—The relative weight of a given volume of oil compared with the weight of the same volume of water at the same temperature or at some standard temperature is known as its specific gravity. The oils and fats are universally lighter than water, and in the comparison the unit weight of water is assumed to be unity or 100 or 1000—usually unity or 1000. If the relative weight of water is unity, then the relative weight or specific gravity of oil is expressed as a decimal fraction. For instance, if water is taken as unity the specific gravity of oil equals .912; if the relative weight of water is assumed to be one thousand then the specific gravity expressed above is 912. Unless it is stated otherwise, in all references to specific gravity of these oils it is assumed that the comparison is between the unit weight of water and oil at the same temperature. This is the most convenient form for comparison for general use, though for strictly scientific purposes it is customary to refer all specific gravity numbers to water at the temperature of its maximum density, namely 4 degrees C. (39 degrees F.). At this temperature a given weight of water has its smallest volume, in other words its greatest density. When water is raised to a temperature above that mentioned, it expands and its volume becomes larger. When it is cooled to a temperature below four degrees C., its volume also expands.
The variations in the specific gravity of the common oils is not very great, and therefore the specific gravity is not the most valuable indication in discriminating between these oils.
Edible Vegetable Oils.
While there is very little chemical difference between the fats of animals and the oils of plants, the difference is sufficiently distinguished to secure a proper degree of identification and classification. Both classes of bodies are composed of the fatty acids combined with glycerine. The three fatty acids which are most important from the edible point of view and also from the chemical are oleic, stearic, and palmitic. When these acids are united with glycerine as the basic element, they form three classes of oils or fats to which the names olein, stearin, and palmitin are respectively given. A distinction may also be made between a fat and an oil by observing its physical consistence at ordinary room temperature of approximately from 70 to 80 degrees F. It is usual to speak of the bodies which are liquid at such temperature as oils, while those that are solid under like conditions are known as fats. A compound of this description does not pass suddenly from one state to another. In the case of a fat, for instance, which is solid at ordinary temperature, it passes by gradual stages from that condition to a slowly softening mass and then to a complete liquid as the temperature is raised. On the other hand, an oil passes gradually through the same stages to the condition of a solid body as the temperature is lowered. Of the different constituents the olein has the lowest melting point, pure olein being still liquid at quite a low temperature, approaching even the freezing point of water. Stearin and palmitin on the contrary, if in a pure state, are solid at a temperature even above that of the room and above that of blood heat. In the mixture of these bodies it is evident that a complicated structure must be present which is composed of different bodies of varying melting points and passing through all different degrees of temperature from a solid to a liquid state or vice versâ. It is evident that an oil has a larger proportion of olein in its composition and a fat a larger proportion of stearin and palmitin.
Animal fats are composed chiefly of olein and stearin, while strictly vegetable oils are principally olein, and palm oil is composed chiefly of stearin and palmitin.
In butter fat there is introduced an important additional compound of a fatty acid with glycerine, namely butyrin, which is made up of a union of glycerine with butyric acid. Butter also contains other components or glycerids, but in small quantities. Oleic, stearic, and palmitic acids are insoluble in water and not volatile at the boiling point of water. Butyric acid is soluble in water and is volatile at the boiling point of water. The first kinds of acid are therefore called “fixed” and the second “volatile.”
The edible vegetable oils like the animal fats are highly nutritious in the sense that they afford to a greater degree than any other kind of food product the elements necessary to the production of heat and energy. The average number of calories to one gram of edible oil is in round numbers 9,300. When this number is compared with the average number of calories in one gram of sugar or starch, namely 4,000, it is seen that fats and oils are two and one-fourth times as valuable as sugar in the production of heat and energy. Since the greater part of the food consumed by an animal is utilized in the production of heat and energy, it is seen that the fats and oils must be classed as the most concentrated and in that sense the most valuable human foods.
The use of edible vegetable oils is also advisable for hygienic purposes. They are readily assimilated and digested, and they produce a physical effect upon the process of digestion which is a matter of importance. The free use of edible vegetable oils is to be recommended in cases of constipation or where there are mechanical difficulties in the digestive process. In these cases it is consumed in larger quantities than would ordinarily be the case.
Use of Edible Oils.
—The edible oils are used most extensively on the table as the base of salad-dressing. Many succulent vegetables, as has already been stated, are eaten very commonly with condimental substances such as vinegar, salt, spices, etc., and as a vehicle for these condimental substances there is nothing superior or even equal to the edible vegetable oils. Vinegar, itself, owes its active principle, namely, its acid, to a member of the fatty acid series, so that the mixture of vinegar with oil is not a bringing together of two wholly different substances but of two substances belonging to the same general family. Vinegar itself has no value as a food, but is useful solely for condimental purposes. On the other hand the edible oil is not only condimental, increasing the pleasant taste of the compound, but also has a high food value. Edible oils may also be used in the place of lard and other animal fats in the preparation of bread and pastry, serving the purpose of shortening. Edible oils are also highly useful as a vehicle for frying foods, such as oysters, croquettes, doughnuts, etc.