78 LIPIDES (fats and RELATED SUBSTANCES) 



an increase in the number of carbon atoms in the molecule. The change 

 in physical state is accompanied by a decrease in solubility. Stearic 

 acid, which melts at 70°C., is practically insoluble in water. 



The fatty acids up to and including capric are easily removed from 

 solutions by distillation with steam and hence are known as volatile fatty 

 acids. The determination of the volatile fatty acids is a matter of con- 

 siderable importance in the analysis of fats, as it aids in chstinguishing 

 one type of fat from another. Butter fat, for example, gves a higher 

 proportion of volatile acids than any other fat or oil. 



The volatile fatty acids likewise have a decided odor. Butyric acid 

 has a strong odor similar to that of rancid butter. Caproic, caprylic, 

 and capric acids have a pronounced animal odor and are sometimes spoken 

 of as the goat acids. When fats become rancid, a small amount of these 

 volatile fatty acids is formed and gives to the fats a particularly objec- 

 tionable odor and taste. Thus the objectionable odor of rancid butter 

 is due largely to the presence of free butyric acid. 



y 



Unsaturated fatty acids 



If a fatty acid contains a pair of carbon atoms that are joined together 

 by two bonds instead of one, that is, a double bond, it is said to be 

 unsaturated. Such an acid can take up hydrogen, iodine, bromine, oxy- 

 gen, or other elements by the breaking open of one of these two bonds. 

 This leaves an open position on each carbon atom. Therefore, for each 

 double bond, two hydrogens can combine with the compound, which would 

 then be regarded as a saturated compound. This condition of unsatura- 

 tion has a unique relation to the physical state of the fatty acid and 

 likewise to the glycerides of the fatty acid. For example, oleic acid 

 (C17H33COOH), which contains one double bond, and therefore two 

 fewer hydrogens than stearic acid, is a liquid, whereas stearic acid 

 (Ci7H35COOH)is a solid. Oleic acid melts at 14°, whereas stearic acid 

 melts at 70°. If the fatty acid contains more than one double bond, 

 it will have a correspondingly lower melting point; linoleic acid, which 

 contains two double bonds, has a melting point of —18°. 



The physical state of the fatty acids is carried over to the glycerides 

 of these acids. Oleic acid and olein are liquids, and stearic acid and 

 stearin are solids. Linolein and linolenin, as expected, are liquid glycer- 

 ides. Since fats are mixtures of glycerides, a fat will be soft or hard 

 depending upon the proportion of liquid glycerides it contains. Oils 

 differ from fats in that they contain a larger proportion of liquid glycer- 

 ides. As a general statement, it might be said that oils contain about 

 80 per cent of unsaturated glycerides, whereas solid fats do not contain 

 more than 40 to 50 per cent. Unsaturation is a fundamental property 

 and, in most fats, is the key to the whole question of their physical state. 



