238 III. CHEMISTRY OF NEUTRAL FATS 



cause of the failure of many of the earher investigators to reahze this fact, 

 many of the melting points reported in the literature are unreliable. 



Although the melting points of the simple triglycerides composed of even- 

 chain saturated acids are directly proportional to the chain length (and 

 vary inversely with the saponification number), the presence of even 

 minor amounts of the unsaturated fatty acids may lower the melting point 

 profoundly. The effectiveness of such an unsaturated acid in decreasing 

 the melting point must depend, not only on the degree of unsaturation, but 

 also on its position in the mixed triglyceride molecule. The melting point 

 can be caused to vary in a natural fat without altering its fatty acid com- 

 position, by rearrangement of the fatty acids. Thus, a limpid cottonseed 

 oil can be changed to a semisolid form by interesterification, while a 

 product solid at ordinary room temperature can be prepared by so-called 

 "directed" interesterification. All of the natural fats listed in Table 24 

 with iodine numbers exceeding 65 are liquid at ordinary temperatures. 

 However, unnatural products having considerably higher iodine numbers 

 may be solid. Thus, the solid fat prepared from cottonseed oil by directed 

 interesterification has the same iodine value as does limpid cottonseed oil, 

 namely, 105 to 115. 



(2) Specific Gravity 



Rather wide variations in specific gravity obtain between the different 

 fats. The values vary from a maximum of 0.960 for cacao butter to 0.915 

 for lard oil and rapeseed oil. The calculated values for beef and mutton 

 tallow are much lower than those for the liquid fats, being 0.860. The 

 differences in specific gravity are to be ascribed to the component fatty 

 acids. In general, they increase with the increasing molecular weight of the 

 combined acids; they also increase with larger proportions of unsaturated 

 or hydroxy-acids. The specific gravity tends to increase when rancidity 

 develops. 



In the case of fats which are ordinarily solid at room temperature, the 

 specific gravity is best determined at a point considerably above the melt- 

 ing point, usually at 40° or 50°C. The Sprengel tube is conveniently used 

 for such a test.^-"-^ The value for the specific gravity at 15.5°C. can be cal- 

 culated by the following formula : 



*Spi5.5 ^ Sp, + k {t - 15.5) 



where Spt is the figure obtained at temperature t, and fc is a constant. 

 Some values for k for some ordinary fats are given by Woodman^" as fol- 

 lows: butter fat, 0.000617; cacao butter, 0.000717; coconut oil, 0.000642; 

 lard, 0.000650; palm oil, 0.000657; tallow, 0.000673. 



'22 A. G. Woodman, Food Analysis, 4th ed., McGraw-Hill, New York, 1941. 



