SOUR MILK. 191 



the lowering of the specific gravity of the fresh milk, the correc- 

 tion to be made for the ammonia is deduced ; this is usually 

 0-0026 or 0-0027, or 2-6 to 2-7 lactometer degrees. 



de Koningh proposes the use of a solution of caustic soda of 

 specific gravity 1-030 instead of the ammonia ; he finds that 

 on mixing 5 c.c. of this solution to 100 c.c. of milk a lowering 

 of the specific gravity by 0-0008 or 0-8 degree takes place. The 

 author and Harrison find that with fresh milks the lowering is 

 less, 0-0003 on the average, and increases with the acidity of the 

 milk ; the cause of this lowering is shown by their experiments 

 to be due to the fadrthat the specific gravity of a solution of a 

 sodium salt is always less than the sum of the specific gravities 

 of equivalent solutions of caustic soda and an acid (less 1). The 

 following procedure has given good results in the author's hands : 

 Add sufficient solution of caustic soda (sp. gr. 1 -032) to render 

 the mixture with the sour milk distinctly alkaline to phenol- 

 phthalein ; determine the specific gravity of the mixture and the 

 acidity of the sour milk (see p. 179). To the figure found for the 

 specific gravity add the acidity in degrees multiplied by 0-000022, 

 and subtract 0-0001 ; thus, if the specific gravity found be 

 1-0305, and the acidity be 70, the corrected specific gravity is 



1-0305 + 70 x 0-000022 - 0-0001 = 1-0305 + 0-00154 - 0-0001 - 1-0319. 



Estimation of Proteins. The proteins precipitated by the acid 

 developed in the milk are filtered off, and either weighed, or 

 the nitrogen is determined in them ; in the filtrate albumin 

 is estimated as on p. 175. in the filtrate from this, albumoses 

 are estimated by precipitation with tannin or phospho-tungstic 

 acid, and determining the nitrogen in the precipitate. 



The total nitrogen is estimated by Kjeldahl's method on a 

 weighed portion. 



Certain changes take place in the proteins in sour milk, and 

 at the Government Laboratory to estimate ammonia 2 grammes 

 of the milk are made up to 100 c.c. with distilled water, and 

 filtered to a clear solution. Ten c.c. of the filtrate, increased 

 to 50 c.c. by the addition of distilled water, are nesslerised against 

 NH 4 C1 solution, equivalent to 0-01 milligramme of NH 3 in each 

 c.c. As the Nessler colour produced in the presence of milk 

 differs somewhat from that of pure saline ammonia, the blank 

 experiment is carried out with the addition of 10 c.c. of the 

 filtrate from 2 grammes of new milk slightly acidified, and 

 diluted to the same extent as the sour milk. The quantity of 

 test ammonia solution required varies from 0-5 to 4-0 c.c. In 

 the case of a milk containing ammonia equal to 2-6 c.c. of the 

 test solution, the ammonia is calculated as follows : 

 0-01 X 2-6 x 500 = 0-013 per cent, ammonia. 



