GERMS IN RELATION TO MILK 15 



milk at this temperature for the time suitable to the temperature. 

 Also the machine must work reliably and be easy to clean. Com- 

 mercial pasteurizers should also have an automatic, temperature- 

 regulating device which is self-registering, and this is now de- 

 manded by Boards of Health (p. 217). 



Imperfect pasteurization prevents milk from souring quickly 

 because the germs which cause milk to sour are those most readily 

 succumbing to heat (since they are generally not spore-bearing 

 germs). The general effect of imperfect pasteurization is simply to 

 check — for a longer or shorter time — the growth of germs. They 

 are retarded in their development, not killed. Disease germs may 

 not be wholly destroyed in the process. Experiments which I have 

 conducted with the pasteurized milk of the general market (in cans) 

 showed that while containing but 15,000 germs to the cubic centi- 

 meter, soon after emerging from the pasteurizer on the delivery 

 wagon, in twenty-four hours the same milk contained several million 

 germs to the cubic centimeter. Drs. Bergey and Pennington found 

 much the same result in Philadelphia ; that raw and recently pasteur- 

 ized milk contained respectively 1,260 and 12 bacteria, but, at the 

 end of J2 hours, the numbers were 17,000,000 and 148,000,000 

 germs. Also the harmless lactic acid germs of raw milk are killed by 

 heat, and the more dangerous germs from dirty bottles, corks and 

 dust contaminate the improperly pasteurized milk. A commercial 

 pasteurizer which fulfils the scientific requirements for heating milk, 

 i. e.j one in which milk may be heated to a given temperature and 

 held at that point for a given time, is the Willmann Perfect Pasteur- 

 izer made by the Dairy Machinery and Construction Co., of Shelton, 

 Conn. In this machine the milk is first heated to 145 ° F. and then 

 passes into an automatic holding machine, where it is maintained 

 at this temperature by an automatic device for 30 minutes, when 

 it is discharged automatically back into the original regenerative 

 pasteurizer and thence on to a cooler to be cooled to 38 or 40 F. 

 If pasteurization is done thoroughly the lactic acid bacilli (sour 



