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 ? 



