Chapter VII: Salad Oils

Salad Oils—Oils used for Culinary and Confectionery Purposes— Chocolate Fats.

=Olive Oil.=—No oil is so extensively used in the preparation of salad dressings as olive oil, and the more closely its characteristics can be imitated by other salad oils the greater is the demand for the latter.

There are several species of the olive tree, some of which are successfully cultivated in hot climates, but the oil used in Europe is mainly derived from different varieties of _Olea europæa_. The fruit produced by these varies considerably in size and in colour, but the oils that they yield show a close similarity in chemical and physical characteristics, though they differ from one another in flavour and in colour.

Apart from the influence of soil and climate and of the variety of the olive tree, the flavour of the oil also depends upon the stage of growth at which the fruit is gathered and upon the method of separating the oil from the pulp.

Fruit that is picked before it is quite ripe yields an oil with a somewhat bitter flavour, and therefore in preparing the finest grade of oil the olives are specially selected and pressed by hand between cloths. The resulting oil is washed with water to remove impurities, then decanted from the aqueous layer, and sold under the name of _virgin oil_.

The bulk of fine commercial olive oil, however, is separated by expression, the ripe olives being superficially crushed between mill-stones, and then expressed at a low pressure. The residue left in the press is broken up, mixed with hot water, and the liquid, consisting of the mixed oil and water, expelled by stronger pressure, and allowed to stand for the oil to separate. The flavour and colour of this oil is inferior to that of the cold-drawn product.

Subsequent grinding of this second residue, followed by another expression with hot water, yields an additional quantity of oil, while the amount still present is often extracted by means of carbon bisulphide or other volatile solvent. The oils finally obtained are generally utilised as lubricants or in the manufacture of soap.

In most districts care is taken not to crush the olive stones until after the finest oil has been expressed, although according to Klein (_Zeit. angew. Chem._, 1898, 847) there is no disadvantage in expressing the mixed kernel and fruit oil, provided the fruit is freshly picked. The general opinion of the trade, however, is that the flavour of the pulp oil is superior to that derived from the mixed pulp and kernels.

Oils that have been extracted by means of carbon bisulphide are known as “sulphocarbon” oils, while those which have been separated by means of petroleum spirit are termed “pyrene” oils.

The oils derived from olives that have been left for some time before expression are of an inferior kind, and are termed _huiles d’enfer_, _huiles tournantes_, etc. They contain a large amount of free fatty acids, and have a sharp, unpleasant flavour which renders them unsuitable for food.

The finest qualities of oil are derived from the districts round Lucca and Leghorn, and from Provence. Large quantities of excellent oil are also exported from Spain, Portugal, Algiers, and Tunis, while California has now become an important oil-producing country.

The flavour is the chief criterion of the quality of olive oil, and a trained palate is able to detect slight differences which could not be recognised by any chemical tests.

The so-called _virgin oil_ is pale yellowish-green in colour, and has but little odour, whereas the lower grades of oil obtained from the _marc_ left in the press, as described above, vary in colour from greenish-yellow to light brown, and have a perceptible odour and a somewhat bitter flavour.

Even the purest olive oil will yield a deposit of “stearin” when exposed to a low temperature, but the amount varies with the kind of olive which yielded the oil. Thus it is particularly abundant in the case of Tunisian and Algerian oils, and it is therefore usual to remove a portion of this solid fat before putting the oil upon the market, under the name of “demargarinated” oil. The amount of “stearin” thus removed depends upon the temperature to which the oil is chilled before being pressed in a filter-press. Oils thus treated are sometimes described as “winter oils” (see Chapter III.).

Olive oil consists chiefly of olein, with smaller quantities of the glycerides of linolic acid and of various solid fatty acids (mainly palmitic acid) which form the insoluble deposit—the so-called “stearin.”

It belongs to the class of non-drying oils, and hence does not form a skin on its surface when exposed to the air, and when spread in a thin film upon glass takes many days to dry up. In this respect it differs from the semi-drying oils—cotton-seed, sesame, and maize oils—which are frequently sold as salad oils, sometimes under descriptions that suggest that they are olive oil.

The chief analytical characteristics of a typical olive oil are as follows:—

+--------+----------------+--------+--------+---------------+
| | | | | Melting-point |
|Sp. gr. | Saponification | Iodine | Hehner | of |
| | Value. | Value. | Value. | Fatty Acids. |
+--------+----------------+--------+--------+---------------+
| 0·918 | 191 | 82 | 95 | 24° C. |
+--------+----------------+--------+--------+---------------+

Olive oil is very frequently adulterated, the principal substances used for the purpose being cotton-seed, sesame, maize, and lard oils. The vegetable oils may be detected by their general analytical values, and the two first by the characteristic colour reaction (see Chapter VIII. pp. 105 and 107), whilst lard oil may be detected by the cholesterol test (p. 101).

=Arachis Oil.=—The chief centre of the arachis oil industry is the South of France, and at Marseilles in particular enormous quantities of the nuts are expressed.

The oil obtained by the first expression in the cold is a pale yellow fluid which is extensively used as a salad oil. A large proportion of the oil obtained on subsequent expression is sold as a lower grade of salad oil, while the remainder, and also the oil obtained by hot pressure, is manufactured into soap. The total yield of oil is from 45 to 50 per cent.

As a rule, the salad oils (which are sometimes sold as “nut salad oil”) have a distinct odour and flavour of the nut, but in the very finest grades, which also have but little colour, this flavour is hardly noticeable in the case of the freshly prepared product. Arachis oil becomes turbid at a much higher temperature and throws down a more abundant deposit of “stearin” than olive oil, and this has greatly interfered with its popularity as a salad oil in this country. The solidification point is usually about +2 to +3° C. Chemically it consists of the glycerides of hypogæic, oleic, linolic, palmitic, stearic, arachidic, and lignoceric acids. The most characteristic constituents are the two last fatty acids (amounting to about 5 per cent.). They are only slightly soluble in cold alcohol, and on this property are based several methods of separating them and of calculating from the result the proportion of arachis oil in, for example, an adulterated olive oil.

The physical and chemical constants of arachis oil vary considerably, as is shown in the following figures recorded by Sadtler and by Crossley and Le Sueur for oils of different origin:—

LEGEND:
(A) = Saponification Value.
(B) = Reichert-Meissl Value.
+----------+---------+-------+--------+-------+-----------+--------+
| | | | | | |Melting |
| | Sp. gr. | | | | Free Acid |point of|
| Oil. | at | (A) | Iodine | (B) | (as | Fatty |
| | 15° C. | | Value. | |Oleic Acid)| Acids, |
| | | | | | per cent. | °C. |
+----------+---------+-------+--------+-------+-----------+--------+
|Virginian | 0·917 | 192·5 | 91·7 | 0·48 | 0·55 | 29 |
|Spanish | 0·9175 | 190·7 | 94·2 | 1·60 | 0·79 | 34 |
|African | 0·911 | 194·0 | 85·6 | | 0·62 | 30 |
|Indian | 0·9223 | 190·1 | 98·5 | Nil. | 1·45 | |
+----------+---------+-------+--------+-------+-----------+--------+

The highest figures recorded for the iodine value of arachis oil are 101-105 (Oliveri), whereas the lowest values fall well within the limits of those of olive oil.

Arachis oil is sometimes adulterated with cheaper oils, and notably with sesame and cotton-seed oils. The determination of the analytical values and the characteristic colour reactions will probably afford information in such cases, while at the same time there would be a decrease in the amount of arachidic acid that could be separated.

The methods of separating and determining this acid are described in