Principles of Milk Preservation. 91 



served best by first pasteurizing it, then freezing it and 

 shipping in this condition. 1 Another process is Casse's 

 method in which a quantity of milk is frozen and placed 

 in large cans. The intervening space is then filled with 

 fresh milk and the whole closed air tight. It is claimed 

 this milk keeps for weeks in this condition. 2 One of the 

 difficulties of simple freezing is that the milk constitu- 

 ents separate out so thoroughly that it is difficult to per- 

 fectly re- incorporate them upon melting. 



90. Condensed milks. Milks may be preserved for 

 an indefinite period of time by condensing them. The 

 keeping quality of such milk often depends, however, upon 

 the action of another principle, viz., the inhibition of bac- 

 terial development by reason of the concentration of the 

 medium. This degree of concentration is reached in 

 either of two ways, by the addition of sugar, or by evap- 

 orating the watery solution by boiling it down either in 

 open air or preferably in a vacuum pan. In the milks 

 preserved by addition of sugar, the bacteria are not nec- 

 essarily destroyed but they are unable to grow on account 

 of the concentrated condition of the medium. Such 

 milks diluted with several volumes of water, even sterile 

 water, not infrequently undergo the ordinary decomposi- 

 tion changes. In some cases, the heating process also 

 destroys in part the contained germ life. 



9 1 . High temperatures. Heat has long been used as 

 a preserving agent. Milk has been scalded or cooked 

 from time immemorial to keep it. Heat may be used at 

 different temperatures, and when so applied exerts a 

 varying effect, depending upon temperature employed. 

 All methods of preservation by heat rest, however, upon 

 the use of the two following principles: 



1 Milch Zeit.., No. 9, 1895. 



2 Milch Zeit., No. 33, p. 527, 1897. 



