COAGULATION OF MILK AND SOURING OF CREAM. 99 



cases, perfectly sterilized, lias been recently called permanent milk. 

 In its preparation different kinds of steaming apparatus are in use, 

 among them that of Neuhaus, Gronwald, and (Ehlmann is very 

 popular. This apparatus renders it possible during heating to expel 

 the air from the milk and the bottle, and after the heating has 

 been finished to close the patent bottles by means of a lever in the 

 apparatus itself before its cover is removed. 



41. The Spontaneous Coagulation of Milk and the Souring of 

 Cream. The so-called spontaneous coagulation of milk takes place, 

 as has been already explained in 7, as soon as a certain quantity 

 of lactic acid is formed by lactic fermentation. The amount of 

 lactic acid produced depends on the original condition of the milk, 

 and the quantity of ferments present. It is dependent also on 

 the temperature. It has been already noticed that there are a 

 comparatively large number of forms of genuine lactic bacteria very 

 similar to one another both in their form and properties, which 

 together are able to effect the formation of lactic acid and the spon- 

 taneous coagulation of milk. Some, and this especially applies to 

 the bacillus acidi lactis of Hueppe, split up the molecule of milk- 

 sugar with comparative ease into four molecules of lactic acid, and 

 produce at the same time an extremely slight evolution of carbonic 

 acid. Others produce small quantities of secondary bye-products, 

 especially alcohol, and others, again, develop in addition very minute 

 quantities of odorous bodies, regarding which very little else is 

 known. Various indications, as has been pointed out, show that 

 in the spontaneous coagulation of milk the caseous matter does not 

 seem to remain unchanged, as is the case in the artificial precipita- 

 tion by addition of acids, but undergoes slight changes. 



The most important practical application of lactic fermentation 

 is seen in the souring of cream for the manufacture of butter, an 

 operation which takes place every day in dairies. Bacteriology has 

 already annexed this operation as a suitable field for investigation. 

 Ever since it has been shown to be probable that all kinds of lactic 

 bacilli are not equally well adapted to act as ferments in effecting 

 this change, the attempt has been made to isolate and to cultivate in 

 pure cultures the particular varieties which are believed to produce 

 the best butter with the finest aroma. In order that this may be 

 accomplished, it is necessary to describe exactly how a fresh and 

 pure daily supply of the souring liquid, or, as it is called, the acid 

 generator, is obtained. It has been recommended to infect with a 



