VOLATILE ACIDS. 193 



At the Government Laboratory the percentage of alcohol is 

 deduced by multiplying the difference between 1 and the specific 

 gravity of the distillate by 1,000 X 1-16, and this gives it as 

 percentages of proof spirit ; as proof spirit contains 49-5 per cent, 

 alcohol by weight, it is evident that the factor 1,000 X 0-572 

 will give the percentage of alcohol by weight. 



The total acidity to litmus paper may be calculated as lactic 

 acid ; from this an amount equivalent to the volatile acids is 

 subtracted. 



The method used at the Government Laboratory for the 

 estimation of volatile acids is as follows : 



Ten grammes of milk are neutralised to the extent of one-half 



N 

 the total acidity with r NaOH, and a little phenol- phthalein 



added. The mixture is then evaporated to dryness on a water- 

 bath with frequent stirring, and, after treatment with 20 c.c. 

 of boiling distilled water, so as to break up and detach the 

 milk solids thoroughly from the capsule, a further addition of 



N 



NaOH is made, until the neutral point is reached. The 



difference between the original acidity of the milk and that of 

 the evaporated portion is regarded as acetic acid. The number 

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

 percentage of acetic acid. 



Example 

 Acidit 

 Acidity of evaporated portion . . =9-2 



N 

 Acidity of original milk . . . = 11-6 c.c NaOH. 



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 0-1 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 then it is preferable to employ the following 

 modification of Duclaux's method : 



Add to the milk from which the alcohol has been distilled 

 a quantity 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 off till the 

 distillate is practically neutral. 



The mixed distillates neutralised to phenol-phthalein are made 

 up to 250 c.c., and an aliquot portion, preferably taking 20 to 



13 



