TRANSPORTATION PROBLEMS. 173 
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 Pasteur 
for treating wine, consists in heating the milk to a moderate tempera- 
ture only, and then rapidly cooling it. The temperatures chosen 
have varied. Sometimes a temperature as low as 140 F. is adopted, 
and continued for from twenty minutes to half an hour or more; 
sometimes 150 to 160 is used for about ten minutes, and some- 
times 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 pasteuri- 
zation no attempt is made to sterilize it, but simply (i) 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 chemi- 
cal changes that are pioduced 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 milk at all, 
while higher temperatures will bring them about. For this reason 
the lower temperatures are better. But will these moderate 
temperatures accomplish the desired ends ? 
