63 



had been settled, the practical question before the investigators 

 was how it would be possible to repress one class and encourage 

 the other. Prof. Fjord had, in the meantime, perfected his pasteur- 

 izing apparatus, which has been figured and described elsewhere. 

 This was invented chiefly with a view to improve the keeping 

 qualities of skim milk so that it might reach the patrons and other 

 consumers in a fresh and sweet condition. This was accom- 

 plished by heating it to a temperature of upwards of 150 F. It 

 was found that this temperature destroyed enough of the bacteria 

 which caused the milk to change to make it possible to keep it 

 sweet from twelve to twenty-four hours longer than when it is not 

 thus heated. The next step was to apply the same treatment to 

 the cream, or to the sweet milk before it was separated, with the 

 same results. Heated to a temperature of 160 F., it was found 

 that the most active forms of the bacteria were killed, and that by 

 again cooling the cream to about 75 or 80 F., at which bacterial 

 life becomes active, the cream could be inoculated by any desirable 

 form of bacteria if these could be obtained from pure cultures, anj 

 that the forms with which the cream was thus impregnated, meet- 

 ing with no opposition from other forms, would develop rapidly in 

 enormous numbers and give their peculiar characteristics to the 

 butter. This is exactly what is now done in practice. The cream 

 is not always pasteurized, because if there are no injurious bacteria 

 present, or at least such numbers as to cause a deterioration of the 

 product, there is no necessity for attempting to kill them, and the 

 addition in sufficient quantity of a pure culture would at once give 

 the latter the upper hand, and their peculiarities would become 

 prominent. A rather more thorough sterilization would be 

 effected if the milk were raised to the boiling point; but a tempera- 

 ture of much over 160 F. gives the. characteristic boiled taste to 

 both milk and cream, which is objectionable and must be avoided. 

 It is found in practice that this temperature destroys nearly all 

 the organisms in active growth. The spores will survive this tem- 

 perature, but it takes them longer to develop and by the time they 

 become ready the pure culture has the mastery. 



" As a higher degree of heat than that required for their normal 

 development is destructive to the bacteria, so in like manner a 

 reduction of the temperature retards their growth. It does not 

 destroy them, if not below the freezing point, but it stops their 



