follows that oils and fats are the most valuable constituents of food in respect of the production of heat and energy.
Crystalline Characteristics.—The forms of crystals which the fats assume on solidifying are valuable indicators of the nature of the oil. While these crystal forms are not in all cases distinct, yet they are influenced to a greater or less extent by the nature of the oil itself. Thus the presence of any particular oil may very often be ascertained by the examination of the crystals produced by lowering the temperature very slowly or by dissolving the oil in a volatile solvent and gradually evaporating the solvent. Tests of even greater delicacy may be obtained by first saponifying the fat or oil, separating the fatty acid, and subjecting it to crystallization.
Distribution of Oils in Plants.—In nearly all cases the part of the plant which contains the most oil is the seeds. In fact all of the vegetable oils which are used for edible purposes are extracted from the seed of the plant. In the case of olives the meaty portion around the seed yields the edible oil of highest value, but in all other cases of edible oils they are derived from the seeds themselves. It is a mistake to suppose that the seeds are the only parts of the plant that contain oil. It is found in all parts of vegetable substances, but is usually concentrated in the seed. It is rather an interesting fact to know that in the seeds of plants both the protein and fats or oils are found, as a rule, in a highly concentrated state, while the carbohydrates are not found chiefly in the seed itself, that is the germ, but distributed in the fleshy envelope surrounding it or in roots or tubers.
The oils and fats are almost all soluble in ether and petroleum ether, though there are some exceptions to this, as in the case of castor oil, which is also insoluble in petroleum ether or gasoline. On the contrary, oils and fats, as a rule, are not soluble in alcohol, but the fatty acids derived from them are. Castor oil is also an exception to this rule, since it is quite soluble in pure alcohol.
Drying and Non-drying Vegetable Oils.—It might be supposed that if one vegetable oil be edible they all would be. This would probably be the case if vegetable oils were all composed almost exclusively of the three classes of glycerids, which have been mentioned above, but such is not the case. There are other fatty acids in combination with the glycerids which exist in vegetable oils, and chief among these may be mentioned linoleic acid, which exists in considerable quantities in the oil of flax seed, and gives to it its valuable property of a drying oil which makes it so useful in the manufacture of paints. Whenever vegetable oils and fats contain any especial quantity of linoleic acid, or any other fatty acid which has drying properties, they are rendered more or less unfit for human consumption. The number of drying oils is very great, but the most important are linseed oil, hempseed oil, and poppyseed oil. Other vegetable oils have, to a certain degree, drying
