FISH MEAL AND OIL 487 



One of the problems in connection with this process is the digestion of the 

 bones. Little difficulty is experienced with small bones, but the larger ones are not 

 suflBciently digested. It is necessary to remove them by some form of straining, 

 followed by either digesting with stronger acid and enzyme action or grinding. 

 Bones are highly desirable in feed as they furnish the needed calcium and 

 phosphorus for a properly balanced diet. 



Fish Meal 



In general fish meal is now classified as an animal feed rather than as a fertilizer. 

 This change has been a result of improved methods of manufacture as well as 

 recognition of the concentrated high quality protein value. It is usually mixed 

 with other feed materials, such as alfalfa meal, bran, or other vegetable materials, 

 and an analysis of the food value is placed on the container. Fish meal is sold 

 on the basis of its protein content, and rated according to the per cent of protein 

 contained in the feed or fish meal. Therefore, producers have all tried to prepare 

 a high protein value meal by improving their methods of manufacture. 



General Characteristics of Fish Oils 



Fish oils are liquid fats and consist chiefly of various proportions of the glyc- 

 erides of saturated and unsaturated fatty acids. Vegetable and animal oils may 

 be divided into three general classes (viz., nondrying, semidrying, and drying), 

 according to their ability to absorb oxygen from the air and thus be converted into 

 solid insoluble compounds. Fish oils belong to the two latter classes. Menhaden, 

 sardine, salmon, herring, bonito, albacore, mackerel, stickleback, akajei, anchovy, 

 and whiting oils are classed as drying, and may be used for the manufacture of 

 paints, varnishes, and other materials. Whereas dab, sprat, hoi, and carp oils have 

 relatively low iodine values and must be classed as semidrying oils, they cannot 

 be used in paints and varnishes since upon drying they do not absorb suflBcient 

 oxygen from the air to form hard nontacky films. 



Fish oils are obtained from all parts of fish, whereas fish-liver oils are prepared 

 from the livers only. The bodies of liver-oil yielding fish, such as cod, are, as a 

 rule, very lean and yield very little oil; whereas the livers of fatty fish usually 

 contain little oil. Fish oils diflFer in chemical composition from liver oils, chiefly 

 in that the latter contain relatively high amounts of an unsaponifiable substance 

 called cholesterol. 



The exact nature of the fatty acids in fish oils is still imperfectly known. Most 

 of the oils deposit small amounts of stearin upon standing in the cold for long 

 periods. Fish-oil stearin usually consists of a mixture of the glycerides of stearic 

 and palmitic acids. It has been found that the fatty acids obtained upon the 

 hydrolysis of menhaden oil contained 22.7 per cent palmitic acid and 1.8 per cent 

 stearic acid, the glycerides of which are solid fats. The fish oils listed above as 

 drying oils contain relatively high proportions of the glycerides of highly unsatu- 

 rated fatty acids; whereas semidrying oils contain lesser amounts of these 

 glycerides. All marine animal oils contain erucic acid 



CsHiT — C< — H 



II 

 H-C- (CH.)u-COOH or (C..H^O.) 



