Chapter VII · SALAD OILS

Salad Oils—Oils used for Culinary and Confectionery Purposes— Chocolate Fats.

Olive Oil.—No oil is so extensively used in the preparation of salad dressings as olive oil, and the more closely its characteristics can be imitated by other salad oils the greater is the demand for the latter.

There are several species of the olive tree, some of which are successfully cultivated in hot climates, but the oil used in Europe is mainly derived from different varieties of Olea europæa. The fruit produced by these varies considerably in size and in colour, but the oils that they yield show a close similarity in chemical and physical characteristics, though they differ from one another in flavour and in colour.

Apart from the influence of soil and climate and of the variety of the olive tree, the flavour of the oil also depends upon the stage of growth at which the fruit is gathered and upon the method of separating the oil from the pulp.

Fruit that is picked before it is quite ripe yields an oil with a somewhat bitter flavour, and therefore in preparing the finest grade of oil the olives are specially selected and pressed by hand between cloths. The resulting oil is washed with water to remove impurities, then decanted from the aqueous layer, and sold under the name of virgin oil.

The bulk of fine commercial olive oil, however, is separated by expression, the ripe olives being superficially crushed between mill-stones, and then expressed at a low pressure. The residue left in the press is broken up, mixed with hot water, and the liquid, consisting of the mixed oil and water, expelled by stronger pressure, and allowed to stand for the oil to separate. The flavour and colour of this oil is inferior to that of the cold-drawn product.

Subsequent grinding of this second residue, followed by another expression with hot water, yields an additional quantity of oil, while the amount still present is often extracted by means of carbon bisulphide or other volatile solvent. The oils finally obtained are generally utilised as lubricants or in the manufacture of soap.

In most districts care is taken not to crush the olive stones until after the finest oil has been expressed, although according to Klein (Zeit. angew. Chem., 1898, 847) there is no disadvantage in expressing the mixed kernel and fruit oil, provided the fruit is freshly picked. The general opinion of the trade, however, is that the flavour of the pulp oil is superior to that derived from the mixed pulp and kernels.

Oils that have been extracted by means of carbon bisulphide are known as “sulphocarbon” oils, while those which have been separated by means of petroleum spirit are termed “pyrene” oils.

The oils derived from olives that have been left for some time before expression are of an inferior kind, and are termed huiles d’enfer, huiles tournantes, etc. They contain a large amount of free fatty acids, and have a sharp, unpleasant flavour which renders them unsuitable for food.

The finest qualities of oil are derived from the districts round Lucca and Leghorn, and from Provence. Large quantities of excellent oil are also exported from Spain, Portugal, Algiers, and Tunis, while California has now become an important oil-producing country.

The flavour is the chief criterion of the quality of olive oil, and a trained palate is able to detect slight differences which could not be recognised by any chemical tests.

The so-called virgin oil is pale yellowish-green in colour, and has but little odour, whereas the lower grades of oil obtained from the marc left in the press, as described above, vary in colour from greenish-yellow to light brown, and have a perceptible odour and a somewhat bitter flavour.

Even the purest olive oil will yield a deposit of “stearin” when exposed to a low temperature, but the amount varies with the kind of olive which yielded the oil. Thus it is particularly abundant in the case of Tunisian and Algerian oils, and it is therefore usual to remove a portion of this solid fat before putting the oil upon the market, under the name of “demargarinated” oil. The amount of “stearin” thus removed depends upon the temperature to which the oil is chilled before being pressed in a filter-press. Oils thus treated are sometimes described as “winter oils” (see Chapter III.).

Olive oil consists chiefly of olein, with smaller quantities of the glycerides of linolic acid and of various solid fatty acids (mainly palmitic acid) which form the insoluble deposit—the so-called “stearin.”

It belongs to the class of non-drying oils, and hence does not form a skin on its surface when exposed to the air, and when spread in a thin film upon glass takes many days to dry up. In this respect it differs from the semi-drying oils—cotton-seed, sesame, and maize oils—which are frequently sold as salad oils, sometimes under descriptions that suggest that they are olive oil.

The chief analytical characteristics of a typical olive oil are as follows:—

Sp. gr. Saponification Value. Iodine Value. Hehner Value. Melting-point of Fatty Acids.

0·918 191 82 95 24° C.

Olive oil is very frequently adulterated, the principal substances used for the purpose being cotton-seed, sesame, maize, and lard oils. The vegetable oils may be detected by their general analytical values, and the two first by the characteristic colour reaction (see Chapter VIII. pp. 105 and 107), whilst lard oil may be detected by the cholesterol test (p. 101).

Arachis Oil.—The chief centre of the arachis oil industry is the South of France, and at Marseilles in particular enormous quantities of the nuts are expressed.

The oil obtained by the first expression in the cold is a pale yellow fluid which is extensively used as a salad oil. A large proportion of the oil obtained on subsequent expression is sold as a lower grade of salad oil, while the remainder, and also the oil obtained by hot pressure, is manufactured into soap. The total yield of oil is from 45 to 50 per cent.

As a rule, the salad oils (which are sometimes sold as “nut salad oil”) have a distinct odour and flavour of the nut, but in the very finest grades, which also have but little colour, this flavour is hardly noticeable in the case of the freshly prepared product. Arachis oil becomes turbid at a much higher temperature and throws down a more abundant deposit of “stearin” than olive oil, and this has greatly interfered with its popularity as a salad oil in this country. The solidification point is usually about +2 to +3° C. Chemically it consists of the glycerides of hypogæic, oleic, linolic, palmitic, stearic, arachidic, and lignoceric acids. The most characteristic constituents are the two last fatty acids (amounting to about 5 per cent.). They are only slightly soluble in cold alcohol, and on this property are based several methods of separating them and of calculating from the result the proportion of arachis oil in, for example, an adulterated olive oil.

The physical and chemical constants of arachis oil vary considerably, as is shown in the following figures recorded by Sadtler and by Crossley and Le Sueur for oils of different origin:—

LEGEND:

(A) = Saponification Value.

(B) = Reichert-Meissl Value.

Oil. Sp. gr. at 15° C. (A) Iodine Value. (B) Free Acid (as Oleic Acid) per cent. Melting-point of Fatty Acids. °C.

Virginian 0·917 192·5 91·7 0·48 0·55 29

Spanish 0·9175 190·7 94·2 1·60 0·79 34

African 0·911 194·0 85·6 0·62 30

Indian 0·9223 190·1 98·5 Nil. 1·45

The highest figures recorded for the iodine value of arachis oil are 101-105 (Oliveri), whereas the lowest values fall well within the limits of those of olive oil.

Arachis oil is sometimes adulterated with cheaper oils, and notably with sesame and cotton-seed oils. The determination of the analytical values and the characteristic colour reactions will probably afford information in such cases, while at the same time there would be a decrease in the amount of arachidic acid that could be separated.

The methods of separating and determining this acid are described in Chapter VIII. p. 108.

Sesame Oil.—This oil also goes by the name of teel oil and gingelly oil, and in commerce is sometimes described as French salad oil.

It belongs to the same class of oils (semi-drying) as cotton-seed oil, which it also resembles in its chemical composition.

It contains the glycerides of oleic, linolic, palmitic, and stearic acid. The solid fatty acids amount to about 14 per cent. of the total fatty acids, while, according to Farnsteiner, the linolic acid is about 12 per cent.

The unsaponifiable matter contains phytosterol, a body termed sesamin, and a compound of the nature of a phenol, termed sesamol. It is to the presence of this substance that the characteristic furfural reaction (see Baudouin’s test, p. 105) is due.

This active constituent may be removed from sesame oil by treatment with animal charcoal, and the oil may also be rendered inert by prolonged heating over boiling water.

Sesame oil has a pale yellow colour and a pleasant odour of the grain. It solidifies at a lower temperature (5° C.) than cotton-seed oil, and does not yield the large proportion of “stearin” given by the latter.

In the elaidin test it gives a reddish-brown partially solid mass of about the same consistency as the elaidin of cotton-seed oil.

Its iodine value (usually about 106) is somewhat lower than that of cotton-seed oil, except in the case of the oil from Russian seed, which has an iodine value of 114-115.

As is mentioned elsewhere, the addition of sesame oil to margarine is compulsory in Germany, Austria, and Belgium, so that the product possesses a “latent colour” which may be developed by the use of the Baudouin reagent.

In addition to its use as a salad oil, and in pharmaceutical preparations, sesame oil is employed as an adulterant of olive and almond oils; while, in turn, it is liable to be adulterated with arachis oil, cotton-seed oil, poppy oil, and rape oil.

The presence of the first may be detected by an estimation of the arachidic acid (p. 108), while Halphen’s test will show the presence of cotton-seed oil.

The other oils are detected by the general physical and chemical characteristics, and, in the case of rape oil, by the characteristics of its principal fatty acid, erucic acid.

Cotton-Seed Oil.—This oil, as has been already mentioned (p. 16), is expressed in enormous quantities in the United States, while there is a steadily increasing production in Egypt and India. In practice, a ton of cotton seed yields about 2½ cwt. of crude oil, which is then separated into “summer oil” and “foots” by refining (p. 23).

The bulk of the cotton-seed oil used for food in this country is derived from Egyptian seed, for the Indian oil is considered to have an unpleasant flavour.

Experiments made by Hooper, however, upon oils derived from American seed grown in India (Ann. Report, Indian Museum, 1910, 26), show that the fault lies with the method of screening the seed and the process of refining the oil.

He finds that by treating Indian oils with the amount of alkali corresponding to the acidity and giving a subsequent thorough washing with water, the whole of the colouring matter, the so-called “bloom,” and the acid taste are removed, and that the product has the light colour and bland flavour of refined Egyptian oil.

Thus by removing all dirt from the seed and delivering the seed in sound condition, and washing the oil to a sufficient extent after the refining with alkali, there appears to be no reason why oils from Indian seed should be in any respect inferior to Egyptian oils. Hooper’s experiments also indicate that as much “stearin” could be separated from Indian oils as from American or Egyptian oils.

Cotton-seed oil consists chiefly of the glycerides of oleic and linolic acids, the latter, as is indicated by the iodine value, being present in greater proportion than in olive oil, and amounting to about 17 or 18 per cent. of the total fatty acids. The glycerides of solid fatty acids which are present in solution, and separate out as “cotton-stearin” on chilling the oil, consist principally of those of palmitic acid with a small proportion of stearic acid (Hehner and Mitchell).

Owing to the presence of the large amount of linolic acid, cotton-seed oil belongs to the class of oils known as “semi-drying” oils, i.e. oils which thicken on exposure to the air, but, unlike the drying oils, do not form a dry film until after the lapse of a long time.

When exposed to a temperature of about 10° to 14° C., cotton-seed oil yields a deposit of the so-called “cotton-stearin” (q.v.), while the entire oil becomes solid at about the freezing-point of water. The stearin, which is separated by filtration (p. 25), is a useful by-product, which is utilised in the preparation of margarine. The oil from which the “stearin” has been separated goes by the name of “winter” cotton-seed oil.

Oils which have been refined by treatment with alkali and separation of “stearin” have a mild taste and are practically devoid of free acids. They are widely employed both as salad oils and as substitutes for lard in cooking, but the bulk of the American oil is made up into margarine or used in the manufacture of soap.

Cotton-seed oil is a common adulterant of olive oil, and, owing to its forming a frequent ingredient of margarine, may find its way into butter.

In addition to the colour reactions, described on p. 107, it may be identified by the relatively high melting-point of its solid fatty acids (32° to 38° C.), and by its iodine value (112-115).

In the elaidin test (p. 107) it yields a butter-like mass of an orange colour, very different from the hard white elaidin produced by olive oil.

Cotton-seed oil contains about 1 per cent. of unsaponifiable matter consisting chiefly of phytosterol. The presence of cotton-seed oil in animal oils and fats may thus be confirmed by the phytosteryl acetate test (p. 101).

Sunflower Oil.—Enormous quantities of sunflower seeds are cultivated in Russia for the production of an edible oil, and most of the oil mills in that country are engaged in the industry.

The method of expression is similar to that used in this country for linseed. The seeds are first “screened” from dirt, particles of stalk, etc., and are then steamed and crushed to a paste, which is wrapped in hair-cloths and expressed in a hydraulic press. Expression by hand is still employed in some of the smaller mills.

When freshly obtained from clean seed, sunflower oil is a pale yellow fluid with a pleasant odour and flavour, but the second (hot) pressings are much darker in colour, and are used for illuminating purposes and in the manufacture of varnish.

Sunflower oil has good drying properties, though these are less pronounced than in the case of poppy or linseed oil. It consists of the glycerides of oleic, linolic, and palmitic acids, and probably contains a small amount of linolenic acid.

Its iodine value (about 130) is considerably higher than that of cotton-seed oil.

When used as a salad oil it has the advantage of keeping fluid to a very low temperature, but its drying capacity, which causes a film to form upon the surface when exposed to the air, is a drawback from which olive and similar non-drying oils are free.

Sunflower oil is occasionally employed as an adulterant of olive oil. It would be detected by the increase in the iodine value, and, according to Jean, by its having a slight reducing action upon silver nitrate in Bechi’s test (q.v.).

Poppy Oil.—The oil expressed from the seeds of the poppy (Papaver somniferum), which is cultivated in Egypt and Asia Minor, has a mild, bland taste, and is sometimes used as an edible oil.

Its chief use, however, is as a drying oil for paints, for which its good drying capacity and its light colour make it particularly suitable.

It consists of the glycerides of oleic, linolic, and linolenic acids, with a small proportion of those of solid fatty acids, including stearic and palmitic acids.

It bears considerable resemblance to sunflower oil in its chemical and physical characteristics, as is shown by the following typical values:—

Sp. gr. Saponification Value. Iodine Value. Hehner Value. Solidification point.

0·926 194 140 95·4 -15° to -20° C.

If added in any considerable quantity to olive oil there would be an increase in the sp. gr. and iodine value of the latter.

Maize Oil.—This oil, which is also known as corn oil, is obtained from the germs of the maize or Indian corn (Zea mais), and is used to a limited extent as an edible oil.

It has a pale yellow colour, and a fragrant odour recalling that of the fresh grain.

It resembles cotton-seed oil in its composition, and contains the glycerides of oleic, linolic, and palmitic acids, together with an appreciable quantity of those of volatile fatty acids, as is indicated by its relatively high Reichert-Meissl value (4 to 4·5).

Like cotton-seed oil it belongs to the class of semi-drying oils, slowly forming a dry skin when exposed to the air in a thin film.

The following values have been recorded:—

Sp. gr. Saponification Value. Iodine Value. Hehner Value. Melting-point of Fatty Acids.

0·9245 190 122 93·6 17°-20° C.

The presence of cotton-seed oil, which is sometimes used to adulterate maize oil, would be shown by the characteristic colour reactions (p. 107). For the detection of maize oil in lard or in butter reliance would have to be placed upon the general analytical values and upon the results of the phytosteryl acetate test.

Chocolate Fats

Cacao Butter.—In the manufacture of cocoa and chocolate a large proportion of the fat contained in the cocoa-bean is expressed, and forms a valuable by-product.

This fat, which is usually termed cacao butter, comes into the market in moulded slabs, weighing several pounds each.

It is a hard substance of a yellowish colour, having an aroma of cocoa, and, when broken, shows signs of crystalline structure.

Its flavour and high melting-point render it particularly suitable for the “cream” of chocolate creams, and large quantities of it are used for this purpose.

The chemical composition of cacao butter resembles that of other hard fats, the differences being due to a different proportion of the various glycerides.

It contains stearic, palmitic, lauric, and oleic acids, while arachidic, and a fatty acid termed theobromic acid, are also said to have been identified, though the presence of either is doubtful.

The hardness and high melting-point of the fat are due to the large proportion (about 40 per cent.) of stearic acid it contains.

The melting-point of the commercial product ranges from about 27° to 34° C.

The relatively low iodine value (usually between 32 and 36) indicates the presence of a much smaller amount of liquid fatty acids than of solid fatty acids, the former, according to Farnsteiner, consisting of oleic acid, and constituting 31 per cent. of the total fatty acids.

It has frequently been stated that cacao butter is not liable to become rancid. This, however, is not the case, for the fat, like other fats, does gradually decompose on exposure to light and air, though owing to its consistency and the low proportion of glycerides of liquid fatty acids present the process is not rapid, as in the case, for example, of cocoanut oil.

Cacao butter is frequently adulterated, the principal substances liable to be found being stearic acid, cocoanut oil, paraffin wax, beeswax, and various vegetable oils.

Cacao butter fetches too high a price to be used in the lowest grade of chocolate creams, and various substitutes are now sold.

Cocoanut and Palm-Kernel Oil Stearins.—Among the most common of these cheaper products are the “stearins,” obtained by expression from cocoanut oil and palm-kernel oil, both of which in their original condition are too soft, and melt at too low a temperature to be suitable for this purpose.

In preparing more solid fats these oils are melted and refined, then chilled down, and subjected to pressure in the cold in a hydraulic press. A fractionation is thus effected, the expressed portion, known as cocoanut or palm-nut olein, being much more fluid than the stearin left in the press, while the latter is more consistent than the original fat.

Thus while the melting-point of cocoanut oil is usually about 23° C., that of the separate is about 30° C.

The following results were obtained by Sachs (Chem. Rev. Fett- u. Harz.-Ind., 1908, xv. 30) in the examination of commercial samples of these stearins:—

LEGEND:

(A) = Saponification Value.

(B) = Reichert-Meissl Value.

(C) = Melting-point of Fatty Acids.

Melting-point, °C. Sp. gr. at 100° C. (A) (B) Iodine Value. (C)

Hard cocoanut stearin 29·3 to 29·5 0·8700 252 3·4 4·0 to 4·5 28·1

Palm-nut stearin 31·5 to 32 0·8700 242 2·2 8 28·5-29·5

Frequently these products are rendered still more consistent by the addition of a small proportion of an animal stearin or some vegetable fat of higher melting-point, such as Japan wax.

Thus a commercial preparation consisting of a mixture of cocoanut stearin with 25 per cent. of Japan wax, melted at 34° to 35°·5 C., or approximately at the same temperature as a good specimen of cacao butter.

Other Vegetable Fats.—Of late years various exotic vegetable fats have been put upon the market as chocolate fats either in their pure state or in admixture with cocoanut or palm-nut stearins. These fats include dika fat, tankawang fat (Borneo tallow) and Illipé fat.

Dika fat, which is obtained from the seed kernels of Mangifera gabonensis and other members of the same family, growing on the West Coast of Africa, is a hard fat, melting at about 39° C. (or higher than cacao butter), and having an iodine value (5) about the same as that of cocoanut oil.

Borneo tallow (tankawang fat) derived from the seed kernels of Shorea aptera and other members of the Dipterocarpi. The native method of separating the fat is to suspend the kernels in baskets above boiling water, and when soft, to express them in primitive presses. According to Sachs (loc. cit.), Borneo tallow melts at 37°·5 C., and has an iodine value of 30-31.

Illipé butter (Mahua butter), which is derived from the seeds of Bassia latifolia, is a yellow fat, large quantities of which are eaten in India.

Nine samples of the Indian product examined by Crossley and Le Sueur had melting-points ranging from 23° to 29° C., and iodine values of 58·4 to 67·8. The relatively low melting-point appears to depend upon the large proportion of glycerides of liquid fatty acids present, and this fat would probably yield a hard stearin on expression.

Other fats that might be used for this purpose, if they could be obtained in sufficient quantity, are shea butter, from the seeds of Bassia Parkii (melting at about 25° C.); Mafura tallow, melting at 35° to 42° C.; Mkani fat, melting at about 40° C.; and Malabar or Piney tallow, produced by the East Indian tree, Vateria indica (melting at about 37° C.).

Sachs (loc. cit.) states that favourite substitutes for cacao butter consist of a mixture of two-thirds of palm-nut stearin with one-third of cocoanut stearin, and of a mixture of 40 per cent. of Borneo tallow with 60 per cent. of cocoanut stearin.