PHYSICAL PROPERTIES OF MILK 91 



quantities than in cow's milk, and was also found in traces in 

 human milk. 



viscosity or consistency of milk (Babcock and Russell) 1 



The consistency of milk or cream is made up of two factors, 

 one dependent on the inherent characteristics of the solution 

 (the milk-serum) and the other belonging to the matter sus- 

 pended in this solution (fat, casein, and so on). 



The effect of the first factor we term viscosity in order to dis- 

 criminate between substances in solution and in suspension. 

 The combined effect of these two factors is what makes the 

 body or consistency of milk or cream. 



The consistency of the milk-serum is due to the viscosity 

 imparted by substances such as the sugar and the ash constitu- 

 ents which are in a state of perfect solution and also by the 

 physical state of the casein and insoluble phosphates which are 

 doubtless suspended in a semi-colloidal condition. These 

 substances differ in their ability to impart consistency to milk- 

 serum, the nitrogenous matter having about three times the in- 

 fluence of the milk-sugar in normal milk. 



The influence of the fat on the consistency is purely mechani- 

 cal, it being wholly in suspension. In part, this is undoubtedly 

 due to the inherent characteristics of the fat globule itself, 

 such as size and its relation to surface tension, and so on, but 

 to a much larger extent to the aggregation of the fat globules 

 into small groups or clots, a condition which is always found 

 to a greater or less extent in normal milk. The effect of these 

 factors is greatly modified in a variety of ways. For example, 

 centrifugal force in separating cream diminishes its consistency 

 to a marked degree; on the other hand, the development of 

 acid in spontaneous souring materially increases the body. 

 The thickening of cream observed in the churn is a phenomenon 



1 Wisconsin Report, 1896, p. 73. 



