THE PHYSICAL PROPERTIES OF MILK 67 



The expansion of rich milk is greater than that of poor milk. 

 Since temperature causes the specific gravity to vary, it should 

 always be corrected accordingly. The usual temperature for the 

 standard is 15° C. or 60° F. 



The specific gravity increases immediately after milking for 

 about twelve hours, due perhaps to the loss of air-bubbles, but 

 also ascribed by Recknagel to the contraction of the casein. This 

 phenomenon is spoken of as the Recknagel phenomenon. At 

 low temperature this rise in specific gravity is greater than at high 

 temperature. Richmond thinks the change is due to an increase 

 in density caused by the contraction of the fat. Milk reaches the 

 point of greatest density at a temperature slightly below the 

 freezing-point of water and somewhat above the freezing-point of 

 milk. 



Freezing-point. — The freezing-point of milk is 0.54° to 0.57° C. 

 lower than that of water. Variations from these figures due to 

 difference in composition are slight. After acidity has developed 

 the freezing-point is still lower. 



It is possible to determine the addition of water to milk by 

 finding out the freezing-point, although dilution with less than 

 10 per cent, cannot be detected with certainty. However, if 

 salt solution (0.9 per cent.) is used for diluting, the freezing-point 

 is not ' affected, as salt solution of this strength has the same 

 freezing-point as milk. 



Sometimes variations in the freezing-point of milk from the 

 same animal have been observed. Probably the food consumed 

 contains a variable amount of mineral matter, which may have 

 such an influence on the freezing-point. Boiled milk has a lower 

 freezing-point than raw milk. 



Electric Conductivity. — Milk, hke other solutions containing 

 salts, has electric conductivity showing the presence of ions. It 

 has been thought that the degree of conductivity might serve as 

 a measure of dilution with water, but this has been a disappoint- 

 ment because of the great variability of conductivity in different 

 milks. No standard can be established in view of this variability. 

 The conductivity varies in milk from different animals and changes 

 with the progress of lactation. Creaming increases electric con- 

 ductivity, while boiling reduces it. Udder diseases and souring of 

 milk increase the conductivity. 



Refractivity. — The refractive index of milk is usually deter- 

 mined in milk serum, prepared either by boiling the milk with 

 acetic acid or precipitating the casein with a solution of calcium 

 chlorid. The purpose of this is the detection of dilution with 

 water. To obtain comparable results the serum must always be 

 prepared according to the same method. Fluctuations of the re- 



