170. Butter Churning. [Sept., 



As the milk becomes decidedly sour, another change commen- 

 ces. The envelopes of the fat globules are attacked by the acid, 

 and are gradually dissolved. As they thin off they begin to burst, 

 the particles of fat are liberated and gradually unite into the visi- 

 ble particles of solid butter. It has been found by experiment 

 that the envelopes are dissolved by acetic acid. I think it quite 

 as likely, therefore, that they are soluble in the acid of milk. 



3. Chemical changes produced during churning, with access of 

 air. — The only change of circumstances here is, that the oxygen 

 of the atmosphere may exercise an influence upon the several in- 

 gredients which the milk contains. 



Supposing other matters to proceed as in the close vessel, 

 there are three ways in which the presence of oxygen may affect 

 the process of churning, or the results obtained from it. 



a. When fresh moist curd is exposed for a time to the air, it 

 undergoes upon its surface a series of chemical alterations or suc- 

 cessive decompositions, which it is unnecessary here to describe. 

 When these take place in the milk itself, they so alter the curd 

 as to promote its action upon the sugar, and to hasten the pro- 

 duction of lactic acid. 



h. By exposure to the air, under a variety of circumstances, the 

 protein compounds, of which both the curd and the envelopes of 

 the globules consist, absorb oxygen, and are more or less com- 

 pletely changed into soluble oxifles of protein. It is probable, 

 therefore, that churning w^th access of air may cause the envel- 

 opes to absorb oxygen, to become partially soluble, to thin off, 

 and finally to burst, and thus to liberate the fatty matter they con- 

 tain. 



c. The constantly renewed and intimate contact between the 

 air and the particles of fat, as they are liberated, may enable the 

 latter to become paitially oxidised also, to give off carbonic acid, 

 the evolution of which is said to have been actually -observed, and 

 thus to produce the solid or margaric acid of butter in larger pro- 

 portion than that in which it existed naturally in the milk. The 

 theory of this operation or change has been explained in the pre- 

 ceding article. 



In all chemical transformations which it is in our power to 

 produce by art, the aid of a specific temperature is necessary to 

 ensure the speediest, purest and most certain res-.ilts. It can 

 scarcely be doubted that the temperature found in practice to be 

 most favorable to the churning of milk and cream, and to the 

 production of butter of good qualify, is so favorable, because it 

 promotes the more rapid and effectual performance of the changes 

 which 1 have above indicated and explained. 



While, therefore, it is conceded that milk and cream may be 

 completely churned in close vessels, and that the oxygen of the 



