186 CONTROL OF THE MILK SUPPLY 
somewhat less easy of digestion and assimilation. The heat pro- 
duces several important changes which result in its being less 
easily handled by the digestive organs. The difference is not very 
great, and a healthy individual is able to digest such milk well 
enough; but delicate children and invalids are not so well nourished 
upon boiled milk as upon raw milk. 
4. Boiling the milk is a treatment which has not proved 
practical to adopt on a large scale at a central source of supply. 
It offers, therefore, no assistance either to the producer or to the 
distributer in enabling him to furnish milk which, since it keeps 
longer, gives greater satisfaction. 
The practice of boiling milk in order to “‘sterilize”’ it is widely 
adopted in private families, but the objections urged against it 
have led to its being less and less recommended. In its place has 
come an extended use of the second method of treating milk by 
heat. 
2. Pasteurization.—This method, originally devised by Pas- 
teur for treating wine, consists in heating the milk to a moderate 
temperature only, and then rapidly cooling it. The temperatures 
chosen have varied. The method now preferred is to use a 
temperature as low as 140° F., and to continue for from twenty 
minutes to half an hour or more; sometimes 150° to 160° is used 
for about ten minutes, and sometimes as high a temperature as 
180° is used, the milk being just brought to this point and then 
cooled at once. Any of these methods is called pasteurization. 
Such temperatures are manifestly not sufficient to sterilize the 
milk, since they are even less efficient than boiling. But in pas- 
teurization no attempt is made to sterilize it, but simply (1) to 
destroy the large majority of bacteria and (2) to destroy the disease 
germs that are liable to be in the milk, and therefore to render it 
safe for drinking. A low temperature is chosen in order to avoid 
the chemical changes that are produced by boiling and that show 
themselves in the boiled taste. These changes begin to appear at 
about 156°, and any temperature below this scarcely changes the 
