142 SCIENCE AND PRACTICE OF DAIRYING. 



force is best ascertained by the percentage of fat in the skim-milk 

 obtained. Considering the efficiency with which separators at 

 present do their work, one is justified in demanding that in dairies 

 where separators are in use the coefficient of cream-raising should 

 be such that a percentage of *2 to '3 on an average '25 of fat is 

 obtained in the skim-milk. 



It is only under very exceptional circumstances that the skim-milk 

 obtained by separators contains as little as ! per cent of fat. Just as in 

 the case of whole-milk which has been evaporated down to dryness, the 

 fat is less easily extracted by ether, so it is found that, in the gravimetric 

 determination of fat in skim-milk, if not done with care, the percentage 

 of fat may quite easily be placed too low. Examples of skim-milk obtained 

 by separators under ordinary conditions containing less than '15 per cent, 

 or much less than ! per cent of fat, are, therefore, to be viewed with 

 suspicion. 



70. The Conditions which Influence the Cream-raising Coefficient 

 in connection with Separators. The coefficient of cream-raising ob- 

 tained with milk-separators depends on the following conditions: 



(1) On the strength of the centrifugal force used to separate the 

 milk, or on the rapidity of the revolutions of the drum. As has 

 already been pointed out, the centrifugal force increases with the 

 square of the number of revolutions made by the drum in a minute. 

 If the drum of a separator does not revolve quickly enough, or up 

 to the required speed, much fat will remain behind in the skim-milk, 

 which might, with greater care, be easily obtained in the cream. 



(2) On the time during which the milk is submitted to centri- 

 fugal force, or on the quantity of milk which is creamed per hour. 

 The more milk that is creamed in a given time, the less favourable 

 will the coefficient of cream-raising be. 



(3) On the temperature at which cream-raising takes place. 

 The warmer the milk the better does it cream. From 5 to 25 C. 

 upwards, the percentage of fat in the skim-rnilk rapidly decreases, 

 and from that temperature always more and more slowly up to the 

 boiling point of milk. 



These three conditions are of enormous importance, and since 

 they are always under control, it may be said that the success of 

 cream-raising depends on the art and method in which separators 

 are worked. It is further influenced by 



(4) The construction and nature of the separator. For example, 



