246 POPULAR SCIENCE MONTHLY. 



it in virtue of a rennet-like ferment, whilst a third class precipitate 

 and dissolve the casein of the milk, along with the development of 

 butyric acid. The process whereby milk is submitted to a heat of 

 65° to 70° C. for twenty minutes is known as pasteurization, and the 

 milk so treated is familiar to us all as pasteurized milk. Whilst the 

 pasteurizing process weeds out the lactic acid bacteria from the milk, 

 a temperature of 100° C. for one hour is necessary to destroy the 

 butyric acid organisms: and even when this has been accomplished 

 there still remain in the milk the spores of organisms which 

 are only killed after a temperature of 100° C. for three to six hours. 

 It will, therefore, be seen that pasteurization produces a partial, not 

 a complete sterilization of the milk as regards its usual bacterial in- 

 habitants. The sterilization to be absolute would require six hours 

 at boiling point. But for all ordinary practical purposes pasteur- 

 ization is an adequate procedure. All practical hygienic require- 

 ments are likewise adequately met by pasteurization, if it is properly 

 carried out, and the milk is subsequently cooled. Milk may carry 

 the infection of diphtheria, cholera, typhoid and scarlet fevers, 

 as well as the tubercle bacillus from a diseased animal to the human 

 subject. For the purpose of rendering the milk innocuous, freez- 

 ing and the addition of preservatives are inadequate methods of 

 procedure. The one efficient and trustworthy agent we possess is 

 heat. Heat and cold are the agents to be jointly employed in the 

 process, viz., a temperature sufficiently high to be fatal to organisms 

 producing a rapid decomposition of milk, as well as to those which 

 produce disease in man; this is to be followed by a rapid cooling to 

 preserve the fresh flavor and to prevent an increase of the bacteria 

 that still remain alive. The pasteurized process fulfils these require- 

 ments. 



In conjunction with Dr. Hewlett, I had occasion to investigate in 

 how far the best pasteurizing results might be obtained. We found 

 that 60° to 68° C. applied for twenty minutes weeded out about 

 90 per cent, of the organisms present in the milk, leaving a 10 per 

 cent, residue of resistant forms. It was found advisable to fix the 

 pasteurizing temperature at 68° C, in order to make certain of killing 

 any pathogenic organisms that may happen to be present. We' passed 

 milk in a thin stream through a coil of metal piping, which 

 was heated on its outer surface by water. By regulating the length 

 of the coil, or the size of the tubing, or the rate of flow of the milk, 

 almost any desired temperature could be obtained. The temperature 

 we ultimately fixed at 70° C. The cooling was carried out in similar 

 coils placed in iced water. The thin stream of milk was quickly 

 heated and quickly cooled as it passed through the heated and cooled 

 tubing, and whilst it retained its natural flavor, the apparatus ac- 



