Fixed Oils
Olive oil designed for table use is frequently adulterated with the oils of poppy, sesamé, cotton-seed, pea-nuts, and other nuts; olive oil, intended for manufacturing purposes, is often mixed with colza and nut oils.
The tests used are of a rather unsatisfactory character. In all instances, when the chemist is called upon to pronounce as to the adulteration of an oil, it is necessary to execute comparative experiments with the pure oil, and with admixtures arbitrarily prepared: it is only when this is done that the indications obtained are of value.
Examination of Olive Oil Intended for Table Use a. The density of the oil is determined by means of a hydrometer (oleometer) provided with a scale giving the densities from 0.8 to 0.94, for the temperature of 15.° Pure olive oil possesses a specific gravity of 0.917; poppy oil one of 0.925; a mixture of the two, an intermediate density. Since the fixed oils are not definite chemical compounds, this test is seldom conclusive.
b. Two or three cubic centimetres of concentrated nitric acid, containing nitric peroxide in solution (or a solution of mercury in strong nitric acid), are added to the oil to be examined, as well as to a sample of pure olive oil. The two samples are then allowed to stand in a room where the temperature does not exceed 10.° The oleine of the olive oil is converted into solid elaidine, and the mixture after some time becomes sufficiently thick to remain in the vessel upon inversion. If the sample under examination is free from adulteration, it will solidify at the same time as the pure oil; whereas, the presence of one per cent. of poppy oil, or of other drying oils, suffices to retard the solidification for forty minutes.
c. Fifteen grammes of the oil are mixed in a glass vessel with the same amount of strong sulphuric acid, the temperature of the two liquids being previously observed. The mixture is stirred with a thermometer, and the maximum temperature noted: pure olive oil produces an elevation of temperature of 37.°7; pure poppy oil, an elevation of 70.°5; and a mixture of the two an elevation of temperature intermediate between 37.°7 and 70.°5.
d. One volume of nitric acid of sp. gr. 1.33 is agitated with 5 grammes of the oil, and notice taken of the coloration produced after the lapse of five minutes. If the olive oil is pure, it acquires a pale green color; in case it is mixed with sesamé or nut oil, a deep-red color appears: poppy oil also communicates a reddish coloration, but one less deep than the preceding.
If an acid of sp. gr. 1.22 is taken, it is still less difficult to distinguish between sesamé, nut and poppy oils; the latter assumes, in this case, a pale yellowish-red color.
Pea-nut oil fails to exhibit a coloration; but can be recognized by its conversion into a white solid, when mixed with 1/5 of its volume of a solution of caustic soda of sp. gr. 1.34.
Examination of Olive Oil Intended for Manufacturing Purposes
The chief adulterations are colza and nut oils. The latter is detected by means of the reaction with nitric acid, as described above. Colza oil is recognized by mixing 5 volumes of the sample to be examined, with 1 volume of sulphuric acid of sp. gr. 1.655: if colza or nut oils are present, a brown coloration ensues; under the same circumstances, pure olive oil assumes a pale greenish hue. In case the sample acquires a brown color when treated with sulphuric acid, and a red coloration is produced by the addition of nitric acid, it contains nut oil; if sulphuric acid produces a brown coloration, and nitric acid fails to change it, the presence of oil of colza is indicated.
Examination of Hempseed Oil
This oil is frequently adulterated with linseed oil. The reactions exhibited by these oils are nearly identical, and the detection of the admixture is extremely difficult. It is advisable to mix the suspected oil with sulphuric acid, notice being taken of the elevation of temperature produced, and to treat it with nitric acid and with dilute potassa solution, subjecting, at the same time, an artificial mixture of the two pure oils to the same treatment, and comparing the results obtained.