CHAPTER XXVII. 



CREAM. 



Skim Milk. The term " skim milk " is applied to milk from 

 which the bulk of the cream has been abstracted. Two ways of 

 removing the cream are practised : (1) by allowing the milk to 

 stand, taking advantage of the force of the earth's gravity to 

 separate the cream ; (2) by employing centrifugal force to attain 

 the same object. 



Distinction between Skimmed and Separated Milk. 

 A distinction has been drawn between skim milk obtained by 

 these two methods ; that obtained by setting the milk being 

 called " skimmed milk," and that obtained by centrifugal force 

 " separated milk." The distinction is one of degree, not of kind, 

 as, were it possible to keep milk without chemical change for an 

 indefinite period, the same ultimate result would be obtained by 

 either method. 



The following are the characteristics of skimmed and separated 

 milks : 



SKIMMED MILK SEPARATED MILK 



Contains the solids not fat of the Contains the solids not fat ot the 



whole milk, partially changed by whole milk, practically un- 



the action of micro-organisms. changed. 



Contains usually more than 0-4 per Contains usually less than 0-3 per 



cent, of fat. cent, of fat. 



Contains a portion of the solid im- Is free from the solid impurities 



purities of the milk. of the milk. 



The term machine-skimmed milk is adopted in the Sale of 

 Food and Drugs Act, 1899. 



Rising of Fat Globules. The globules of fat rise through 

 the milk because they are lighter than the milk serum. 



If we have a globule of fat of radius r, the force impelling it to 

 rise will be proportional to the weight of an equal bulk of milk 

 serum less the weight of the globule, or 



6 = k . .1 TT r 8 . (d, - d f ) . g, 



where k is a constant varying with the units adopted for r, d s and df. 

 TT is the ratio of the circumference of a circle to its diameter, 

 r is the radius of the globule, and 

 g is the acceleration due to gravitation. 

 d s and df are the specific gravities of the serum and fat. 

 408 



