This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
The olive is a small tree widely distributed by cultivation, especially in the countries bordering on the Mediterranean, and has been introduced into America (California), where it promises so well and is increasing so rapidly that that country will probably soon be independent of Europe for her supply of olive oil.
The tree produces a small, ovoid, drupaceous fruit about 2 or 3 cm. in length, which, whilst unripe, is green in colour and in this state is pickled and used as a table relish. As the fruits ripen the cells of the mesocarp become filled with a fixed oil, the colour at the same time changing from green to purple. They are collected in the winter and spring (December to April), the ripe sorted out, crushed, and subjected to a moderate pressure. The crude oil that flows from the press is run into tubs and mixed with water; the latter removes colouring matter and other impurities from the oil, and the oil, after it has completely separated from the water by standing, is skimmed off and filtered; in this way the finest quality of olive oil (virgin oil) is obtained. The marc is again ground, mixed with hot water, and again pressed, this time more strongly, and thus a second quality of oil is obtained. The residue still contains a little oil which may be obtained by a third pressing or by extraction with solvents; such oil is usually used for soap-making, etc. Sometimes all the fruits are thrown into heaps and allowed to ferment; on pressing, the whole of the oil is obtained, but it is of inferior quality, and is used principally for technical purposes.
For medicinal use only the first pressings, obtained without heat, should be employed. Such oil has a pale yellow or greenish yellow colour, a slight characteristic odour, and a bland taste without rancidity. Its specific gravity varies from 0.915 to 0.918. It is liquid at ordinary temperatures, but when cooled to 10° it often assumes a pasty consistence, from deposition of solid fats, and at 0° it becomes a nearly solid, granular mass.
Olive oil consists chiefly of olein and a little palmitin, together with linolein and traces of arachin, the palmitin and arachin separating out in the solid form when the oil is cooled.
By saponification these compounds yield respectively oleic, palmitic, linolic, and arachic acids together with glycerin.
Olive oil has nutritive and laxative properties. Applied externally it is emollient and soothing.
Valuable indications of purity are to be found in the specific gravity (0.915 to 0.918) and in the iodine value (79 to 87). The free acid present should not exceed 1 per cent., calculated as oleic acid, lower qualities may contain up to 5 per cent. of acid, and oils for technical purposes as much as 30 per cent. Cotton-seed oil, a frequent adulterant, is best detected by Halphen's (Bevan's) test which consists in warming 2 c.c. of the oil mixed with 1 c.c. of amylic alcohol and 1 c.c. of a 1 per cent. solution of sulphur in carbon disulphide for ten minutes in a water-bath when no red colour should be developed.
Sesame oil [from the seeds of Sesamum indicum, Linne (N.O. Pedalineoe), India] is tested for by mixing 2 c.c. with 1 c.c. of hydrochloric acid containing 1 per cent. of sucrose, shaking for half an hour, adding 3 c.c. of water, again shaking and setting aside, when the aqueous liquid should not become pink.
The oil expressed from the seeds of the tea plant (tea seed oil) closely resembles olive oil and is said to be used for adulterating it.
For details of other tests for ascertaining the purity of olive oil, reference should be made to works on analytical chemistry, such as Allen's ' Commercial Organic Analysis,' or Lewkowitsch, 'Chemical Technology and Analysis of Oil, Fats and Waxes.'