SOME PHYSICO-CHEMICAL MEASUREMENTS ON MILK. 181 



The mean increase in conductivity from Ave experiments 

 for the removal of 5 per cent, of fat was 11*4 per cent, of 

 the conductivity. It was also found that the decrease in 

 conductivity for the addition of 5 per cent, of water cor- 

 responded to a decrease of 3*6 per cent. This shows that 

 the increase in conductivity due to the removal of 5 per 

 cent, of fat is greater than the increase which would be 

 due to a removal of 5 per cent, of water, which can be 

 explained by attributing to the fat globules an obstructing 

 influence on the ions in their passage through the solution. 



The Hydrogen Ion concentration of milk. 

 The presence of H ion in milk is due to the hydrolysis 

 of certain of the salts present in it and the dissociation of 

 the lactic acid produced in its decomposition. This decom- 

 position is brought about, in the case of healthy cows, by 

 the action of those bacteria which have gained access to 

 the milk either after or during the process of milking. 



Among the bacteria which gain access are those having 

 the power to form lactic acid from the milk sugar; these 

 are of two kinds: — 



(1) E. lactis cerogenes and its allies, 



(2) B. acidi lactici or Streptococcus lacticus. 



The former forms in addition to lactic acid, volatile acids, 

 ethyl alcohol etc., while the latter forms almost exclusively 

 pure lactic acid. It has been shown by Heinemann 1 that 

 milk allowed to sour naturally at about 20° 0. contains 

 chiefly dextro-lactic acid, that soured at 37° O. chiefly 

 racemic acid with laevo in excess if allowed to stand for 

 several days. The lactic acid formed by these bacilli is 

 not produced directly from the lactose but from the simpler 

 sugars glucose and galactose, formed from the lactose by 

 the action of certain enzymes. The decomposition may 

 also be carried to butyric acid with the evolution of hydrogen 

 and carbon dioxide. 



1 Journal of Biological Chemistry, Vol. n, No. 6, p. 603, 1907. 



