Physical Characteristics.
—The difference in the physical characteristics of vegetable fats and oils is even greater than in their chemical composition. Unfortunately for the chemist, the vegetable fats and oils naturally have about the same color or at least very slight variations therefrom, namely, an amber tint, so that, as a rule, it is impossible to discriminate between these oils by their mere color alone. The edible oils also have very much the same taste, so that this physical property is not of any very great diagnostic value. Some of the more important physical properties by which the oils are distinguished are the following:
Refractive Index.
—The well-known phenomenon which is shown by water of bending sharply a ray of light falling upon it in a direction oblique to its surface is known as refraction, and the degree of deflection of the ray is a measure of the refractive index. This is easily illustrated by putting a straight stick or rod into still water at an angle to its surface. The stick or rod will appear to be broken or bent at the surface. Oils have a higher faculty of deflecting the ray of light than water. For instance, if in round numbers the refractive index of water is represented by 1.33, the refractive index of oil may be represented by 1.44. The oils differ greatly among themselves in the magnitude of the refractive index, but these indexes are all approximately of the magnitude last mentioned. Hence a determination of the refractive index is a valuable means of helping to discriminate between oils of different kinds.
Reichert-Meissl Number.
—Attention was called above to the fact that in addition to three special forms of fatty acids there were many others present in oils in small quantities. Among these are found acids which are volatile in a current of steam, which is not the case with the oleic, palmitic, and stearic acids. Among the most important of the volatile acids is the one which exists in large quantities in butter, namely butyric acid. The quantity of volatile acid is determined arbitrarily by the amount of a standard alkali solution which will be neutralized by the volatile acid from five grams of fat. In the case of butter, for instance, it may be said that in round numbers it requires 28 cubic centimeters of standard alkali to neutralize the volatile acid produced according to the above method of procedure. In cottonseed oil the amount of standard solution required to neutralize the volatile acid obtained in the same way is extremely minute, amounting to less than one-half cubic centimeter.
I have given above a brief description of some of the physical and chemical characteristics of oils and fats in order that the reader not specially trained in chemistry may understand thoroughly the references made to these properties in the general description given of vegetable fats and oils. It is not necessary to be a skilled chemist in order to have a general knowledge of some of the points which are of most interest in this respect.
Saponification Value.
—As is well known, one of the most common uses of oils and fats is in soap making. Soap consists of the products of chemical reactions by means of which the glycerine contained in an oil or fat are set free and a mineral or other base substituted therefor. For instance, lye consists of the hydrate or carbonate of potash and soda. When an oil is heated with a lye the fatty acid leaves the glycerine in the oil and combines with the potash or soda of the lye. The number of milligrams of potash or soda required to saponify one gram of fat or oil is called its saponification value. For instance, in the case of cottonseed oil it requires, in round numbers, 190 milligrams of potash or hydrate of potash (KOH) to replace the glycerine in one gram of oil. The quantity of potash required for an edible oil to make a complete saponification varies, and hence this number becomes one of the means of distinguishing between them.