248 III. CHEMISTRY OF NE UTRAL FATS 



CsoHgfi. This subatance occurs only in extremely small amounts in other 

 oil (peanut, 0.07%; rapeseed, 0.05%; apricot kernel, 0.02%). The type of 

 sterol may also be of considerable assistance in the identification of a fat. 

 The chemical behavior of the other compounds is discussed in later sec- 

 tions. 



In most cases the total N.S.F. amounts to less than 1%. However, the 

 results may be somewhat higher in cod-liver oil (0.54-2.68) and corn oil 

 (1.5-2.8), while wool fat, which consists chiefly of cholesterol esters' and 

 triterpenes, is composed of 39 to 44% of unsaponifiable extract. ^°^ 



6. Physical Properties of Fats and Oils 



(1) Polymorphism and Melting Point 



Since the classical work of Chevreul, it has been recognized that fats ex- 

 hibit a peculiarity in melting point not shown by other organic compounds. 

 Dutfy,-^^ in 1853, noted that stearin prepared from mutton tallow had 

 three melting points; this inspired the suggestion that isomeric stearins 

 exist. For some years it has been known that tristearin which has been 

 quickly chilled will melt at 55 °C., after which it resolidifies as the tempera- 

 ture is increased, and then remelts at 71.5°C.2^ Smits and Bokhorst^^" 

 noted that, if the chilled sample was allowed to stand for some time at 

 room temperature, it spontaneously changed to the higher melting modi- 

 fication. These workers attributed the existence of two melting points in 

 the triglyceride to the presence of two different crystalline modifications 

 which are interchangeable. That more polymorphic forms may exist in 

 the case of mixed triglycerides is shown by the fact that /3-lauro-a,a'- 

 distearin is reported by various workers to have such widely divergent 

 melting points as 36°, 52.5°, 56.5°, 58°, 59.8°, 60.5°, and 68.5°C. 



The explanation for these results has become clearer as a result of the 

 studies of Malkin and his collaborators. ^^^~^^^ These workers have de- 

 monstrated that simple triglycerides can exist in three forms (7, a, and /3), 

 while an additional modification (jS ') exists in the case of the mixed tri- 

 glycerides and of the simple triglycerides composed of odd-chain acids. 

 Each phase has its own melting point, so that it is necessary to know what 

 particular modification is involved before the melting point can be used as a 

 method for identification. These forms are to some extent interconvertible 

 by the technic usually employed in determining the melting points. The 



^^ A. Smits and S. C. Bokhorst, Proc. Akad. Wetenschappen, Amsterdam, 15, 681-683 

 (1912); J. Chem.. Soc, IO4, I, 157A (1913). 



'«! M. G. R. Carter and T. L. Malkin, J. Chem. Soc, 1939, 1518-1521. 

 !'«2 C. E. Clarkson and T. Malkin, J. Chem. Soc, 193/,, 666-671. 

 363 M. G. R. Garter and T. Malkin, ./. Chem. Soc, 1939, 577-581. 

 36* T. Malkin and M. L. Meara, J. Chem. Soc, 1939, 103-108. 

 365 T. Malkin and M. L. Meara, /. Chem. Soc, 1939, 1141-1144. 



