170 Milk and Its Products 



till its maximum density is reached at about 40 F., 

 at which temperature the difference between the 

 specific gravity of the water or milk serum and the 

 fat will be at its greatest, and the separation of the 

 fat for that reason promoted. But while the water 

 does cool faster than the fat, the fat shrinks or 

 increases in density much faster than the water, 

 so that the difference in specific gravity between the 

 two is no greater, and in fact less, at low tempera- 

 tures than at high ones. Further, the viscosity of 

 the milk serum is much increased at low tempera- 

 tures, so that the favorable influence of low tempera- 

 tures cannot be explained upon these grounds. 

 Arnold* attempted to explain the favorable influence 

 wholly upon the relative contraction of the fat and 

 serum, as follows : 



Water is a better conductor of heat than fat ; hence when 

 the temperature of milk varies either up or down, the water 

 in the milk feels the effect of heat or cold sooner than the 

 fat in the cream does, therefore the cream is always a little 

 behind the water in swelling with heat or shrinking with cold, 

 thus diminishing the difference between the specific gravity of 

 the milk and cream when the temperature is rising, and in- 

 creasing it when the temperature is falling. 



But that this explanation is not sufficient, Bab- 

 cock t has shown thus : 



Though it is true that water is a better conductor of heat 

 than fat, the small size of the fat globules renders it impossible 

 that, under any circumstances, there can be more than a small 

 fraction of a degree difference between the temperature of the 

 fat and that of the milk serum. Moreover, within the limits 

 of temperature practical for creaming (90 F. to 40 F.), the 



* American Dairying, p. 210. 



f Wisconsin Agricultural Experiment Station, Bull. 18, p. 24. 



