170 



THE FKEEZING POINT OF MILK. 



proportion of 0-5 per cent, nitrogen and 0-7 per cent, ash,, 

 found in normal milk), is higher than that estimated from 

 the freezing point of the .sample. 



We have not done sufficient work to enable us to say 

 whether any definite relationshix) exists between the. 

 depression of the freezing point and the increase of acidity. 

 It is very doubtful if any constant factor could be obtained 

 to correct for the acidity as the fermentation jproducts 

 would vary with the nature of the ferment and the time and 

 temperature of the reaction. Barthel states that the 

 percentage of water can be determined fairly accurately 

 if the acidity is 20° (Thorner) by adding 5 per cent, to the 

 result of Winter's formula. Figures obtained by us, on a 

 small number of samples only, indicate that 1 per cent, 

 increase of acidity covers the addition of 0-5 per cent, of 

 water. It is to be noted that any eiTor introduced by the 

 increased acidity is entirely in favour of the milk vendor. 



TABLE A. 



The acidity of milk is determined by Dr. Chairman's 

 method, using 25 c.cs. of milk, 100 c.cs. of Avater, and l.c.c. 

 of 0*1 per cent, of phenol phthalein solution and titrating 

 with -p^ NaOH. The normal acidity of milk by this method 

 is about 13*5 c.cs. .^ NaOH per 100 c.cs. of milk. 



