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:—
| Olive oil | ·914 to ·917 |
| Poppy oil | ·924 „ ·927 |
| Cotton-seed oil | ·922 „ ·930 |
| Sweet almond oil | ·914 „ ·920 |
| Arachis oil | ·916 „ ·920 |
| Colza oil | ·914 „ ·916 |
| Sesamé oil | ·921 „ ·924 |
| Rape-seed oil | ·914 „ ·916 |
| Lard oil | ·915 |
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:—
| Oil. | Coloration. | Minutes for Solidification. |
| Olive | Pale yellowish green | 60 |
| Almond | White | 90 |
| Arachis | Pale reddish | 105 |
| Rape | Orange | 200 |
| Cotton-seed | Orange red | 105 |
| Sesamé | Yellowish orange | 150 |
| Beech-nut | Reddish orange | 360 |
| Poppy | Red | Remains fluid. |
(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.