95 



Decomposition of oils and fats is usually caused by ,, hydro- 

 lysis", viz. the formation of fatty acids and glycerine. This 

 may be caused by : 



1. the action of chemicals (alkalis, acids, etc.), 



2. the action of enzymes, 



3. a microbial influence. 



Various opinions prevail in the voluminous literature on the 

 true cause of rancidity of fats, which opinions cannot be expa- 

 tiated upon in this treatise. Chiefly they come to this : the 

 rancidity is ascribed to the influence of divers factors such as 

 the presence of the oxygen in the air, of moisture, the influence 

 of light or of chemicals, or combinations of all these factors. 



In many cases oils and fats under the influence of oxidation 

 get a sharp odour and taste, known by the name of , .rancid" 

 and this is especially so when the oils contain water and are 

 placed in the light. This we can call a purely chemical change. 



It has been discovered that with the rancidity of vegetable 

 margarine we have to deal with a microbiological process, 

 similar to that described for pure butter by Orla JENSEN 1). 

 The rancidity of vegetable margarine, containing cocoa nut oil 

 or palm kernel oil and also of pure butter, is recognised by a 

 typical ester odour appearing as soon as the margarine begins 

 to go bad. At the time the rancid odour occurs — and 

 this may be taken as the best test for rancidity — a great 

 change of taste is observed ; the flavour becomes disagreeable, 

 caused through the formed ester products and the free fatty 

 acids which always arise when rancidity occurs. 



From experience we know that especially those fats containing 

 volatile fatty acids (cocoa nut oil, palm kernel fat and butter 

 fat) show this form of rancidity. 



For a long time the amount of free butyric acid was consi- 

 dered a measure for the degree of rancidity of butter. This 

 opinion however is not correct, seeing that a fat may contain 

 free volatile fatty acids without being rancid. 



Jensen was the first to study rancidity of pure butter and 

 succeeded in detecting its causes. 



Briefly his results are the following : 



1) Jensen, O. Centralbl. f. Bakteriologie, 2e Abt. Bd. VIII, 1902. S. 11, etc. 



