This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
Cod-liver oil is the oil extracted from the fresh liver of the cod, Gadus morrhua, Linne (Phylum Chordata, Sub-phylum Craniata, Class Pisces, Order Teleostei).
The cod inhabits the North Atlantic Ocean in great numbers, leaving the deeper seas and approaching the coasts, chiefly of Norway and Newfoundland, towards spawning time - that is, from January to April. During that time immense quantities of the fish are taken both by nets and lines. The livers are cut out whilst the fish is quite fresh, the healthy ones selected, and, after the removal of the gall bladders, subjected to a gentle heat, usually about 70° (according to the British Pharmacopoeia not exceeding 85°). The oil that separates is drawn off and exposed to a low temperature (about - 5°), at which a considerable quantity of solid fat separates. This is removed by filtration and pressure, and the oil thus purified forms the finest medicinal oil.
Inferior qualities of the oil are obtained from the residual livers (to which are added the unhealthy and injured livers previously rejected), by subjecting them to a higher temperature. Much oil, also of inferior quality but suitable for many technical purposes, is obtained by keeping the livers until they are partially decomposed, skimming off the oil that has separated and heating and pressing the residue; such oil is usually of a brownish colour.
After the livers have been removed and the offal separated the fish are dried, and form an important article of commerce. The chief seats of the cod fishery are the shallow banks off Newfoundland, and the Loffoden Islands near the north-west coast of Norway.
Cod-liver oil should be of a pale yellow colour, and have a slight, fishy, but not rancid odour. Its specific gravity should vary from 0.920 to 0.930; it is readily soluble in ether and chloroform, but sparingly in alcohol.
Cod-liver oil consists chiefly of jecolein and therapin associated with palmitin and probably other fat-acids combined with glyceryl. Traces of biliary acids, alkaloids (morrhuine, aselline), cholesterol, iodine, etc, are also present.
Jecolein and therapin consist of jecoleic and therapic acids combined with glyceryl. Jecoleic acid is a very unstable acid belonging to the oleic acid series.
Cod-liver oil is employed as a nutritive and is a food rather than a drug.
Cod-liver oil is liable to adulteration with other fish-liver oils and with seal oil, the detection of which is exceedingly difficult. The following characters have been suggested for a pure cod-liver oil for medicinal use: -
Slightly fishy, not rancid
Bland, not rancid
0.925 to 0.931
Saponification value .
179 to 198
Free fat-acid calculated as oleic
Not over 1.5 per cent.
Melting-point of fat-acids .
23° to 26°
Iodine value .
154° to 170°
Refractive index (20°)
Not under 1.4790
If three drops of nitric acid (sp. gr. 1.4) are added to 15 drops of cod-liver oil and the mixture vigorously stirred, it should develop a bright, rose-red colour.
Whale oil (Baloena sp.), seal oil (Phoca sp.), dolphin oil (Delphinus sp.), and shark oil (Carcharias sp.) are used as illuminants, lubricants, for leather dressing and soap making. Large quantities of whale oil are converted by hydrogenation into bland edible fats of varying degrees of hardness. (Compare Allen's 'Commercial Organic Analysis,' Vol. II).