70 MASS. EXPERIMENT STATION BULLETIN 151. 



free fatty acids in a commercial product, however, does not necessarily 

 preclude the presence of monoglycerides and diglycerides. 



Solid alcohols of the cyclic series (sterols) occur in oils and fats both in 

 combination as esters and as free alcohols.^ The amount of cholesterol 

 or phytosterol is generally small, often inappreciable, and is indicated 

 approximately by the unsaponifiable matter which it characterizes. 

 Alcohols of the ethane and other series, free and in combination, compose 

 a considerable proportion of waxes. 



Oils and fats, therefore, may contain glycerides of monohydroxy and 

 dihydroxy acids, possibly free hydroxy acids, monoglycerides and di- 

 glycerides and free alcohols; and the insoluble acids, separated from the 

 oils and fats, may contain monohydroxy and dihydroxy acids and free 

 alcohols. A portion, at least, of the free alcohols found in the insoluble 

 acids probably occurred in the fat as esters. With the exclusion of the 

 natural glycerides of hydroxy acids and a small amount of free alcohols, 

 the acetyl number of many oils and fats may be deemed an index of 

 quality, and when considered in conjunction with the acid and iodine 

 numbers may serve to measure (more or less imperfectly, to be sure) the 

 amount of hydrolysis and of oxidation the product has undergone. To 

 differentiate between products of hydrolysis and of oxidation the acetyl 

 number of the insoluble acids should also be determined. 



Earlier Methods. 

 The several analytical processes that have been offered are based on 

 the same chemical reactions, but differ in application and in details of 

 procedure. The original method was devised by Benedikt and Ulzer- 

 and applied to the insoluble acids. The acetyl number indicated the 

 milligrams of potassium hydroxide required to neutralize the acetic acid 

 obtained on saponifying one gram of acetylated insoluble fatty acids, 

 and was determined by the difference between the acid and saponification 

 numbers of the acetylated acids (acetyl ether number). The actual pro- 

 cedure consisted in saponifying the acetylated acids after neutralizing 

 in alcohol. Lewkowitsch^ has shown, however, that the results so ob- 

 tained were generally in excess of the true values, due to the conversion 

 of a part of the fatty acids on heating with a large excess of acetic an- 

 hydride into their anhydrides, as illustrated by the following equation: — 



2 RCOOH + (CH3CO)20 = (RC0)20 + 2 CH3COOH 



fatty acid acetic anhydride of acetic acid 



anhydride fatty acid 



These fatty anhydrides are fairly stable compounds, but may become 

 hydrolyzed to some extent on washing with boiling water. Subsequent 

 treatment with cold alcohol in the determination of the acetyl acid 



1 See numerous references: Abderhalden, Physiological Chemistry (1908); Hammarsten, Physi- 

 ological Chemistry (1911); Leathes, The Fats (1910). 



2 Monatsh. Chem., 8, pp. 41-48 (1887). 



3 Analysis of Oils, Fats and Waxes, 1. pp. 344, 345 (1909). 



