mentioned above have been isolated from the oils and identified.
This first step has led to the synthetical production of the most characteristic substances of essential oils in the laboratory, and the synthetical manufacture of essential oils bade fair to rival in importance the production of tar colours from the hydrocarbons obtained on distilling coal. One of the earliest triumphs of synthetical chemistry in this direction was the production of terpineol, the artificial lilac scent, from oil of turpentine. At present it is almost a by-product in the manufacture of artificial camphor. This was followed by the production of heliotropin, coumarin and vanillin, and later on by the artificial preparation of ionone, the most characteristic constituent of the violet scent. At present the manufacture of artificial camphor may be considered a solved problem, although it is doubtful whether such camphor will be able to compete in price with the natural product in the future. The aim of the chemist to produce essential oils on a manufacturing scale is naturally confined at present to the more expensive oils. For so long as the great bulk of oils is so cheaply produced in nature’s laboratory, the natural products will hold their field for a long time to come.
Applications.—Essential oils have an extensive range of uses, of which the principal are their various applications in perfumery (q.v.). Next to that they play an important part in connexion with food. The value of flavouring herbs, condiments and spices is due in a large measure to the essential oils contained in them. The commercial value of tea, coffee, wine and other beverages may be said to depend largely on the delicate aroma which they owe to the presence of minute quantities of ethereal oils. Hence, essential oils are extensively used for the flavouring of liqueurs, aerated beverages and other drinks. Nor is their employment less considerable in the manufacture of confectionery and in the preparation of many dietetic articles. Most fruit essences now employed in confectionery are artificially prepared oils, especially is this the case with cheap confectionery (jams, marmalades, &c.) in which the artificial fruit esters to a large extent replace the natural fruity flavour. Thus amyl acetate is used as an imitation of the jargonelle-pear flavour; amyl valerate replaces apple flavour, and a mixture of ethyl and propyl butyrates yields the so-called pine-apple flavour. Formic ether gives a peach-like odour, and is used for flavouring fictitious rum. Many of the essential oils find extensive use in medicine. In the arts, oil of turpentine is used on the largest scale in the manufacture of varnishes, and in smaller quantities for the production of terpineol and of artificial camphor. Oil of cloves is used in the silvering of mirror glasses. Oils of lavender and of spike are used as vehicles for painting, more especially for the painting of pottery and glass.
The examination of essential oils is by no means an easy task. Each oil requires almost a special method, but with the progress of chemistry the extensive adulteration that used to be practised with fatty oils has almost disappeared, as the presence of fatty oils is readily detected. Adulteration of expensive oil with cheaper oils is now more extensively practised, and such tests as the determination ​of the saponification value (see above) and of the optical rotation, and in special cases the isolation and quantitative determination of characteristic substances, leads in very many cases to reliable results. The colour, the boiling-point, the specific gravity and solubility in alcohol serve as most valuable adjuncts in the examination with a view to form an estimate of the genuineness and value of a sample. Quite apart from the genuineness of a sample, its special aroma constitutes the value of an oil, and in this respect the judging of the value of a given oil may, apart from the purity, be more readily solved by an experienced perfumer than by the chemist. Thus roses of different origin or even of different years will yield rose oils of widely different value. The cultivation of plants for essential oils has become a large industry, and is especially practised as an industry in the south of France (Grasse, Nice, Cannes). The rose oil industry, which had been for centuries located in the valleys of Bulgaria, has now been taken up in Germany (near Leipzig), where roses are specially cultivated for the production of rose oil. India and China are also very large producers of essential oils. Owing to the climate other countries are less favoured, although lavender and peppermint are largely cultivated at Mitcham in Surrey, in Hertfordshire and Bedfordshire. Lavender and peppermint oils of English origin rank as the best qualities. As an illustration of the extent to which this part of the industry suffers from the climate, it may be stated that oil from lavender plants grown in England never produces more than 7 to 10% linalool acetate, which gives the characteristic scent to lavender oil, whilst oil from lavender grown in the south of France frequently yields as much as 35% of the ester. The proof that this is due mainly to climatic influences is furnished by the fact that Mitcham lavender transplanted to France produces an oil which year by year approximates more closely in respect of its contents of linalool acetate to the product of the French plant.
Bibliography.—For the fixed oils, fats and waxes, see C. R. A. Wright, Fixed Oils, Fats, Butters and Waxes (London, 2nd ed. by C. A. Mitchell, 1903); W. Brannt, Animal and Vegetable Fats and Oils (London, 1896); J. Lewkowitsch, Chemical Technology and Analysis of Oils, Fats and Waxes (London, 4th ed., 3 vols., 1909; also German ed., Brunswick, 1905; French ed., Paris, vol. i. 1906. vol. ii. 1908, vol. iii. 1909); Laboratory Companion to Fats and Oil Industries (London, 1902); Cantor Lectures of the Society of Arts, Oils and Fats, their Uses and Applications; Groves and Thorp, Chemical Technology, vol. ii.; A. H. Gill, Oil Analyses (1909); G. Hefter, Technologie der Fette und Öle (Berlin, vol. i. 1906; vol. ii. 1908); L. Ubbelohde, Handbuch der Chemie und Technologie der Öle und Fette (Leipzig, vol. i., 1908); R. Benedikt and F. Ulzer, Analyse der Fette und Wachsarten (Berlin, 1908); J. Fritsch, Les Huiles et graisses d’origine animale (Paris, 1907).
For the essential oils, see F. B. Power, Descriptive Catalogue of Essential Oils; J. C. Sawer, Odorographia (London, 1892 and 1894); E. Gildemeister and F. Hoffmann, Die aetherischen Öle (Berlin, 1899), trans. (1900) by E. Kremers under the title Volatile Oils (Milwaukee, Wisconsin); F. W. Semmler, Die aetherischen Öle nach ihren chemischen Bestandteilen unter Berücksichtigung der geschichtlichen Entwickelung (Leipzig); M. Otto, L’Industrie des parfums (Paris, 1909); O. Aschan, Chemie der alicyklischen Verbindungen (Brunswick, 1905); F. R. Heussler (translated by Pond), The Chemistry of the Terpenes (London, 1904).  (J. Lh.)