SPONTANEOUS CHANGES 29 



liberated in fats which have become rancid, and this is especi- 

 ally so in the case of fats such as butter, which contain acids 

 of low molecular weight, as butyric acid, the smell of which 

 recalls that of rancid butter. It is, however, a fact that a fat 

 may be acid without being rancid ; * coco-butter, for instance, 

 has usually an acid reaction, but very rarely becomes rancid. 



With regard to other constituents found in rancid fats, 

 various authors have from time to time observed the presence 

 of hydroxy-acids, aldehydes, alcohols, and of esters of lower 

 fatty acids, and peroxides, but there appears to be a general 

 consensus of opinion that glycerol does not occur. 



According to Fierz,f in the case of unsaturated fats, oxida- 

 tion may take place at the double bond, in the absence of 

 micro-organisms, with the formation of aldehydes and acids 

 of lower molecular weight which, like butyric acid, have an 

 offensive odour and taste. On the other hand, saturated fats 

 become rancid under the action of Penicilliuni glaucum and 

 Aspergillus niger with the liberation of various odoriferous 

 ketones. This is due to the oxidation of the j8 carbon atom 

 according to the scheme — 



RCH2CH2 . CH2 . COOH + O -^ RCH^CHOH . CH^COOH + O 



-> RCHjCO CH2COOH + HjO 



the latter acid by loss of carbon dioxide giving a ketone— 

 RCH2 COCH2COOH = RCH2 COCH, + CO.. 



By this means caproic, caprylic, and myristic acid, which occur 

 as esters in coco-butter, may be regarded as the precursors of 

 methyl amyl, methyl heptyl, and methyl undecyl ketones 

 respectively, and which have been shown to occur in rancid 

 coco-butter. 



These compounds have been experimentally produced from 

 their respective precursors by growing Penicilliuni glaucum 

 and Aspergillus niger upon the ammonium salts of the relative 

 acids. Methyl heptyl ketone has been isolated from Roquefort 

 cheese. 



Drying and Resinification. — Most fatty oils on exposure to 



* Vintilesco and Popesco : " J. Pharm. Chim.," 1915 [iv.], 12, 318. 

 f Fierz : " Z. angevv. Chem.," 1925, 38, 6. 



