CONSTANTS FOR IDENTIFICATION OF FATS AND OILS 235 



or of the quality of a given fat. 'fhe most common constants which are of 

 vakie inckide the melting point, refractive index, specific gravity, saponi- 

 fication number, iodine value, and the Reichert-Meissl number. Table 24 

 lists the commonly accepted constajils for some animal and vegetable fats. 



The figures for the various constants vary considerably for different sam- 

 ples of the same type of fat. They cannot be stated A\ith the precision of 

 chemically pure substances, since the natural fats are mixtures of a number 

 of types of different triglycerides. However, certain arbitrary limits are 

 definefl within which such values fall. The commonly accepted values for 

 some of the better knowii oils are included in Table 24. 



Variations occur in the constants of vegetable oils obtained from the 

 same species gro^vn under different conditions, as well as in related species 

 raised under the same environment. Although such differences are usually 

 minor, they may become of sufficient magnitude to be of considerable com- 

 mercial importance. Thus, the iodine numbers of linseed oils have been 

 sho\\Ti*°^~^°^ to vary from 128 to 209, while that prepared from the Bison 

 variety of flaxseed raised in different localities had iodine values ranging 

 from 155 to 196. Such factors as climatic conditions, high temperature, 

 and insufficient moisture while the seed is ripening apparently produce 

 linseed oils with extremely low iodine numbers. The variation in un- 

 saturation of the linseed oils is considerably greater than is noted with 

 other vegetable fats. 



On the other hand, it has been widely recognized for many years that 

 marked variations may occur in animal fats from the same species of 

 animals. The diet of the animal seems to be the most important factor 

 which causes an alteration in the composition of fat. As early as 1883, 

 Lebedeff'^" demonstrated that the fat of a fasted dog could be profoundly 

 affected by diet. When mutton tallow was fed, a very hard fat was de- 

 posited, while after the administration of linseed oil the fat was practically 

 liquid, with an unsaturated fatty acid content much higher than normal. 

 One year later, it was sho\vn that erucic acid, which is ordinarily not present 

 in dog fat, is laid down after the administration of rapeseed oil, a fat es- 

 pecially rich in this component. ^^^ Although some polyunsaturated acids 

 are present in the tissues of rats on fat-free diets, Rieckehoff and collabora- 

 tors^^'- were able to fuid increased amounts of tetraenoic, pentaenoic, and 



»>« E. P. Painter, Oil & Soap, 21, 343-346 (1944). 



307 E. P. Painter and L. L. Nesbitt, Ind. Eng. Chem., Anal. Ed., 15, 123-128 (1943). 



30* E. P. Painter and L. L. Nesbitt, Oil & Soap, 20, 208-211 (1943). 



*" C. D. Hodgman, Handbook of Chemistry and Physics, 31st ed., Chem. Rubber Pub. 

 Co., Cleveland, 1949, pp. 1267-1270. 



''« A. Lebedeff, Arch. gcs. Physiol. {Pflngrr's), 31, 11-59 (1883). 



"1 I. Munk, Arch. path. Anat. Physiol. {Virchow's), 95, 407-467 (1884). 



5'«I. (i. RieckehofT, R. T. Ilolman, and G. O. Burr, Arch. Biochem., 20, 331-340 

 (1949). 



