SEED OILS 
Vegetable oils are found naturally in the seeds of plants, and 
in many species the oil accumulates in considerable quantities. 
Many of these seed oils are edible, while others are useful for 
their medicinal properties or for the manufacture of paints, 
soaps, candles, or other practical purposes. In general, to ob- 
tain these oils, the seeds are first shelled. Although the oil may 
be obtained usually from the shelled seeds by extraction with 
organic solvents such as ether, the more general method is to 
subject the dried, shelled seeds to pressure. This process expels 
the oil, leaving a dried, crushed meal known as oil cake. The 
cake may be used for various purposes, depending upon its com- 
position. If it does not contain injurious substances, it may be 
utilized for cattle food. Sometimes the oil cake is also em- 
ployed as fertilizer or fuel. The expelled oil is filtered and, if 
necessary, subjected to further methods of refinement. 
Edible oils usually contain both solid and liquid fats. Since 
fat is generally recognized as an indispensable constituent of 
human food, these edible oils are consequently substances of con- 
siderable importance. The fatty oils are sometimes called fixed 
oils, because when a drop of one of these oils is placed on wood 
it forms a rather permanent spot which does not evaporate 
readily on exposure to the air. 
Fats (glycerides) consist essentially of glycerol (glycerin) 
combined with certain fatty acids such as oleic, palmitic, and 
stearic. The fats are usually insoluble in water, but dissolve 
readily in organic solvents. When boiled with an alkali solu- 
tion they are decomposed (saponified) and converted into the 
alkali salts (soaps) of the fatty acids present, and glycerol. 
When fats, or edible oils containing fats, are exposed to light 
and air for a considerable length of time, they gradually de- 
compose, forming free acids and other products, which have 
an unpleasant taste and odor. Oil which has been decomposed 
in this manner is said to be rancid. When decomposition has 
begun, the presence of micro-organisms appears to hasten 
these chemical changes. If the dried seeds are prepared prop- 
erly and the oil obtained from them properly preserved, this 
decomposition can be hindered greatly and in some cases prac- 
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