TYPKS OK IJACTKKIA FOTM) IN MILK 23 



would be indicated by the simple equation above. Its exact 

 nature is not understood even by chemists, and it is sufficient 

 for the dairyman to know that milk sugar is changed to lactic 

 acid. The lactic acid causes the milk, first, to turn sour, as in- 

 dicated by chemical test, or by its taste; and, later, when the 

 acid rises to .7 or .9 per cent., the milk curdles. This phenomenon 

 is also a somewhat complicated one. Milk contains casein in 

 a state of partial solution. As long as the liquid is slightly 

 alkaline, this partial solution is maintained, but the casein will 

 not remain dissolved in an acid solution. As soon as the liquid 

 becomes sufficiently acid, the casein, by a well known chemical 

 law, is precipitated and the milk curdles. Any other acid will 

 produce exactly the same result, as may be proved easily by 

 pouring into milk a little vinegar, or any other acid. The 

 curdling of milk is produced by the appearance of an acid re- 

 action. Curiously enough the three factors, bacteria growth, 

 milk sugar destruction and acid- formation, though causally 

 connected, do not run parallel, sometimes one and sometimes 

 another of the three running ahead of the others. In other 

 words, it is impossible to determine the amount of acid from 

 the number of bacteria present. * The reasons for this are not 

 well understood. 



Souring of Milk and Thunder Storms. 2 We have just no- 

 ticed that the souring of milk is always the result of bacterial 

 action. There has been a wide-spread belief that thunder storms 

 bring about the souring and curdling of milk, and the general 

 opinion has been that this is due in some way to the direct 

 action of thunder or lightning. It frequently does happen that 

 milk will sour and curdle during a thunder storm, but the con- 

 clusion that there is any real connection between the two is a 

 mistake. The suggestion that electricity has some direct effect 

 upon milk was advanced some years ago and carefully tested; 



1 Haacke. Arch. f. Hyg. xliv., p. 16, 1902. 

 3 Treadwell, Science, xvii., p. 178, 1894. 



