OF THE RANCIDITY OF BUTTER. 663 



The outyric acid thus produced is a colourless transparent volatile 

 liquid, which emits a mingled odour of vinegar and of rancid butter. 

 To the production and presence of this acid, therefore, in the milk or 

 cream or in the manufactured butter, the rancidity of this important 

 dairy product is partly to be ascribed. 



2°. Relation to the fatty bodies. — It is probable that in certain cir- 

 cumstances the casein of milk is capable of inducing chemical changes 

 in the fatty bodies as well as in the sugars, but this conjecture has not, 

 as yet, been verified by rigorous experimental investigation. 



3°. Relation to fats and sugars mixed. — It is known, however, to act 

 upon fatty bodies when mixed with sugar. Thus, if a small quantity 

 of casein be added to a solution of sugar, lactic acid is produced for a 

 certain length of time, but it ceases to be sensibly formed before the 

 whole of the sugar is transformed into this acid. If now a quantity of 

 oily matter be added to the mixture, the production of lactic acid will re- 

 commence, and may continue till all the sugar is changed. If more 

 sugar be added by degrees, the formation of acid will go on again, and, 

 after a while, will cease. The introduction of a little more oil will again 

 give rise to the production of acid, and, at length, the acid will cease to 

 be formed, while both sugar and oil are present. The casein originally 

 added has now produced its full effect (Lehmann). 



It appears, therefore, that in the presence of sugar, casein is capable 

 of changing or decomposing the fatty bodies also, and of giving birth to 

 oily acids of various kinds. Now, in milk, in cream, and in butter, the 

 casein is mixed with the sugar of the milk and the fats of the butter, and 

 thus is in a condition for (^hanging at one and the same time both the 

 sugar into lactic or butyric acid, and the butter into other acids of a 

 fatty kind. Among those latter into which the butter-oil is convertible 

 may probably be reckoned the capric and c;aproic acids, which are still 

 more unpleasant to the smell and taste than the butyric acid, and which 

 are known to be present in rancid butter- 



§ 16. Of the rancidity and preservation of butter. 



We are now prepared, in some measure, to understand the. changes 

 that take place when butter becomes rancid — and the way in which those 

 substances act which are usually employed for preserving it in a sweet 

 and natural state. ' • 



1°. When butter becomes rancid, there are two substances which 

 change — the fatty matters and the milk sugar with which they are mixed. 

 There are also two agencies by which these changes are induced — the 

 casein present in butter, and the oxygen of the atmosphere. The quantity 

 of casein or cheesy matter which butter usually contains is very small, 

 but, as we have seen, it is the singular property of this substance to in- 

 duce chemical changes of a very remarkable kind, upon other compound 

 bodies, even when mixed with them in very minute quantity. 



2°. As it comes from the cow, this substance, casein, produces no 

 change on the sugar or on the fatty matters of the milk. But after a 



Every chemist is aware, however, that in decompositions of this kind, it is seldom 

 that one single product is obtained aloma. Though the above formula, therefore, represents 

 truly how butyric acid may be produced from lactic acid under the circumstances, yet 

 other substances are not unfrequenily formed during the actual experiment, by which the 

 result is more or less complicated. 



