i MILK 33 



c.c. of Dornic's solution are equivalent to 10 c.c. JV/10 acid. As 

 the molecular weight of lactic acid is 90, O'l c.c. of Dornic's 

 solution is exactly equal to 1 mg. lactic acid. Dornic has 

 constructed a special burette so graduated that one division 

 = 01 c.c., so that the amount of lactic acid in milligrams can 

 be directly read. The number of burette divisions of caustic 

 soda solution required to neutralise 10 c.c. of milk is known as 

 the degrees of acidity, according to Dornic, of that particular 

 milk. As indicator, 5 drops of a 2 per cent, alcoholic solution 

 of phenolphthalein are used. 



Fresh cow's milk has an acidity of 16 20 degrees (Dornic). 



II. DETERMINATION OF THE DRY MATTER OR 

 TOTAL SOLIDS. 



To determine the dry matter in milk, a weighed quantity of 

 it is dried at 100 C. until the weight is constant. The loss of 

 weight represents the water which has evaporated, the remaining 

 weight is that of the dry matter. 



The desiccation of the milk is usually done in a basin 

 containing ignited sand or pumice stone. In recent times the 

 flat nickel basins advocated by Soxhlet are much used. The 

 basin with the sand is first weighed after the whole has been 

 dried for an hour at 100 C. in the drying-oven. About 10 c.c. 

 of the milk under investigation are put into a small beaker 

 provided with a watch-glass for a cover. After the beaker and 

 the contained milk have been weighed together, the latter is 

 poured on to the sand in the basin, and the empty beaker again 

 weighed. The difference between the two weights gives the 

 weight of milk used. 



The basin is then placed in the water- oven in which the 

 drying, which generally takes several hours, is to be done. 

 Under no circumstances must the drying be regarded as com- 

 plete until two successive weighings give the same result. 



With the aid of Soxhlet's glycerine drying-oven (Fig. 11) 

 the milk can be dried more rapidly, for this apparatus is heated 

 at 105 C., and a strong current of air passes through it. In 

 some laboratories a vacuum drying-oven is used for the 

 determination of water in milk. 



According to Fleischmann l a loss of dry matter can arise 

 1 Lehrbuch der Mildiwirthschaft. Fourth Edition. Leipsic, 1907. 



D 



