270 MILK, BUTTER, AND CHEESE [chap. 



milk sugar forms another 4J per cent. In addition to 

 these three carbon compounds, the milk solids contain 

 certain inorganic materials, chiefly phosphates and 

 chlorides of calcium and potassium and sodium, some 

 of which inorganic materials are, however, combined 

 with the proteins. 



Milk Fat. — The fat of milk is there present in a 

 state of finely divided globules varying in size from 

 o-oi mm. to o-ooi mm., i.e. from 25^00 of an inch to yV 

 of that diameter. When examined under the micro- 

 scope these minute globules appear to have a skin upon 

 the surface, but this skin probably consists of nothing 

 more than adhering particles of the casein, which also 

 is not truly dissolved in the water of the milk. The fat 

 globules vary in size, the milk of Jersey cows contain 

 the largest, in the Guernseys and in the other Channel 

 Island breeds they are almost as large, and they are 

 also well above the average size in the milk of Kerrys. 

 In the milk of the Shorthorns, globules of various sizes 

 may be found, but the small predominate ; the Welsh 

 and South Devon, the Dutch and Holstein races, 

 yield milk containing rather small globules. In these 

 globules the fat is present in the liquid state, although 

 the temperature of the milk may have cooled down 

 below the temperature at which the fat solidifies in 

 bulk. In such small particles, however, the fat remains 

 liquid in a supercooled state, only assuming a solid 

 condition when the globules are beaten together, as in 

 the act of churning. The fat is lighter than water, 

 possessing a specific gravity of 0-93. In consequence, 

 the globules tend to rise upwards through the heavier 

 milk serum, and they are only hindered from rising 

 rapidly by their smallness and the high viscosity of the 

 milk serum. The larger the particles, however, the 

 more quickly do they rise, as may be seen in the quicker 



