THE RELATION OF MICROORGANISMS TO MILK. 



39 



leuco-compound. Through the action of the oxygen of the air the litmus 

 is slowly reoxidized and the pink layer, which immediately after curdling is 

 but a few millimeters in depth, is slowly extended until the entire mass of 

 curd has a uniform pink color. Saccharose, dextrose, maltose, and 

 mannit are fermented. 



The maximum amount of acid produced by organisms that are most 

 typical of the group is determined by the composition of the medium. 

 It is often said that the organisms causing the normal souring of milk 

 represent a group that can grow in a strongly acid medium. This is true 

 as far as acid salts are concerned, but free acid totally inhibits growth. In 

 a culture medium, which contains no substance that can combine with the 

 acid formed and thus remove it from the sphere of action, no growth or 

 but very slight growth occurs. In sugar bouillon and in milk, the amount 

 of acid formed is determined by the content of substances in these liquids 

 that can combine with the acid. In milk such compounds are the casein 

 and some of the ash constituents, especially the phosphates. In normal 

 milk, the maximum acidity attained ranges from 0.9 to 1.25 per cent cal- 

 culated as lactic acid. If the content of neutralizing compounds per unit 

 volume is varied by concentration, dilution, or by the addition of such 

 substances as calcium phosphate, the maximum amount of acid produced 

 by typical cultures will be changed. In sugar bouillon the maximum 

 acidity produced rarely exceeds 0.25 per cent. 



The fermentation of lactose is usually expressed as follows: 



Ci 2 H 22 Oii + H 2 O = 4C 3 H 6 O 3 



Thus 342 parts of lactose should yield 360 parts of lactic acid. The 

 theoretical yield of lactic acid is never obtained, for the action of the organ- 

 ism on the carbohydrate is much more complex than is represented by the 

 equation given. In the following table are given data obtained by a num- 

 ber of investigators: 



