1847.] Butter Churning. 169 



tirely mechanical, and as due solely to the mechanical action in 

 churning. It may, however, and more reasonably, I think, be 

 regarded as including also a chemical action, which I shall pres- 

 ently explain. 



2. Chemical changes produced during churning in close vessels. 

 — The first change observed is the souring of the milk. If pure 

 sugar of milk be dissolved in water, it is not transformed into lac- 

 tic acid by any length of agitation in close vessels. In the churn- 

 ing, therefore, it is not the mere mechanical action which turns 

 the milk sour. But if, with this solution of sugar of milk, a little 

 fresh curd be mixed, and the mixture be then exposed to the air 

 for a length of time with occasional stirring, the sugar is gradual- 

 ly changed into lactic acid. The presence of a little chalk in the 

 mixture facilitates and ensures the change, lactate of lime being 

 pro(kiced. The curd of milk, therefore, in favorable circumstances, 

 can persuade or induce the sugar of milk to become transformed 

 into lactic acid. In milk itself, the cuid without doubt possesses 

 a similar power, and agitation, with a slightly elevated tempera- 

 ture, may facilitate the exercise of it. 



Milk contains curd or casein, in two states. The greater part 

 is in a state of solution, but a small part of it is in the coagulated 

 or undissolved state, according to the latest observers. Milk in 

 its natural condition, as it comes from a cow in good health and 

 in a state of repose, is alkaline. It loses this character, however, 

 almost immediately, and becomes acid, though not sufficiently so 

 to be perceived by the taste, after a very brief exposure to the 

 air. The process of souring — of converting the sugar into the 

 acid of milk — has already begun. 



I have said, perhaps, because in an alkaline state of the milk, 

 it does not appear likely that any of the common curd or casein 

 should be in a coagulated state, since it is by the soda of the milk 

 that the whole is supposed to be held in solution. 



The course of the changes, therefore, which take place when 

 sweet milk or cream is churned in a close vessel, is most probably 

 as follows: 



The action, perhaps of the coagulated portion of the curd, aid- 

 ed by the free alkali of the milk, causes the production of acid to 

 begin; this acid combines with the free alkali, and with apoition 

 of that by which the curd is held in solution. An additional 

 small quantity of curd is thus coagulated, and this is repeated up- 

 on every fresh production of acid. The agitation and constant in- 

 termixture of the particles of the milk brings the acid, the alkali 

 the curd, and the sugar, into frequent and close contact with each 

 other, and thus promotes their mutual action, while the slightly 

 elevated temperature at which churning succeeds best, assists and 

 promotes this action. 



