220 MILK AND DAIRY PRODUCTS PART 



(a) Qualitative Estimation. 



The best way to detect and separate the different volatile 

 fatty acids is to form the silver salts as in the method worked 

 out by Orla Jensen. 1 These silver salts can easily be pre- 

 pared from solutions of the free acids and silver carbonate, or 

 by precipitation of the alkali salts with silver nitrate. They do 

 not contain water of crystallisation, are easily dried in vacuum 

 over sulphuric acid, and by careful ignition the silver is quickly 

 obtained in a pure state. In the following table are given the 

 solubilities of the silver salts, arid the percentages of silver 

 as found by Orla Jensen : 



It will be seen that the solubility of the silver salts decreases 

 rapidly with the rise of molecular weight, and therefore frac- 

 tional crystallisation of the salts is an excellent way of 

 separating the volatile fatty acid. 



If the procedure now to be described is followed, very satis- 

 factory results are obtained. When the milk culture cannot easily 

 be filtered through paper, the casein is precipitated with a little 

 dilute sulphuric acid, and the filtration is then performed without 

 difficulty. Sufficient sulphuric acid is added to the filtrate to 

 liberate the volatile acids, and they are then distilled off with 

 steam, the volume of liquid in the flask being kept constant, 

 so that from 200 c.c. of the filtrate 1 litre of distillate may 

 be got. Ordinary tap water, which has been boiled with 

 sulphuric acid to drive off all the carbon dioxide, is used as the 

 generator of steam. The distillate is titrated with baryta 



" Biologische Studien iiber den Kasereifungsprogress unter spezieller 

 Beriicksichtigung der fliichtigen Fettsaiiren," Landwirtschaftliches Jahrbuch 

 der Schweiz, 1904, p. 319. 



