88 LIPIDES (fats and RELATED SUBSTANCES) 



Various water-insoluble soaps of metals other than sodium or potassium 

 have important industrial uses. Soaps of the higher saturated or slightly 

 unsaturated fatty acids such as stearic, palmitic, and oleic acids, with 

 such metals as aluminum, calcium, lead, barium, lithium and others, are 

 used in making lubricating greases. When combined with lubricating 

 oils, these soaps produce semisolid gels, or greases. More highly un- 

 saturated acids such as linoleic or eleostearic are combined with lead, 

 manganese, or cobalt to produce ''driers" for use in paints, varnishes, 

 and enamels. These soaps catalyze the oxidation processes, which cause 

 the films to "dry" or harden. Zinc oleate and stearate are used as 

 antiseptics and astringents in medicinal preparations. Of the above soaps, 

 aluminum and zinc stearates are quantitatively the most important, being 

 produced annually to the extent of some 10 million lb. each. 



Acrolein test 



When glycerol is heated strongly, and especially if a deliydrating agent 

 such as potassium bisulfate is present, it decomposes into water and 

 acrolein: 



CsHjCOH), ^^gQ^ > CH2=CHCH0 + 2H2O 



The unsaturated aldehyde, acrolein, has a characteristic sharp, irritating 

 odor and is partially responsible for the smell of burnt fat. 



Fats likewise give the acrolein test since on heating to a sufficiently 

 high temperature the glycerides in the fat are partially broken down with 

 the eventual formation of acrolein. 



Iodine number 



The unsaturation of a fat is determined by means of iodine, which gives 

 the so-called iodine number of a fat. The iodine number is the per- 

 centage of iodine by weight that the fat will absorb; for example, if a 

 fat has an iodine number of 100, one gram of the fat will absorb one 

 gram of iodine. The following table shows how the iodine number gener- 

 ally varies with the physical state of the fats. It will be noted that 

 the hard fat, tallow, has a low iodine number, 35^5, whereas lard, a 

 soft fat, has an iodine number of 50-70, and the oils have iodine numbers 

 ranging from 80 to 200. If judged by the low iodine number, butter, 

 and especially coconut oil, should be hard fats. The low melting point 

 of coconut oil (about 25°) as compared with that of tallow (about 45°) is 

 due to the presence of large quantities of glycerides of the lower saturated 

 fatty acids. The softness of butter is caused by two factors, unsaturation 

 and glycerides of low molecular weight. In most fats and oils only the 

 first of these two factors plays a part. 



