FATTY OILS AND WAXES
Another type of oil that occurs in plants is the fatty oil. The fatty oils are also called fixed oils because, unlike the essential oils, they do not evaporate or become volatile, and they cannot be distilled without being decomposed. Chemically these vegetable fatty oils are close to animal fats. They consist of glycerin in combination with a fatty acid. The so-called oils are liquid at ordinary temperatures and usually contain oleic acid. The fats, on the other hand, are solid at ordinary temperatures and contain stearic or palmitic acid. The fatty oils are insoluble in water, but soluble in various organic solvents. When fats break down, they yield the fatty acids and glycerin, of which they are composed, and usually develop a rancid odor and taste. When a fat is boiled with an alkali, it decomposes and the fatty acid unites with the alkali to form soap. If potash or lye is used, a soft soap is obtained; if soda is used, a hard soap is the result.
Fatty oils are produced in many families of plants, both tropical and temperate. They are stored up, often in large amounts, in seeds (Fig. 98) and, to a less extent, in fruits, tubers, stems, and other plant organs; they are often associated with proteins. This type of reserve food material is available as a source of energy for the processes involved in the germination of the seed. The fatty oils are bland and lack the strong taste and odor and the antiseptic qualities of the essential oils. Consequently they are available as food for man. These edible oils contain both solid and liquid fats and form indispensable articles of human food. The demand for edible oils has so increased in recent years that various processes have been developed whereby the nonedible oils have been rendered available. This is usually done by hydrogenation, the adding of hydrogen.
The method of extraction of the oils varies in different cases. Usually the seed coats have to be removed, and then the material is reduced to a fine meal. The oils are removed by solvents or by subjecting the meal to screw or hydraulic pressure. This latter method is used primarily for the edible oils. The residue is rich in proteins and is valuable as a fertilizer and as a cattle feed. The pressure causes the cell walls to break and the fats escape. The extracted oils are filtered and may be