Page:Foods and their adulteration; origin, manufacture, and composition of food products; description of common adulterations, food standards, and national food laws and regulations (IA foodstheiradulte02wile).pdf/428

 properties, and among those which are most marked in this particular may be mentioned cottonseed oil, sesamé oil, maize or corn oil, and rapeseed oil. Types of the oils which have the least drying properties and which are regarded as types of non-drying oils are olive oil and peanut oil. The castor oil group is distinguished partially from the other vegetable oils because it contains, or is likely to contain, more or less of a somewhat poisonous substance, namely, ricinolein, which is peculiar to castor oil and to which its purgative value as a medicine is due. The castor bean also contains a very poisonous nitrogenous base, ricin, very small quantities of which may be incorporated in the oil itself.

Melting Point and Solidifying Point.—The oils and fats differ greatly among themselves in the temperature at which they become solid or liquid. If a solid fat or oil is subjected to a gradual rise of temperature it does not pass at once or suddenly from a solid to a liquid state, but there is a gradual liquefying,—thus olein first becomes liquid and the stearin and palmitin become liquid at a higher degree of temperature. The same phenomenon in its inverse order occurs when a liquid fat is cooled until it solidifies. The moment at which the fats become semi-liquid, liquid, or semi-solid, therefore, is not to be determined with absolute precision, but only approximately, and that temperature is designated as the melting or solidifying point respectively. When the process is carefully conducted under standard conditions the different fats and oils have very definite melting or solidifying points, as determined in the manner described above, and these temperatures should be sufficient to make the melting and solidifying points valuable indications of the character or kind of oil.

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