176 MILK 



parturition differs markedly from ordinary milk. It is a thick, 

 slimy, yellow liquid, containing about 4.5 per cent, of casein and 

 15-5 per cent, of albumin, and owing to the large proportion of 

 the latter protein, colostrum coagulates very readily on heating. 

 The fat of colostrum (3-4 per cent.) is poor in volatile acids, and 

 hence gives a low Reichert-Meissl number (p. 200). 



Sampling. 



Owing to the fact that the fat globules present in milk are 

 considerably lighter than the milk serum in which they are sus- 

 pended, it follows that milk that has been standing for some 

 time will be no longer homogeneous, owing to a rising of the fat 

 (cream) to the surface. 



Before carrying out any tests or quantitative operations with 

 milk, even if the milk has been standing for only a short time, 

 it is, therefore, necessary that it be thoroughly mixed, in order 

 that the sample obtained may be a representative one. The 

 mixing is conveniently effected by pouring the milk three or four 

 times from one vessel to another, and then immediately with- 

 drawing the sample before the lighter fat globules have had time 

 to rise to the surface. 



In the case of milk that has been standing for some time, it 

 is often found that the cream layer does not readily mix with the 

 rest of the liquid. By warming to a temperature of about 40° C, 

 however, it will be found that the process of mixing can be readily 

 performed, and a representative sample thus obtained. 



175. Qualitative Examination of Milk. 



(i.) Albumin. — About 20 c.c. of milk are heated in a 

 porcelain basin on the water-bath for a short time, and 

 the skin of albumin formed on the surface is removed, 

 washed with water, and examined by the xantho- 

 proteic and biuret reactions (p. 15). 



(ii.) Mineral Ash. — The liquid remaining in the basin 

 after the removal of the albumin is evaporated to 

 dryness, any skin on the surface being kept broken 

 by stirring with a glass rod. The contents of the dish 

 are then ignited over a Bunsen burner or in the muffle 

 furnace, the carbonaceous residue containing the 



