OLIVE OIL, or, as it is usually called in the household, salad oil, is extracted by pressure from the fruit of the Olea Europoea. This tree is a native of Asia (where it is named zituna) and the northern parts of Africa. It is also grown in France, Italy, and Spain, and requires a calcareous, rocky soil for its cultivation. The fruit of the olive, from which the oil is obtained, is smooth and oval, like a plum, and when ripe, its skin is of a deep violet tint. Inside the fruit is soft and fleshy, and of a whitish colour, containing a hard, long, pointed nut, and is of a bitter disagreeable taste.
The olive tree commences to yield olives in its second year, and continues to improve until it is a centry old, after which it detiorates. The fruit is gathered in November; and great care should be taken that it is sufficiently ripe, otherwise the oil it yields will be bitter: but if the olives should be allowed to be too ripe, the oil will be thick, and containing much mucilage. The very best olive oil will be found to have fruity taste; thuis is due, not so much to the method by which the oil is obtained as to the quality of the olives used.
To obtain the oil; the fruit is usually collected in heaps and allowed to ferment, during which process they rise considerably in temperature, Great care has to be taken, while the olives are fermenting, that the temperature does not reach as high as 100° Fahrenheit, otherwise the fruit will yield an inferior oil, which will have a rancid taste. The use of thus causing the fruit to ferment is to render the olives softer, and enable them to yield their oil the more easily. When the olives have remained in heaps for the proper period, they are crushed into a paste beneath heavy rollers (which are placed so far apart as not to crush the kernels) and then placed in bags or sacks made of rushes, Eighteen or twenty of these bags are then piled up in a powerful press, and compressed as tightly as possible. The oil thus obtained is of the best quality, and is known as virgin oil. When the oil ceases to flow, the olives are removed from. the sacks, broken up in hot water, and again compressed, to extract a further quantity of oil, but which is now of an inferior quality, and which issues from the press mixed with water. The oil is now separated from the water by collecting it in cisterns, and allowing the water which is mixed with the mucilage of the olives to collect at the bottom of the vessel, while the oil floats on the surface, from which it is removed by skimming it off. The olive oil thus obtained is known in commerce as oil of the second quality.
Even after this has been done, the kernels of the olives contain a considerable quantity of oil. For the purpose of extracting this oil, the olives are removed from the sacks and mixed up with water, by which means the broken stones fall to the bottom of the vessel, and allow the kernels to float to the top. They are then collected by means of a sieve, dried at a gentle heat, rolled into a paste, put into bags, and again compressed, when they yield oil of the third quality.
When olive oil is first expressed, it is always found to contain a considerable quantity of mucus. This is got rid of by allowing the oil. to remain at rest for three weeks, at a temperature of about 68° Fahrenheit, when the oil deposits the mucus at the bottom of the vessel, in the form of a white albuminous substance of a fibrous consistence.
The best olive oil is imported into England in flasks, and is then known as salad oil, from being employed in the preparation of that dish. It is packed up in boxes, called half-chests. Sometimes—and this is especially the case with oil of an inferior quality—it is sent over in jars.
Most of the olive oil imported into England is sent from the port of Gallipoli, where it is stored in huge cisterns cut out of the solid rock. It is found that the quality of the oil is much improved by this treatment, and that when thus stored it has kept good for more than seven years. The best olive oil is said to be made in Provence, and its great excellence is supposed to be due to the care taken in cleaning the olives previous to using them.A considerable quantity of the oil imported into England, however, comes from Lucca and Florence. Olive oil is also sent over from Sicily, but is of an inferior quality, and has a resinous taste, which has been attributed to the nature of the soil on which the olives were grown.
The oil is a compound of two principles, named oleine and margarine. Oleine is a thin and oily liquid, of which seventy-two parts are contained in each hundred of oil. Margarine is a solid substance, which, although soluble in oleine at the ordinary temperature of the atmosphere, is deposited when the oil is exposed to cold. This principle constitutes twenty-eight per cent. of olive oil.
When olive oil is kept for a long period—and especially if exposed to the action of the atmosphere at a warm temperature—it becomes partially decomposed, sebacic acid is formed, and the oil acquires a disagreeable taste and odour, and becomes what is commonly known as rancid. And, at the same time, the oil deposits stearine, becomes becomes thick, and of a dark brown colour. This is especially the case if it has been adulterated with poppy oil.
Olive oil, when of good quality and free from adulteration should have scarcely any smell, and have a bland and pleasant taste. Owing, however, to its value, olive oil is much adulterated with poppy oil, cocoa-nut oil, and rape oil. The oil procnred by expression from the seeds of the Sesamum orientate, and which is commonly known as teel, sesame, or gingelly oil, is also employed for adulteration.
The most ready way ef detecting these adulterations and also of ascertaining the nature of the oils employed is to place a small quantity—from ten to twenty drops—on the middle of a slip of colourless glass, and drop on the centre of it a very small drop of sulphuric acid. If the olive oil is perfectly pure, it will assume a pale yellow appearance, which gradually changes to a yellowish green. If the olive oil has been adulterated with poppy oil, it becomes of a deeper yellow, which at length becomes almost opaque. When rape oil has been used, a ring of greenish blue forms round the acid, with some yellow-brown streaks in the centre, If whale oil (train oil) has been employed, the addition of the acid produces a movement in the centre, gradually extending over it, and at length assuming a red tint, with violet at the edges. Or if lard, or tallow oil, has been added, the liquid becomes of a brown appearance, which, if linseed oil had been used, would be almost black.
Another way to detect whether olive oil has been adulterated with other oils, is to mix it with a solutionof the nitrate of mercury. This solution is prepared by adding eleven parts of mercury to eight parts of nitric acid in a small flask or test tube. A gentle heat should applied until the metal is dissolved, and then fourteen parts of water are to be added. To use this solution, a small stoppered phial must be half filled with the suspected oil; then half that quantity of the solution of nitrate of mercury is to be added, the stopper placed it the mouth of the phial, and the whole well shaken together for five minutes. The vessel is then to be allowed to remain at rest for a short time. again well shaken and afterwards placed on one side for some hours. At the end of that time, if the oil is perfectly pure, it will be found. converted into a solid mass; but if it contains one part in twenty of other oils, it will remain fluid. The solid mass thus produced by the action of the nitrate of mercury is due to the production of a solid fat named elaidine.
Cassells Household Guide, c.1880