NUTRITIVE VATATE OF FISH AND SHELLFISH 533 



quickly and would melt at a lower tem|)erature than suck fats as 

 beef or mutton tallow, which contain a high percentage of the glyc- 

 eride of stearic acid. 



The fats from land sources consist largely of the glycerldes of oleic, 

 palmitic, and stearic acids. In addition to these, fish oils contain 

 others to a greater or less extent. Various investigators have sub- 

 mitted experimental evidence to show that from fish fats one may 

 obtain jecoric acid, jecoleic acid, therapic acid, and culpanodonic 

 acid. The discovery of the presence of the glycerldes of culpano- 

 donic acid in marine animal fats was of more than scientific interest, 

 for Tsujimoto, a Japanese investigator, has apparently shown that 

 the characteristic fishy odor of these fats is due very largely to this 

 substance. When the glyceride of culpanodonic acid was removed 

 from marine fats, or by hydrogena tion was transformed into a gly- 

 ceride of some other fatty acids, the characteristic odor of fish fats 

 disappeared. 



The edible fats obtained from, land animals and plants are naturally 

 divided into two groups — those that are solid and those that are liquid 

 at ordinary temperature. In general, the fats of animal origin, such 

 as lard, butter, and beef and mutton suet, are solid at ordinary tem- 

 perature. With a few exceptions, such as cocoa butter and coconut 

 fat, those from the vegetable kingdom are liquid at ordinary tem- 

 peratures. In addition, there are, to be sure, a few border-line fats, 

 such as chicken fat, that are semisolid at ordinary temperature. 



In comparing the nature of fish fats with those obtained from land 

 plants and animals it will be found that a much larger proportion of 

 the former are liquid at ordinary temperature than is the case with 

 fats of land origin. In fact, with the exception of that obtained from 

 the Pacific coast eulachon, all fats obtained from fish are liquid at 

 ordinary temperature. 



Crude ifish-hver oils, such as cod-liver oil, are veiy nearly on the 

 border line between liquid and solid fats, for if the temperature is 

 lowered stearin begins to solidify and separate from crude cod-liver 

 oil. 



In considering the nature of fish fats there is one factor that should 

 be given attention — their tendency to oxidize and become rancid. 

 Considered chemically, fats may be divided into three groups — drying, 

 semidrying, and nondrying oils. This classification of oils is based on 

 the power of oils to take up oxygen. Oils that can absorb relatively 

 large amounts of oxygen are called drying oils and are in demand for 

 use in paints. The best-known member of this group is linseed oil. 

 Next comes the semidrying oils, or those which can take up some 

 oxygen but not enough to make them good paint oils. The non- 

 drying oils are characterized by a low iodine number and do not 

 absorb oxygen readily. 



While fish and marine animal oils differ from the terrestrial animal 

 oils to some extent as regards color, odor, and viscosity, they are quite 

 different chemically. The terrestrial animal oils resemble closely the 

 nondrying oils in that they do not easily absorb oxygen. The fish 

 and marine animal oils resemble the drying oils and have the pov/er 

 to absorb oxygen. Due to this chemical difference a cook would 

 have relatively little difficulty in keeping beef or mutton fat, lard, 

 chicken fat, or butter without oxidizing and becoming rancid, but 



