v DECOMPOSITION PRODUCTS 231 



A more accurate idea of the degree of rancidity of butter is 

 obtained by distillation of the volatile fatty acids of the butter 

 fat, and the amount of these, as well as of the esters, deter- 

 mined. Amthor l employs the following method for this 

 purpose : 



Ten g. of butter are distilled with steam until 500 c.c. of 

 distillate are collected. The volatile fatty acids contained in 

 the distillate are titrated with N/W alkali and then 50 c.c. of 

 N/10 alkali are added, the mixture heated for half an hour 

 with a reflux condenser, and the free alkali titrated back. 

 The difference gives the amount of alkali required to saponify 

 the ester-like substances. Amthor calls the number of c.c. 

 of N/W alkali required to saponify the ester-like substances 

 from 100 g. of butter the " ester number." 



The fact that normal butter also contains volatile substances 

 which are saponifiable with ca,ustic potash makes it impos- 

 sible, as Amthor himself agrees, to regard the ester number as 

 strictly quantitative, and this necessitates a determination of 

 the butyric acid ester. 



Orla Jensen 2 has also proved that the more or less pro- 

 nounced smell of the ester which is developed in butter is not 

 always in proportion to the quantity of volatile, saponifiable 

 substances. He therefore disregards quantitative determina- 

 tions, and contents himself with the " total acid number " on 

 the one hand, and on the other with the " volatile acid 

 number," by which last he understands the number of c.c. 

 of normal alkali which are required to neutralise the dis- 

 tillate from 100 g. of butter. Jensen employs Amthor's method 

 of distillation, and in addition washes out the condenser at the 

 end of the operation with 100 c.c. neutral absolute alcohol, in 

 order to get the higher soluble fatty acids. Where the decom- 

 position of the butter is far advanced, more alcohol is needed 

 to bring all the fatty acids into solution before titrating. 



It is important to notice that Orla Jensen has proved that, al- 

 though the free volatile fatty acids in rancid butter only make up 

 a small part of the total free fatty acids, the conclusion cannot be 

 drawn that the glycerides of the non- volatile acids are attacked 

 more readily by the micro-organisms than are the glycerides of 



1 Ref., p. 229. 3 Ref., p. 229. 



