576 ANNUAL. REPORT OF THE Off. Doe. 



favorable conditions are present. This method also has the advant- 

 age of greatly diminishing the amount of exposiue to air, thus pre- 

 venting additional bacteria getting into the milk from the air dnr 

 iug the creaming process. Owing to the low temperature employed, 

 the growth of bacteria in the milk is greatly retarded. To use this 

 system satisfactorily, ice is necessary. The cream raised by the 

 deep-setting method is not very rich, usually- containing between 15 

 and 20 per cent, of fat. 



In the gravity method of cream-raising, the rapidity and complete- 

 ness of separation of cream are dependent upon several different 

 factors, among which may be mentioned (a) the size of the fat-glo 

 bules, (b) the amount of solids in the milk-serum (the solids-not-fat), 

 and (c) the relative proportions of the dilTerent solids in the serum. 

 The larger fat-globules separate from the milk more quickly than 

 the smaller ones do. The size of fat-globules varies with individual 

 cows, with breeds, with advance of the period of lactation, and 

 with other conditions. The fat-globules in the milk of cows fresh 

 in milk are larger than later in the lactation period, decreasing m 

 size as the cow is farther from the beginning of her period of lac- 

 tation. The solids-not-fat in milk, that is, the casein, albumin, sugar 

 and mineral salts cause milk to have the property of what is called 

 viscosity; by this is meant the power of adhering to other solid 

 tilings. The presence of these solids in milk gives milk the power 

 of adhering to fat-globules, thus offering resistance to their tend 

 enc}' to rise or move in any direction. Now, the greater the amount 

 of these solids-not-fat iu milk, the more opposition do the fat-globules 

 have to overcome in rising, and the more slowly and incompletely 

 does such milk cream. It is w^ell known that, as a cow gets farther 

 along in her lactation period, the casein and albumin increase more 

 rapidly than do other constituents and they greatly increase this 

 viscosity of the milk. Hence, with advance of lactation, we have 

 two different factors increasing all the time in the milk that inter- 

 fere with the raising of the cream, viz: decrease in size of fat-globules 

 and increase of viscosity. This explains why the gravity method 

 of creaming gives less satisfactory results when a cow is far along 

 in milk than when she is fresh. To overcome these adverse condi- 

 tions, various expedients have been tried, the most common being 

 dilution of the milk by water, both warm and cold. The advantages 

 secured in this way are doubtful and the disadvantages accompany- 

 ing such a practice are considerable. 



19. Separation of Cream by Centrifugal Force. 



It is not our purpose to discuss the details of the mechanism of 

 centrifugal machines or to describe the different varieties in the 

 market. We intend simply to make a few statements in regard to 



