436 Transactions of the American Institute. 



neutralizes the soda that holds in combination the bulk of the casein 

 and causes tlie latter to coagulate, as is seen in sour or lobbered 

 milk, and this thickening of the milk prevents the further rising 

 of the cream. The best results are said to be attained when tlie 

 milk has a depth of from one and a half or two inches in the pans. 

 Zine pans possess the .curious property of retarding the souring of 

 milk from four to five hours as compared with that kept in tin or 

 wooden vessels. In the absence of definite knowledge upon the 

 matter the reason of this may be inferred to be that the lactic acid 

 first formed is neutralized by combination with the basic oxyd on 

 the surfjice of the metal. The use of zinc for this purpose is, how- 

 ever, highly reprehensible because the salts of zinc are poisonous in 

 a high degree, and cases have been known \yhere persons who have 

 used butter made from milk and kept in zinc vessels have suffered 

 severe and nauseating attacks from this cause. Glass and tin, which 

 are practically incorrosive, and wood, when kept perfectly clean, 

 are the best materials for vessels in which milk or cream is to be 

 held. The cream being skimmed and collected in sufficient quantity 

 is ready for conversion into butter, pending the full consideration 

 of which a few paragraphs may be devoted to 



The Theory of Chukning. 

 We have seen that butter exists in milk in the form of minute 

 globules, each enveloped in a minute cyst or sac. The operation of 

 churning consists, first, in breaking these sacks to liberate the butter ; 

 and, second, in causing the butter particles to adhere together, that 

 the product may assume consistency and solidity. The rupture 

 of the globules is secured by friction, partially by their rubbing 

 against each other, and partially by the rubbing upon them 

 of the surface of the churning mechanism. The kneading 

 together of the liberated butter is done by bringing them forcibly in 

 contact, and for this purpose the means employed to bring about the 

 first stage of the churning process is commonly sufficient. When we 

 speak, however, of the liberation of the butter by friction, it must 

 not be supposed that ordinary churning depends wholly on mechani- 

 cal agencies. Air admitted to the milk or cream while being agitated^ 

 the oxygen acts upon the milk-sugar, and changes it into lactic acid, 

 which, as previously explained, coagulates the casein in clots, such as 

 are seen in the buttermilk and acts indirectly to facilitate the rup- 

 ture of the globules. As a matter of curiosity, it may be mentioned 



