Preservation of Milk. 



109 



ent temperatures, and when so applied exerts a varying 

 effect, depending upon temperature employed. All meth- 

 ods of preservation by heat rest, however, upon the applica- 

 tion of the heat under the following conditions: 



1. A temperature above the maximum growing-point 

 (105-115 F.) and below the thermal death-point (130- 

 140 F.) will prevent further growth, and consequently 

 fermentative action. 



2. A temperature above the thermal death-point destroys 

 bacteria, and thereby stops all changes. This temperature 

 varies, however, with the condition of the bacteria, and for 

 spores is much higher than for vegetative forms. 



Attempts have been made to employ the first principle 

 in shipping milk by rail, viz., prolonged heating above 



growing temperature, 

 but when milk is so 

 heated, its physical ap- 

 pearance is changed. 1 

 The methods of heating 

 most satisfactorily used 

 are known as steriliza- 

 tion and pasteurization, 

 in which a degree of 

 temperature is used ap- 

 proximating the boiling 

 and scalding points re- 

 spectively. 



Effect of heat on milk. 



When milk is subjected 

 to the action of heat, a 

 number of changes in its 

 physical and chemical properties are to be noted. 





FIG. 23. Microscopic appearance of milk 

 heated above 140 F., showing the homo- 

 geneous distribution of fat-globules. The 

 physical change noted in comparison with 

 Fig. 22 causes the diminished consistency of 

 pasteurized cream. 



i Bernstein, Milch Zeit., 1894, pp. 184, 200. 



