232 MILK AND DAIRY PRODUCTS PART 



the volatile fatty acids. The fact really is, as Harms and Stocky 1 

 have proved, that the volatile acids freed by hydroly tic cleavage 

 are attacked more greedily by the micro-organisms. Likewise, 

 the greater part of the free volatile fatty acids fail to be esti- 

 mated because they, more than the solid acids, unite with the 

 ammonia present in the drops of brine of the butter. 



Immediately after the butter has been made, the " total acid 

 number " generally varies between 1 and 3. In very rancid 

 butter the number may rise to 20 or 30 in the exterior 

 portions, whilst in the interior it may be about 10. The 

 " volatile acid number " immediately after the butter has been 

 made = 0, or, at most, 'l-'2, whilst in the outer portion of a 

 very rancid butter the number never exceeds 1. 



2 Determination of the Degree of Oxidation. 



As already mentioned, the oleic acid in butter is oxidised 

 under the action of sunlight, and imparts a tallowy flavour. A 

 determination of the iodine number (p. 153) serves as a measure 

 of the advance of the process of oxidation. Not much use, 

 however, can be made of this determination unless the figure 

 obtained is very low, for the iodine number is known to vary 

 within wide limits, even with normal butter. 



On this account, the distillate from butter must be tested 

 also for aldehyde, either by the amrnoniacal silver test or some 

 other reagent. By operating in the following mariner, the 

 sensitiveness of the ammoniacal silver reaction can be increased. 

 A few drops of caustic potash or caustic soda solution are added 

 to the silver nitrate solution, and the precipitated Ag 2 O after- 

 wards dissolved in ammonia. The solution obtained in this 

 way is used for the test. 



Schiffs reagent, which is a solution of fuchsin that has been 

 decolorised by sulphur dioxide, is also used as a test for 

 aldehyde. The reagent becomes violet in presence of alde- 

 hyde, and the test is made particularly sensitive by following 

 Gayon's 2 directions, which are to mix 



1000 c.c. aqueous fuchsin solution, 1 : 1000. 

 20 sodium bisulphite, 30 B. 

 10 pure concentrated hydrochloric acid. 



1 Zeitschrift fur U liter suchung der Nahr. -mid Genutumittel, 1900, No. 19. 



2 Compt. rend, de I' A cad. des Sciences, Vol. IV, 1887. 



