Page 6 - ALLEN'S COMMERCIAL ORGANIC ANALYSIS A TREATISE ON THE PROPERTIES, MODES OF ASSAYING... VOL II

EXTRACTION AND PURIFICATION. 3
varnish.' Such oils (e. g., linseed, walnut, hempseed, and poppy-seed
oils) are called drying oils.
r2. The non-drying oils behave in a different manner on exposure
to air. They gradually become rancid; that is, lose their colour (and to
a certain extent their fluidity), and acquire an acrid, disagreeable taste,
• and acid reaction to litmus-paper. This alteration is primarily an
oxidation process brought about by the action of air and light, and is
accompanied by the liberation of free fatty acids and other bodies. It
may be accelerated by the presence of foreign matters, such as the
cellular substance of the animal or plant from which the oil was ex-
tracted. These substances furnish nourishment for bacteria, which
probably cause further changes when once the decomposition process
has begun. By agitating such rancid oil with hot water, and subse-
quently treating it with a cold and dilute solution of sodium carbonate,
the products of decomposition may often be removed and the fat re-
stored to its original state.

EXTRACTION AND PURIFICATION OF FIXED OILS AND
FATS.

The method of extraction and subsequent treatment have con-
siderable influence upon the analytical characteristics of the product.
For the extraction of oils and fats from animal tissues it is often sufficient
to allow the material (e. g., cod liver) to become somewhat putrid,
when some of the oil drains from it, or may be obtained by slight
pressure. A further quantity can beextracted by warming or boiling
the tissue with water, as is done with blubber. In the case of lard and
tallow, it is merely necessary to heat the substance alone, and strain
the melted fat away from the membranous matter. From compact
tissue, such as bone, the whole of the fat can be extracted by a solvent
only.
The extraction of the fat or oil from vegetable tissue may be effected
by boiling the crushed substance with water or by subjecting it to
powerful pressure, either at the ordinary temperature or between plates
heated to slightly above the m. p. of the fat. The product obtained in
the last manner will usually contain more "stearin" or solid fat than
the "cold-drawn" oil. In either case a certain quantity of the fat is
teer certain conditions, as when cotton-waste, shoddy, or hemp is moistened with
oil and exposed to the air, the oxidation of the oil becomes so energetic as to lead to con-
siderable elevation of temperature, and even actual inflammation (see p. 38).