ANALYSIS OF SOUR MILK. 143 



of c.c. of soda shown, when multiplied by 0'06, will give the 

 percentage of acetic acid. 



Exa mple — 



Acidity of original milk . . . = ii-e c.c. — NaOH. 



Acidity of evaporated portion . . = 9-2 „ 



Difference . . . = 2-4 „ 



or, 2-4 X 0-006 X 10 = 0-144 per cent, of acetic acid. 



The author and Miller have shown that this method is only- 

 accurate when the volatile acidity lies between O'l and 0-2 per 

 cent, of acetic acid, as is the case in the majority of sour 

 milks. 



It sometimes happens that a considerable quantity of butyric 

 acid is formed, and it is then preferable to employ the following 

 modification of Duclaux's method : — 



.Vdd to the milk from which the alcohol had been distilled 

 a (|uantity of acid exactly equal to the soda used for half neut- 

 ralising, and distil this to a small bulk ; water is then added 

 in successive quantities of about 25 c.c, and distilled ofi till the 

 distillate is practically neutral. 



The mixed distillates neutralised to phouolphthaleiu are made 

 up to '250 c.c, and an aliquot portion taken for fractional dis- 

 tillation ; to this is added a quantity of N sulphuric acid, very 

 slightly greater than is necessary to neutralise the soda used. 

 Eif^'ht successive fractions, each about one-ninth of the total 



N 

 volume, are distilled, and separately titrated with — strontia ; 



2.") c.c. of water is added to the residue, and a further 25 c.c. 

 distilled and titrated, and, if desired, this treatment may be 

 repeated. From the last titration the total quantity of volatile 

 acid is obtained by extrapolation of the results ; the total quantity 

 is always slightly less than the total acidity of the first distillate, 

 due, no doubt, to the presence of a little lactic acid. As much 

 as possible of the first distillate is made up to a convenient bulk, 

 after adding a quantity of N sulphuric acid in slight excess of 

 that required to neutralise the soda, and exactly one-third is 

 distilled ; this is made up to a convenient bulk, and exactly 

 one-third is again distilled. The last distillate is made up to 

 100 c.c, and eight fractions of about 11 c.c each are distilled 

 and separately titrated. The residue in the flask and condenser 

 is washed out and titrated to obtain the total quantity. 



