222 ILLINOIS STATE DAIRYMEN'S ASSOCIATION. 



by a short exposure to the same temperature which is fatal to 

 them in a longer exposure. In dry air much higher degrees of 

 heat than those named are necessary to kill bacteria. For this 

 reason steam is generally used instead of dry heat for sterilizing 

 utensils. 



Bacteria also require moisture. It is well known that dead 

 organic matter quickly disintegrates when it is in a moist condi- 

 tion and its changes are arrested when it is dried. Milk being 

 a fluid, all the moisture that is necessary for micro-organisms is 

 at hand. There is no danger of food being too dilute for bac- 

 teria; some forms do well even in distilled water. In milk, germs 

 seem to find ideal conditions. 



The chief agents that are antagonistic to bacterial increase 

 are, together with light, the opposites to the first three favorable 

 conditions mentioned above, viz., lack of food, extremes of tem- 

 perature, and dryness. These are the dairyman's most impor- 

 tant weapons, and when he has learned to use them properly 

 he need have no fear of milk souring too soon or being other- 

 wise affected by germs. 



The operator of a creamery or factory is also sometimes able 

 to take advantage of the fact that certain species of bacteria are 

 antagonistic to each other and can not grow well together if they 

 are in the milk at the same time. In such a case there is a 

 battle for existence, the kind having the smaller number to start 

 with, or being less favored than than the other by temperature 

 or other conditions, is usually overcome. Thus one can at times 

 cut off the effects of undesirable bacteria by giving advantages to 

 other desirable or harmless forms that are hostile to them. This 

 is what takes place when the butter maker adds a "starter" to 

 his cream and ripens it at a high temperature as rapidly as pos- 

 sible to prevent the increase of a taint which he may discover in 

 the milk. A starter is a preparation or culture containing large 

 numbers of the peculiar kind of bacteria that ripen cream; by its 

 use proper forms of fermentation are started in milk or cream. 



When micro-organisms are growing, new products are form- 

 ed from the constituents of the medium by which they are sur- 



