366 DEDUCTIONS FROM ANALYSIS. 



it keeps indefinitely without change ; but in the presence of 

 light and air it becomes oxidised. 



The general course of change may be indicated roughly thus 



(1) The fat is partly hydrolysed into fatty acids and glycerol. 



(2) The glycerol is oxidised to fatty acids of low molecular weight. 



(3) The unsaturated acids are oxidised, forming hydroxy-acids. 



The general effect of these changes is 



The volatile and soluble acids are increased, the soluble in greater pro- 

 portion than the volatile. 

 The insoluble acids are decreased. 

 The iodine absorption is lowered. 

 The density and refractive index are increased. 

 The potash absorption is increased. 



If the butter has been kept in its natural state, the butter fat 

 obtained on melting may have properties differing materially 

 from those indicated above, owing to the solubility of some of 

 the products in the water still left in the butter. The soluble 

 and volatile acids in the filtered fat may be lowered from this 

 cause, and the insoluble acids increased. 



The change is not very rapid, and in the course of several 

 weeks the changes are often not very pronounced. 



Bell has recorded the following figures for the changes in the 

 insoluble fatty acids ; the butter in this case was kept for the 

 times indicated : 



No. of weeks kept, 12 7 7 6 86 



Before keeping, per cent., . 87-30 87-80 85-50 87-40 87-72 87-65 

 After . 88-97 90-00 85-72 87-97 88-40 88-00 



Vieth has made analyses showing the change in the insoluble 

 fatty acids produced when butter fat is kept. In each case about 

 a year had elapsed between the two analyses. 



Per cent. Per cent. Per cent. Per cent. 



Original insoluble fatty acids, . 87-43 88-33 87-61 87-72 

 Insoluble fatty aeids, after keeping, 85-07 85-97 84-41 83-82 



The same observer has also examined old butter fat and old 

 butter (kept for about ten years) which had not been previously 

 analysed. 



The old butter fat was divided into two portions one, com- 

 pletely bleached, contained 83-52 per cent, of insoluble fatty 

 acids ; and the other, which still retained a trace of its natural 

 colour, yielded 83-90 per cent. 



The results with the old butter were as follows : 



Lower portion, . . . 89-28 per cent, insoluble fatty acids. 



washed, . 89-33 , 



Upper portion, . . . 90-94 



