320 



PHYSICAL PROPERTIES OF MILK 



and nitrogen. From a litre of milk, Pfliiger (1869) obtained 1 

 oxygen, 76 cc. carbon dioxide and 7 cc. nitrogen. 



cc. 



COWS' MILK. Cows' milk, being the most important from a 

 commercial and agricultural standpoint, has been studied much more 

 thoroughly than the milk of other animals. 



Cows' milk is a white, or yellowish white, opaque liquid of sweet 

 taste. Its specific gravity varies usually between 1/027 and 1*034, 

 but in certain cases may be outside these limits. 



When freshly drawn and quickly cooled, milk has a certain 

 specific gravity, but shows a decided increase in density (about '0005) 

 on keeping at the same temperature for some hours. This phenome- 

 non, known as Eecknagel's phenomenon, 1 has been attributed to a 

 molecular change in the casein and to the presence of air bubbles, 

 which gradually escape ; but is more likely to be due, as suggested by 

 Eichmond, to the fact that the fat globules, liquid at the temperature 

 of the cow, do not at once solidify on cooling, but remain for some 

 time in a super-cooled liquid condition. Since contraction occurs 

 when milk fat solidifies, their slow solidification during standing 

 would cause an increase in density. 



The maximum density of milk is, unlike that of water, coincident 

 with its freezing-point, about - 0'55 C. or - 0'3 C. (Fleischmann). 

 It expands when heated at a rate which, naturally, is dependent upon 

 its composition, but is usually about -0002 for each degree Centi- 

 grade. 



When milk is partially frozen, the solid and liquids show consider- 

 able differences in composition. If the milk be kept at rest, the usual 

 rise of cream leads to the uppermost layers of spongy ice containing a 

 high proportion of fat, but the real ice, actually frozen upon the walls 

 of the containing vessel is poorer in fat and much poorer in solids-not- 

 fat than the remaining liquid portion of the milk. 



Mai found 2 that when 10 litres of milk were kept in a vessel ex- 

 posed to a temperature of 15 C. for thirty hours, the various portions 

 had the following composition : 



It is evident that the distribution of the suspended matter, particu- 

 larly the fat, is determined by purely mechanical (gravitational) 



1 Berichte, 14, 2684. 



2 Zeitsch. Nahr. Genussm., 1912, 23,250; Jour. Chern. Soc., 1912, Abstracts, 

 ii. 580. 



