382 



Effective Farming 



193. Separation of cream from milk. The best way to 

 separate cream from whole milk is to use a centrifugal cream 

 separator. The saving of butter-fat by the use of one of these 

 machines soon pays for its cost and upkeep. By adjustment of 

 the screw on the separator, cream of any desired richness, up 

 to a certain limit, can be produced. The milk passes into a 

 bowl on the machine and the bowl is revolved 

 at a high rate of speed. By reason of centrifugal 

 force, the heavier part of the milk the skim- 

 milk is thrown toward the outside and the 

 lighter part the fat with some milk is 

 crowded toward the center of the bowl. Near 

 the side of the bowl at the top is an opening 

 through which the skim-milk passes and near 

 the center of the bowl, one through which the 

 cream passes. The skim-milk and the cream 

 are carried from the separator through spouts 

 and into pails or cans placed near the machine. 

 194. Bacteria in milk. Most of the fermenta- 

 tions that occur in milk are due to bacteria. 

 Not all bacteria are harmful, in fact many are 

 useful, but milk containing large numbers should 

 be looked on with suspicion, as many undesirable 

 in cream test- bacteria are likely to be present with the bene- 

 ones. Bacteria reproduce very rapidly 



FIG. 163. 



to b, nor from a Sit high temperatures and more slowly at low 

 temperatures. At 70 F., the growth is rapid, 

 at 50 F., it is retarded, and at 40 F., it is very slow. Thus in 

 milk not properly cooled, the bacteria multiply very rapidly. 

 This is shown graphically in Fig. 164. One of the most 

 numerous kinds of bacteria in milk causes lactic acid to form. 

 The bacteria break down the milk-sugar and produce lactic 

 acid or, in other words, cause the milk to become sour. There 

 are several types of lactic-acid bacteria ; those that grow under 

 70 F. are useful, especially in the manufacture of butter and 



