FISH-LIVER OILS 505 



is added. The digestion is then carried out at a temperature of 110 to 120° F (43— 

 49° C) for 36 to 48 hours, depending upon the completeness of digestion of the 

 Hver material. At the end of this period a sufficient quantity of saturated sodium 

 carbonate solution to adjust the pH to approximately 9.0 is added. The tempera- 

 ture is raised to 175° F (80° C) and digestion is continued for exactly one hoxir. 



It is suggested that this method is superior in the event that exceptionally fresh 

 livers are being handled. In this case it is often difficult to separate the oil from 

 the other components of the emulsion. The theory advanced for this is that the 

 enzyme action and the addition of a mineral acid stops the action of fat- 

 splitting enzymes usually present in the livers. These are responsible, to a con- 

 siderable extent, for the formation of free fatty acids. 



Extraction of Livers of Low-Oil Content and High- Vitamin A Potency. This 

 process is useful in handling such livers as halibut, tuna, and sablefish. The 

 minced or disintegrated livers are covered with a low-potency oil (pilchard or 

 grayfish are extensively used for this) and the temperature is raised to 212° F 

 (100° C). Heating is continued for periods ranging from 30 to 60 minutes, with 

 constant mechanical agitation. At the end of the digestion the enriched oil is 

 separated mechanically or drawn oflF after the solids have settled. Repeated 

 extractions improve the efficiency of the process and recover the vitamin A almost 

 completely. This method can be used at Hver receiving points. The cooked mix- 

 ture can be transferred to the oil refining plants, where the process is completed. 



Solvent Extraction Methods 



The use of solvents for the extraction of vitamin A oils from fish livers has 

 been largely replaced by the alkali-digestion methods. This is chiefly because 

 of the greater care needed in the process and the difficulty in obtaining solvents 

 free from impurities, particularly peroxides, which cause deterioration of vitamin A. 

 The use of solvents results in darker colors, higher viscosities, and foreign odors. 



The livers to be solvent-extracted are steamed at 158-167° F (70-80° C) from 

 30 to 45 minutes with continual stirring. When the cooking is completed they 

 are then drained as completely as possible. The livers are then placed in tight 

 containers and protected from oxidation by covering with carbon dioxide gas or 

 paraffin. After this they are cooled to —10° F (—23° C) or lower, and are ex- 

 tracted with diethyl ether, free from peroxide or some similar solvent. The extract 

 is filtered and the oil is recovered by vacuum distillation. The solvent is recovered 

 for reuse. This process is covered by a patent (Nielsen, 1937). 



Condition of Livers and Processing Procedure 



Obviously the degree of freshness of the fish livers has to be given consideration 

 when deciding upon the type process most likely to produce a high-grade vitamin 

 A oil. Fishermen are conscious of the fact that since the livers are purchased 

 on the basis of the vitamin A potency, it is to their advantage to handle them so 

 that as much of the original vitamin content as possible is retained. The livers are 

 removed from the fish and immediately packed in clean 5-gallon telescope-top tin 

 cans and packed in ice. 



When the livers are landed they are purchased on the basis of the vitamin A 

 potency of each can of livers. The processor then must decide the most efficient 



