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Oils: Animal, Vegetable and Fish
Various organic oils and fats are essential constituents of all forms of plant and animal life. Every species of plant and animal produces some quantity of fat or oil during its life cycle. However, only a relatively few plants and animals produce fats and oils in sufficient quantity to become articles of commerce. The most important oils, therefore, are those that may be obtained in commercial quantities from readily available source raw materials. Of the many animal and vegetable sources, only a relatively few account for 90 per cent of the total quantity produced in all countries of the world.
Generally, the terms "fat" and "oil" are used interchangeably. From a technical viewpoint, the term "fat" refers to those fatty materials that are solid at room temperatures, whereas the term "oil" refers to products that are liquid under those conditions. However, these designation's are somewhat anomalous, since some fatty materials, such as coconut oil, are quite solid under ambient temperatures.
Composition and Structure
In contrast to petroleum or other oils of mineral origin, all animal, vegetable, and marine oils are triglycerides or esters of fatty acids—that is, in their molecular structure they are chemical combinations of one molecule of glycerol with three molecules of fatty acids. When saponified with caustic alkalies, such as caustic soda or potash, they form soaps, which are salts of fatty acids, and glycerol, which is a trihydric alcohol. The fatty-acid component may be one or more of the common fatty acids. See fat.
The fatty-acid portion of the triglyceride structure represents over 92 per cent of the total weight, the balance consisting of the glyceryl radical and minor percentages of nonglyceride constituents, such as cholesterol, phytosterol, phosphatides (lecithin), vitamins, and moisture. The total amount of nonglyceride components present is usually not over 3 per cent of the oil base product.
During the process of recovering the fats and oils, some slight hydrolysis, or saponification, invariably occurs, resulting in the formation of small quantities of free fatty acids. Thus most crude oils contain from 1 per cent to 5 per cent of such free fatty acids, and usually these are removed in refining operations, especially if the oil is destined to be consumed as food. The free fatty-acid content, therefore, bears a rough inverse relationship to the quality of the oil as well as to the care that has been taken in its preparation.
Various systems of classifying fats and oils have been used, the most common, perhaps, being their division into the categories of animal, vegetable, and marine oils. Other systems of classification include divisions into (1) edible and inedible oils and (2) nondrying, semidrying, and drying oils. The latter system is based on the rate of drying of thin films in air. The so-called iodine number of a fat or oil is a measure of the drying rate. The nondrying group includes such oils as coconut, butter, taP low, palm oil, and lard, having iodine numbers ranging from 9 to 65. In the semidrying group are olive, peanut, rapeseed, cottonseed, corn, soybean, sunflower, and sesame oils, with iodine numbers between 85 and 130. The third group, representing the drying oils, comprise the fish oils, tung, linseed, perilla, and oiticica, whose iodine numbers vary between 150 and 200.