106 PHARMACEUTICAL BACTERIOLOGY. 



d. By destroying the non-spore-bearing bacteria, pasteurization some- 

 times allows toxic organisms to grow and produce serious poisons in the 

 milk. (On the other hand, these same poisons are more frequently pro- 

 duced in milk that has not been pasteurized, and thus danger may be ob- 

 viated in pasteurized milk by cooling it quickly, keeping it cold and short- 

 ening the time for distribution.) 



e. Pasteurization is inefficient as a preservative; the milk keeps only 

 twelve to twenty-four hours longer than otherwise. (This is really no dis- 

 advantage, for the quicker bad milk sours, the better.) 



/. Pasteurization injures the taste of the milk. (This is not so, if prop- 

 erly done.) 



g. Pasteurization increases the cost of the milk. (True, but it is the 

 cheapest safeguard, and the expense of pasteurization is offset by the keeping 

 quality of the milk.) 



Rosenau has made extensive tests to determine the thermal death -point 

 of those pathogenic microbes most commonly found in milk. His conclu- 

 sions are summarized as follows: 



Milk heated to 60 C. and maintained at that temperature for two minutes 

 will kill the typhoid bacillus. The great majority of these organisms are 

 killed by the time the temperature reaches 59 C., and few survive to 60 C. 



The diphtheria bacillus succumbs at comparatively low temperatures. 

 Oftentimes it fails to grow after heating to 55 C. Some occasionally sur- 

 vive until the milk reaches 60 C. 



The cholera vibrio is similar to the diphtheria bacillus regarding its 

 thermal death-point. It is usually destroyed when the milk reaches 

 55 C.; only once did it survive to 60 C. under the conditions of the experi- 

 ments. 



The dysentery bacillus is somewhat more resistant to heat than the 

 typhoid bacillus. It sometimes withstands heating at 60 C. for five minutes. 

 All are killed at 60 C. for ten minutes. 



So far as can be judged from the meager evidence at hand, 60 C. for 

 twenty minutes is more than sufficient to destroy the infective principle of 

 Malta fever in milk. M. melitensis is not killed at 55 C. for a short time; 

 the great majority die at 58 C., and at 60 C. all are killed. 



Milk heated to 60 C. and maintained at that temperature for twenty 

 minutes may, therefore, be considered safe so far as conveying infection 

 with the micro-organisms tested is concerned. 



Evaporated, condensed and dry milk are found upon the market and are 

 extensively used. Sugar is frequently added as a preservative. In making 

 condensed milk, it is evaporated in large pans until it assumes a creamy 

 consistency. Dry milk is prepared by spraying the milk on revolving hot 

 cylinders. The thin film of milk is evaporated to dryness in a moment, 



