320 COMPOSITION AND PROPERTIES OF MILK POWDERS 



spray-drying chamber usually has a temperature of from 150 to 

 about 180 degrees F. 



In the spray-drying process it is customary to heat the fluid 

 milk or the condensed milk to not to exceed 150 degrees F., and it 

 appears that when this is done the milk solids are not exposed to 

 temperatures materially higher than 150 degrees F. until they have 

 given off their moisture ; in fact, it is possible that at least during 

 the early stages of desiccation they are actually cooled by their 

 rapid surrender of moisture. 



Experimental study has demonstrated that, when a certain de- 

 gree of concentration has been exceeded, exposure to high heat de- 

 stroys the solubility of the protein constituents of the milk. This 

 is a matter of common knowledge to the operator of the sterilizer 

 in the manufacture of evaporated milk. In the spray-drying process 

 the change from high concentration of the milk in the liquid state, to 

 complete dryness, is so instantaneous that no damage is done and 

 when once dry, exposure to heat does no further harm. 



The solubility of the powder resulting from the spray-drying 

 process may be materially reduced, however, if the fluid milk, 

 prior to desiccation, is heated to temperatures considerably in ex- 

 cess of 150 degrees F. 



The powders of the properly operated spray-drying process are 

 practically completely soluble in cold water. The powders of the 

 film-drying process require hot water for their solution and even in 

 hot water they fall short slightly, but unmistakably, of complete 

 solution. 



The relative solubility of spray- and film-dried powders is 

 shown in the following table : 



