COMPOSITION AND PROPERTIES OF MILK POWDERS 319 



to receive their fluid milk twice daily, and some are using alkaline 

 neutralizers in order to reduce the acidity of the milk before desic- 

 cation. 



One fundamental reason why even slight increases in acidity 

 do very markedly reduce the solubility of the finished powder, lies 

 in the fact that the high degree of concentration necessarily mul- 

 tiplies the percentage of acid, and with it the solubility-destroying 

 effect of the heat of desiccation. 



The process of manufacture controls the solubility of the milk 

 powder chiefly by the degree of heat to which the milk is exposed 

 and by the manner in which the heat is applied. 



In the film process of drying, for instance, the milk is exposed 

 to the heated cylinder charged with steam under pressure, and con- 

 sequently it is subjected to temperatures far exceeding that of the 

 boiling point of water. This high heat does materially reduce the 

 solubility of the resulting powder, though this unfavorable effect 

 may be minimized to some extent by having the cylinders operate 

 in a vacuum chamber under reduced pressure. 



In the case of the spray-drying process, the milk is not exposed 

 to a steam-heated metal surface. The fact that the air entering the 

 spray-drying chamber may have, and usually does have, a tempera- 

 ture of from 275 degrees F. to over 300 degrees F., appears to not 

 materially affect the solubility of the resulting powder. 



In the spray-drying process the evaporation of the moisture in 

 the atomized spray is so rapid that it brings about a marked cooling 

 effect, and it is believed that the milk solids are kept in a relatively 

 cool condition until they have surrendered substantially all of their 

 moisture. 



This protection of the milk against the solubility-destroying 

 action of heat appears to be especially insured by the process of 

 the Gray patent, in which the coolest strata of the heated air "only 

 come in contact with the incoming moisture-laden milk, and by the 

 time the milk particles enter the zone of the hot incoming air they 

 have surrendered the bulk of their moisture. 



That a marked cooling effect does take place in the drying 

 chamber is further borne out by the fact that the moisture-laden air 

 escaping from the drying chamber has a temperature very much 

 lower than the entering air. The outgoing air of a properly operated 



