506 THE PASTEURIZATION OF MILK 



The terms sterilization and pasteurization with reference to milk 

 are today, unfortunately, still used somewhat indiscriminately, and, 

 in fact, no strictly scientific definition of the term pasteurization is in 

 existence. Tjaden, in the chapter on "Sterilization and Pasteur- 

 ization" in Sommerfeld's Manual on Milk, define pasteurization 

 as the heating of milk up to 98 to 99 C.; sterilization as the heating 

 to the actual boiling temperature or beyond it; and Foersterization, as 

 the heating at 60 C. for one hour. Rosenau defines pasteurization 

 as applied to milk in heating it to 60 C. for twenty minutes, followed 

 by rapid cooling. 



As the object of pasteurization, Tjaden designates : 



1. A better and more complete separation of the milk into its com- 

 ponent constituents. 



2. To make milk products more tasty and to impart to them better 

 keeping qualities. 



3. To improve the keeping quality of the milk as a whole. 



4. To destroy disease germs which may possibly be present in milk. 

 It has been ascertained that the yield in butter fat in milk treated 



in the centrifuge is much better at 45 to 80 C. than at lower temper- 

 atures. Milk products derived from heated milk and treated with 

 pure cultures of certain fermentative bacteria, permitted to act upon 

 cream and casein in the manufacture of butter and cheese, were also 

 found to have a much finer taste and flavor than the same edibles 

 prepared from raw milk in which other bacteria present modify the 

 special fermentations. While pasteurization improves the keeping 

 qualities of milk this is only true as long as the milk after heating is 

 cooled rapidly, kept cool, and not dispensed in dirty vessels which 

 would again lead to contamination and the rapid multiplication of 

 bacteria. Pasteurized milk has lost the power to inhibit the growth 

 of bacteria for some time, and those bacteria -which do multiply in 

 it are not the harmless lactic-acid bacteria but the spore-forming, 

 peptonizing, putrefactive bacteria. 



The pasteurizers used in the heating of the milk on a large com- 

 mercial scale are generally of either one of two types. The milk 

 which goes rapidly through an apparatus in a continuous stream, is 

 either heated for a very short time to a comparatively high temper- 

 ature or it remains in the apparatus for a comparatively long time, is 

 agitated during this period, and is kept at a proportionately lower 

 temperature. This is called the discontinuous method. 



The object is always the destruction of most bacteria of any kind, 

 and the destruction of all pathogenic bacteria. The most important 

 of the latter is the tubercle bacillus. Numerous scientific investiga- 

 tions have dealt with temperatures and periods necessary to destroy 

 it in milk. The results have furnished by no means uniform data, but 

 it appears to be generally conceded (Tjaden, Weigmann, and others) 

 that in the continuous method heating to 85 C. (185 F.) for one to 

 two minutes is generally sufficient to kill tubercle bacilli. Tjaden, 



