COMPOSITION AND PROPERTIES OF MILK PowpbERS 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, hes 
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 
