Olive Oil is extracted from the pericarp of the fruit of the Olea Europea. When pure, it exhibits a pale yellow or greenish colour, has a specific gravity of 0·9176, and possesses a faint, pleasant odour and a bland and agreeable taste. It is insoluble in water, very slightly in alcohol, but dissolves in about 1½ parts of ether. Olive oil boils at 315°, and begins to deposit white granules at 10°; at 0°, it solidifies to a solid mass which, by pressure, may be separated into tripalmetine and trioleine. Upon saponification, it is decomposed into oleic, palmetic, and stearic acids and glycerine. The best-known varieties of olive oil met with in commerce, in the order of their quality, are—Provence, Florence, Lucca, Genoa, Gallipoli, Sicily, and Spanish.[136]

Owing to the high price of the pure article, and perhaps to the difficulty experienced in detecting foreign admixtures, olive oil is probably more extensively adulterated than any substance of general consumption. The oils most employed as adulterants are those of cotton-seed, poppy, pea-nut, sesamÉ, rape-seed, arachis, and lard. Although the subject of the adulteration of olive oil has received the attention of numerous chemists, including several of exceptionally high standing, the results obtained, while of service in indicating the presence of some foreign oil, are unfortunately often of but little use in the positive identification of the particular adulterant used. Of the many methods of examination that have been suggested, the following are the most satisfactory:—

1. Specific gravity.—The density of olive oil is lower than that of the majority of the oils with which it is mixed, and it is sometimes possible to detect the presence of the latter by means of this property, especially when they are contained in a considerable proportion. Cotton-seed oil differs more in specific gravity than the other oils generally employed as adulterants. Donny[137] applies the test by placing in the suspected sample a drop of olive oil of known purity which has been dyed with ground alkanet root, and observing whether it remains stationary. A more satisfactory method is to determine the density by the gravity bottle. The following tabulation gives the densities (at 15°) of olive and several other oils liable to be met with as admixtures:—

2. Solidifying point.—Attempts have been made to utilise the fact that some of the oils added to olive congeal at a lower temperature than the pure oil. Thus, cotton-seed oil solidifies at -22°, ground-nut oil at -33°, poppy at -18°.

3. Elaidin and colour tests.—Pure olive oil is converted into a solid mass when treated with various oxidising agents, the change being retarded by the presence of some of its adulterants. The test may be made in several ways:—

(a) Ten grms. of the sample are shaken with 5 grms. of nitric acid (sp. gr. 1·40) and 1 grm. of mercury, and the colour produced and time required for solidification noticed. In this manner the following results have been obtained:—

(b) Or a few pieces of copper foil are added to a mixture of equal parts of the oil and nitric acid, the liquor occasionally stirred, and then set aside. If the oil be pure, it will be converted into a nearly white buttery mass in from three to six hours; sesamÉ oil yields a red, cotton and rape-seed a brown, and beech-nut a reddish-yellow colour, the solidification being delayed from 10 to 20 hours, while poppy oil fails to solidify at all.

(c) Nine parts of the sample are oxidised by heating with one part of concentrated nitric acid, the mixture being well stirred; pure olive oil forms a hard, pale-yellow mass in the course of two hours; seed oils (including cotton-seed) turn orange-red in colour and do not become solid in the same time or manner.

(d) A portion of the sample is well mixed with one-fourth of its weight of chromic acid; if pure, the oil will be converted into an opaque mass.

(e) Introduce 2 c.c. of the sample into a narrow graduated glass cylinder, add 0·1 gramme potassium dichromate, next 2 c.c. of a mixture of sulphuric and nitric acids, shake well, and then add 1 c.c. of ether; shake again and allow the mixture to stand at rest. Lively effervescence and evolution of nitrous fumes soon take place, and the oil rises to the surface, showing a characteristic coloration. Olive oil exhibits a green colour, whereas in presence of 5 per cent. of sesamÉ, arachis, cotton-seed, or poppy oil, the colours will vary from greenish-yellow to yellow or yellowish red. The coloration is more readily observed upon agitating the mixture with water and, setting it aside for a short time.

(f) Several portions of the oil are placed upon a porcelain slab and separately treated with a few drops of concentrated sulphuric acid, nitric acid, and a solution of potassium dichromate in sulphuric acid, and notice taken of the colours produced, comparative tests being simultaneously made with olive oil of undoubted purity.

(g) The presence of sesamÉ oil is readily detected by the formation of a deep green colour when the oil is agitated with a mixture of equal parts of nitric and sulphuric acid.

(h) Upon mixing samples containing cotton-seed oil with an equal volume of nitric acid (40° B.) a coffee-like colour is produced. Olive oil gives a pale green, rape and nut, a pale rose, and sesamÉ oil a white-coloured mixture.

The presence of rape- and cotton-seed oils may also be detected as follows:—Dissolve 0·1 gramme silver nitrate in a very little water, and add about 4 c.c. of absolute alcohol. This solution is added to the sample of olive oil to be tested, the mixture well shaken and put aside for one or two hours; it is then to be heated for a few minutes. If cotton-seed or rape-seed oil is present, the oily stratum which separates on standing will exhibit a brownish-red or blackish colour, due to the reduction of silver. Olive oil fails to cause an appreciable coloration. Experiments made by the author with samples of olive oil containing 10 per cent. of cotton-seed and rape-seed oils furnished the following results:—On standing one hour, without heating, the mixture containing cotton-seed oil showed a slightly dark colour, that adulterated with rape-seed oil a decidedly dark colour; upon the application of heat, the former exhibited a dark-red colour, while the latter turned quite black.

MaumenÉ’s test.—This test is founded upon the fact that the elevation of temperature caused by mixing olive oil with strong sulphuric acid is considerably less than that produced with the oils commonly employed as its adulterants. With these latter an evolution of sulphurous acid generally takes place, which is not the case with pure olive oil. The best method of procedure is as follows:—10 c.c. of sulphuric acid (sp. gr. 1·844) are gradually added to 50 grammes of the sample, the mixture being constantly stirred with a small thermometer, and observations made of the maximum increase of temperature produced, as well as of the evolution of gas. When treated in this manner, genuine olive oil causes an elevation of about 42°; that given by various other oils, often added to it, ranges from 52° to 103°, and it is frequently possible to recognise their presence in admixtures by the high temperature produced. The following are the increases of temperature observed by L. Archbutt:—olive, 41-45; rape, 55-64; arachis, 47-60; sesamÉ, 65; cotton-seed (crude) 70; (refined), 75-76; poppy-seed, 86-88; menhaden, 123-128. In the Paris Municipal Laboratory an acid of 1·834 sp. gr. is used, and the following heating powers are regarded as standards:—For olive oil, 55·5°; for cotton-seed, 69·5° for nut, 62°; for sesamÉ, 66°; for poppy oil, 73°.

The application of Hubl’s test for butter (see p. 75) is one of the most useful means for the detection of foreign oils in olive oil. The iodine absorption number of the pure oil is considerably below that of its most common adulterants.

The prevalence of the adulteration of olive oil has been abundantly demonstrated. Of 232 samples examined by the New York and Massachusetts State Boards of Health, 165 (71 per cent.) were spurious. It is a notorious fact that large quantities of cotton-seed oil are exported from the United States to France and Italy, much of which returns home under the guise of the genuine product of the olive.