THE MILK SITUATION IK THE DISTRICT OF COLUMBIA. 91 



Pasteurization as applied to milk consists properly in heating this 

 commodity for a short period of time at a temperature considerably 

 below the boiling point (212 F.), followed by rapid chilling, the 

 object being not so much to preserve the milk as to destroy the harm- 

 ful bacteria and their products. In this connection caution should 

 be exercised to distinguish between sterilization as commonly under- 

 stood and pasteurization scientifically attained, for in sterilizing the 

 milk is heated at least to the boiling point, a temperature which com- 

 pletely annihilates all germ life, whether pathogenic or otherwise, 

 and which treatment is shown to deprive milk to a certain extent of 

 its nutrition and digestibility. 



TEMPERATURE AND LENGTH OF EXPOSURE BEST SUITED TO PASTEURIZATION. 



Varying temperatures and durations of exposure of milk to the 

 heating process have been suggested by different observers as best 

 calculated to insure the complete destruction of the disease-breeding 

 micro-organisms without at the same time destroying the enzymes or 

 ferments in the milk, which are generally recognized as playing a 

 most important part in facilitating its digestion. Milk treated in 

 the proper manner is rich in enzymes, retains entirely the taste of 

 fresh milk, and is quite as digestible. The following exposures, 

 among others, have been recommended by independent investigators : 

 140 F. for 15 minutes, 140 F. for 20 minutes, 140 F. for 40 minutes, 

 approximately 154 to 156 F. for 30 minutes, 158 F. for 5 to 10 

 minutes, and 158 F. for 30 minutes. 1 



Dr. Coit, who is identified as closely, perhaps, as any other person 

 with the development of sanitary milk production in America, rec- 

 ommends 140 F. for 40 minutes, 150 F. for 30 minutes, 167 F. for 

 20 minutes, or 190 F. for 1 minute, followed by immediate cooling 

 to between 40 and 50 F. Efficient pasteurization at these tempera- 

 tures and durations does not, according to his observations, destroy 

 the digestive enzymes or nutritive principles in milk, though, on the 

 other hand, it is destructive of all ordinary pathogenic bacteria and 

 also of most other contaminating germs. 



The two main considerations which control the temperature and 

 time during which milk should be pasteurized are, first, the thermal 

 death points of (1) the pathogenic (disease-breeding) bacteria, and 

 (2) the ferments in the milk. The first named must be surely killed, 

 so as to eliminate danger, and the latter should not be affected suffi- 

 ciently to " devitalize " the milk. So far as may be judged from 

 present knowledge, the best temperature is 60 C. (corresponding to 

 140 F.) continued for a period of 30 minutes, or 145 F. for 20 min- 

 utes, followed by rapid cooling, which, it should be emphatically 

 noted, is an essentially important part of the process. There is rea- 

 son to believe that a higher degree of heat applied during a shorter 

 interval is not as satisfactory, since in commercial pasteurization the 

 shorter periods are likely to result in imperfect pasteurization, and it 

 is mechanically impracticable to assure the application of a uniform 

 temperature in such cases to the entire body of milk subjected to the 

 process. The pathogenic bacteria succumb to heat at temperatures 

 below those of the ferments in milk, so that in this way the infectious- 



1 Bulletin No. 56, Hygienic Laboratory, Public Health and Marine-Hospital Service, p. 

 650. 



