354 BUTTER. 



Valenta test is also useful, and the density may be used as a 

 corroborative test. 



Margarine. — It is advisable to calculate from tbe mean figures 

 yielded by genuine butters and margarines the apparent per- 

 centage of margarine present. If the percentage thus calculated 

 from the mean combining weight of the insoluble fatty acids, 

 the Valenta value, and the density be less than that calculated 

 from the other determinations and, at the same time, the iodine 

 absorption and refractive index are slightly high, it is probable 

 that the butter is genuine. If the contrary is the case, and the 

 apparent percentages from all the methods give approximately 

 the same value, it is probable that the butter is adulterated, 

 especially if the Ave-Lallemant method shows adulteration. 

 If, in addition, the colour tests for vegetable oils have given 

 distinct reactions, the probability of adulteration is strengthened. 



Though in the present state of science it is not possible 

 definitely to certify many cases of small amounts of adulteration, 

 for dairy control work the task is much simplified. The samples 

 which must be regarded as suspicious can be reported as such, 

 or even as adulterated, with a high degree of probability, and it 

 will be frequently possible to trace such samples to their origin, by 

 examining the fat of the milk of the cows which yielded the butter. 



Influence of Keeping on the Analytical Properties of 

 Butter. — When butter is kept and becomes rancid very pro- 

 nounced changes take place in the composition of the fat. These 

 may be classed under two heads — hydrolysis and oxidation. If 

 butter fat be kept in the dark and out of contact with the air, 

 it keeps indefinitely without change ; but in the presence of 

 light and air it becomes oxidised. 



The general course of change may be roughly indicated 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. 



