84 LIPIDES (fats and RELATED SUBSTANCES) 



However, the more detailed structural formulas should be used by the 

 beginning student until famiharity with them has been gained. It is 

 evident that a great many different individual glycerides might be 

 formed by suitably combining glycerol with the various fatty acids. It 

 has been calculated that ten fatty acids can produce 550 possible com- 

 binations. 



Formerly it was customary to regard the natural fats as consisting 

 chiefly of simple glycerides, but more recent work shows that they con- 

 sist largely of mixed glycerides. It is impossible to say with certainty 

 just how much of any given fatty acid goes to make up simple or mixed 

 glycerides in a fat. In the case of butyric acid, it is known that the 

 acid does not exist as a simple glyceride in butter, but is present in a 

 mixed combination. Tributyrin is a bitter substance; obviously, it cannot 

 be present in butter. It is probably more nearly correct to say that 

 natural fats consist essentially of mixtures of mixed glycerides. 



PHYSICAL PROPERTIES OF FATS 



As already explained, fats may be either solids or liquids at room tem- 

 perature (20°C.). The common animal fats are solids at this tempera- 

 ture, and the majority of vegetable fats (oils) are liquids. Fats and 

 oils are lighter than water and, as a rule, have a specific gravity of about 

 0.8. As usually seen they are noncrystalline, although many fats can 

 be made to crystallize under suitable conditions. (See Fig. 4-1.) They 

 are poor conductors of heat, therefore serving a useful purpose in the 

 insulation of the body. 



Fats are colorless when they are obtained in a pure state. The more 

 or less yellow color common to many fats is due to the presence of a 

 pigment, and not to the fat itself. Butter, for example, varies in color 

 with the season. In June, when cows feed on grass, the butter is highly 

 colored, while in January, when the animals receive a dry ration, the 

 butter is paler in color. The yellow pigment, therefore, is contained in 

 the feed of the animals. This is largely carotene, C40H56, which occurs 

 in all green plants, in the petals of many flowers, such as the narcissus, 

 and in many vegetables such as carrots, squash, etc. In grass it is not 

 evident because the green pigment, chlorophyll, masks the carotene. 

 Xanthophyll, C40H56O2, is another yellow pigment which is widely dis- 

 tributed in plant materials. This is the chief coloring pigment found 

 in egg yolk. It is also contained in butterfat, but in a much smaller 

 percentage than is carotene. The egg yolk likewise varies in color with 

 the feed of the poultry. Lard contains no coloring matter, probably 

 because swine are fed on rations largely free from green material. 



The coloring of fats, which is of importance commercially, has been 

 brought into considerable prominence in connection with the vitamin A 



