230 FISHERY INDUSTRIAL PRODUCTS 



A process useful for the removal of coarse, suspended matter from fish 

 oil involves simply the gravity settling of such matter. Maintaining the 

 oil in a fully liquid state for several days in tanks is often sufficient. A 

 clear oil may be drawn off the top of the tanks leaving the settled impuri- 

 ties on the bottom. 



In many instances, colloidal and small suspended matter require more 

 extensive treatment of the oil. This may involve blowing the oil with wet 

 steam at 100°C for some time and then allowing the oil to settle. Salt may 

 be added at a 1 to 2 per cent level to assist the handling of possible trouble- 

 some emulsions. Following the settling, the lower layers of condensed 

 water and oil-water emulsion are piped off through centrifuges to separate 

 the oil from the water. Such centrifugal separation leads to lower oil losses 

 and a clearer, cleaner oil product. 



Alkali-refining is the process known to be effective for the removal of 

 free fatty acids. Alkaline treatment of the oil is also effective in removing 

 various color-producing materials. In such a process, a solution of caustic 

 soda is gradually mixed with warm fish oil and then heated to about 

 100°C. A slight excess of alkali above that necessary to neutralize all of 

 the free fatty acids is used. The amount of excess depends on the color 

 of the original raw oil. Concentrated solutions of soda ash (sodium car- 

 bonate) may be used in place of caustic soda. The oil, after alkaline treat- 

 ment, is separated from fatty-acid soaps by centrifugal separators. Wash- 

 ing and a second centrifugation improves the yield and quality of the 

 finished oil. 



It is also possible to remove free fatty acids by the process of molecular 

 distillation^^. Owing to the high molecular weight of the glycerides, ordi- 

 nary distillation methods of separation and refinement are not practical. 

 Molecular distillation under high vacuum makes it possible to remove 

 vitamins, steroids, and hydrocarbons (e.g., squalene), as well as fatty 

 acids, from the raw oils leaving glycerides as a residue. This method has 

 been used extensively for the recovery of vitamin A from fish liver oils on 

 an industrial scale. 



The natural coloring compounds found in fish oils vary from species to 

 species. Some of the red oils are known to contain carotinoid 

 pigments, while the greenish oils are believed to contain chlorophylls. To 

 decolorize fish oils, adsorptive bleaching techniques are most common. 

 Chemical methods of bleaching have been less successful with fish oils 

 than with other natural fats and oils, probably due to the greater degree 

 of unsaturation common to commercial fish oils. In common adsorptive 

 bleaching methods fish oil is mixed with a clay-type adsorbent, such as 

 fuller's earth or a chemically activated fuller's earth. Other adsorbents 

 used include activated charcoals and diatomaceous earths. The amounts 

 of adsorbent used vary from 3 to 5 per cent and more for fish oils. The 



