The foregoing description does not apply to the inferior sorts of oil, which congeal more easily, are more or less deep-coloured, have a disagreeable odour and taste, and quickly turn rancid. These inferior oils have their special applications in the arts.
Chemical Composition--The chief constituent of olive oil is Olein or more correctly Triolein, C₃H₅(O·C₁₈H₃₃O)₃, identical so far as at present ascertained with the fluid part of all oils of the non-drying class. The proportion of olein in olive oil, as well as in other oils, is liable to variation, the result partly of natural circumstances and partly of the processes of manufacture. The best oils are rich in olein.
As to the solid part of olive oil, Chevreul believed it to be constituted of Margarin, which he first examined in 1820. But Heintz (1852 and later) showed margarin to be a mixture of palmitin with other compounds of glycerin and fatty acids. Collett in 1854 isolated Palmitic Acid, C₁₆H₃₂O₂, from olive oil; and Heintz and Krug (1857) further proved that Tripalmitin is the chief of the solid constituents of olive oil. They also met with an acid melting at 71°·4 C., which they regarded as Arachic Acid ([p. 187]). As to stearic acid, Heintz and Krug did not fully succeed in evidencing its presence in olive oil.
Lastly, Benecke discovered in olive oil a small quantity of Cholesterin, C₂₆H₄₄O. It may be removed by means of glacial acetic acid or alcohol, which dissolve but very little of the oil.
Commerce—Various sorts of olive oil are distinguished in the English market, as Florence, Gallipoli, Gioja, Spanish (Malaga and Seville), Sicily, Myteline, Corfu and Mogador.
Olive oil was imported into the United Kingdom in the year 1872 to the value of £1,193,064. Nearly half the quantity was shipped from Italy, one-fifth from Spain, and the remainder from other Mediterranean countries.
The average annual production in Italy is estimated at about 3 millions of hectolitres (66 million gallons), but the quantity exported does not reach half that amount.
The statistics of the French Government indicate the annual production of olive oil in France to be not more than 250,000 hectolitres, equivalent in value to 30 millions of francs (£1,200,000).[1553]
Uses—The uses of olive oil in medicine and its immense consumption in the warmer parts of Europe as an article of food, are too well known to require more than a passing allusion.
Adulteration—Olive Oil is the subject of various fraudulent admixtures with less costly oils, the means of detecting which has engaged much attention. Of the various methods by which chemists have endeavoured to ascertain the purity of olive oil, the following are the more noteworthy:—