Principles of Milk Preservation. 95 



curs, a second application of heat is made on the suc- 

 ceeding day, and so on for three or four days. By this 

 intermittent process, all spores are given a chance to 

 germinate, and thus become susceptible to a relatively 

 low heat. 



These methods which necessitate so protracted a treat- 

 ment are manifestly out of the question for commercial 

 purposes unless a product is desired that will keep for a 

 considerable period of time. 



When milk is sterilized commercially, it is usually 

 treated for a single time at a high temperature, and in 

 some of the better types of apparatus is rendered nearly 

 sterile, or at least the small amount of germ-life remain- 

 ing is so weakened by the treatment given that develop- 

 ment is suspended, unless the conditions are very favor- 

 able. In most cases sterilized milks acquire a more or 

 or less pronounced cooked taste. This is more of an ob- 

 jection in America than in Europe, where in most cases 

 milk is usually heated before being consumed. Appar- 

 atus of this class is usually expensive, so that the cost 

 of sterilized milk is higher than the normal product. 

 Machines for this purpose have not been readily adopted 

 in this country, the tendency being to employ the follow- 

 ing process for milk preservation. 



94. Pasteurization. In this method the degree of 

 heat ranges from 140-175 F. and the application is 

 made for only a limited length of time. The process was 

 first extensively used by Pasteur (from whom it derives 

 its name) in combatting the various maladies of beer 

 and wine. Its importance as a means of increasing the 

 keeping quality of milk was not generally recognized 

 until a few years ago ; but the method is now growing 

 rapidly in favor as a means of purifying milk for com- 

 mercial purposes. The method does not destroy all 



