158 MODERN DAIRY PRACTICE. 



lactic-acid bacteria proper had disappeared or else were 

 considerably reduced in number, and that even putrefac- 

 tive bacteria had decreased in number, although not as 

 much as the former. This change in the bacterial content 

 was so striking that it could be observed by a mere micro- 

 scopic examination, and showed that the pure bacilli forms 

 had entirely gained the upper hand after the heating. I 

 am not in possession of sufficient medical knowledge to 

 state the hygienic importance of the disappearance of the 

 lactic-acid bacteria proper from the milk, or at least their 

 great diminution, and of the decrease in the number of 

 bacteria causing fermentations of the casein; but I can 

 testify on the basis of my investigations that this relation 

 plays an important part in regard to the keeping qualities 

 of the milk, since the lactic-acid fermentation usually 

 precedes the fermentation processes caused by the other 

 bacteria.f 



The relative keeping qualities of the milk are thus 

 increased through the disappearance of the lactic-acid 

 bacteria. 



A heating of the milk to 165-185 F. will therefore 



* Bitter found that tubercle bacilli were killed by simple heating 

 for 30 minutes at 154 F. ; Forster (Milchzeitung , 1894, p. 84) ob- 

 tained the same result by heating for 10 minutes at 158 F., for 5 

 minutes at 170 F., and for 1 minute at 203 F. At lower tempera- 

 tures longer time was required, viz., 1 hour at 140 F. and 4 hours 

 at 131 F. Bang found that a temperature of 176-185 was required 

 to kill tubercle bacilli ; see Part IV., Chapter I. W. 



f Duclaux mentions similar observations in the January number 

 of Annales de I' Institute Pasteur for 1891, and ascribes great 

 hygienic importance to the fact that the acid-forming bacteria have 

 disappeared in the heated milk, especially in regard to the applica- 

 tion of the milk for baby-feeding. 



