THE DAIRY. 547 



" The circumstances that influence the rapidity of souring are 

 ^' chiefly temperature and access of air. When milk sours, it is 

 " because of the formation of lactic acid from the milk sugar. This 

 " chemical change is the result of the growth of a microscopic vege- 

 " table organism, which, according to Hallier's late investigations, 

 *' is of the same origin as common yeast. Like common yeast, this 

 " plant requires oxygen for its development. This it gathers from 

 " the air, if the air have access ; but in comparative absence of air, as 

 ** when growing in milk, it decomposes the latter (its sugar) and the 

 *' lactic acid is a chief result of this metamorphosis. If milk which 

 *' by short exposure to the air has had the microscopic germs of the 

 " ferment plant sown in it, be then excluded from the air as much 

 " as possible, the ferment, in its growth, is necessitated to decom- 

 " pose the milk sugar, and hence the milk rapidly sours. On the 

 " other hand, exposure to the air supplies the ferment partly with 

 " free oxygen, and the milk remains sweet for a longer period. 

 " Such is the theory of the change. Miiller's experiments confirm 

 '' this view by demonstrating that free exposure to the air, or, bet- 

 *' ter, a supply of pure oxygen gas, retards the souring of milk ; 

 " while confinement from the air, or replacing it with pure nitro- 

 " gen, hastens this change. That low temperatures should prevent 

 " souring, is in analogy with all we know, both of ordinary chemi- 

 ** cal change and of changes that depend upon vital operations." 

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*' Aeration of the cream during churning is of little importance. 

 " Neither chemically nor mechanically does a stream of air favor 

 '' the separation of the butter in any perceptible degree. On the 

 " contrary, cream that is cold and slightly sour, is thereby con- 

 " verted into a mass of froth, from which it is exceedingly difficult 

 " to make butter." 



******* 



" Washing Butter. — To prepare butter for keeping without 

 " danger of rancidity and loss of its agreeable flavor, great pains 

 "are needful to remove the buttermilk as completely as possible. 

 ** This is very imperfectly accomplished by simply working or 

 *' kneading. As the analysis before quoted shows, salting removes 



