164 Milk and Its Products 
which tends to increase the surface tension or the 
viscosity will act as a hindrance towards the sepa- 
ration of the cream. While the viscosity of the milk 
serum prevents the particles of fat from uniting 
into a mass, still the particles have considerable free- 
dom of movement in the milk, and being of a less 
specific gravity, of course are acted upon with less 
intensity by any force to which the milk is subjected. 
If the milk is allowed to remain at rest in a 
vessel, the force of gravity, acting with different in- 
tensities upon the globules of fat and the milk serum, 
will cause the particles of fat to gather together 
near the surface of the liquid. In so gathering, 
they carry with them certain of the milk con- 
stituents, and the layer of fat globules and adher- 
ing particles we call cream. From time immemo- 
rial, and until within a very recent date, the 
force of gravity, acting in the way indicated, has 
been the only means used for separating cream from 
milk. Now machines are in use that effect a sepa- 
ration of the cream from milk by means of centrif- 
ugal force, and at the present time we have three 
systems of separating cream; namely, by gravity 
acting upon a thin layer of milk in a shallow vessel, 
known as the Shallow Pan System; secondly, by 
gravity acting upon a deeper mass of milk, usually 
submerged or partially submerged in water, known 
as the Deep Setting System; and thirdly, by ma- 
chines making use of centrifugal force, known as 
the Centrifugal or Separator System. The quality 
of the cream for consumption or for purposes of 
