New York Agricultural Experiment Station. (yj 



THIS CHECKING IS DUE TO THE CHANGE OF SUGAR INTO ACID. 



The ability to form acid, as the name imphes, is the point of 

 similarity upon which the classification of the lactic-acid group is 

 based. Shirokich^ has shown that when representatives of this 

 group were grown in milk the nitrogenous portion was not at- 

 tacked to a measurable extent. Chodat & Hoffman-Bang^ 

 analyzed milk cultures of five lactic-acid organisms and found 

 that at the end of five weeks the milk sugar had decreased i to 

 1.25 per ct. A considerable portion of this sugar was converted 

 into acid. While the larger part of this acid was believed to be 

 lactic they determined the presence of small quantities of formic, 

 valeric and acetic acids. 



The destruction of this amount of sugar does not measure the 

 limit of the ability of the germs to attack the sugar but rather 

 shows the limit of the ability of the compounds present to dispose 

 of the acid as formed. In similar milk cultures to which chalk 

 had been added to combine with the excess of acid as formed, all 

 of the 5.77 per ct. of sugar had disappeared in four of the five 

 flasks at the end of five weeks. 



The majority of organisms aside from this group prefer a 

 neutral or even faintly alkaline reaction, while the lactic-acid 

 organisms grow best in a slightly acid reaction. As soon, there- 

 fore, as they break up an appreciable quantity of sugar into acid 

 they place their competitors in the milk at a disadvantage and 

 this disadvantage increases with the increased formation of acid. 



THE EFFECT OF THIS ACID UPON THE CURDLING ACTION OF THE 



RENNET. 



The direct relation between the extent of acid formation and 

 the rate of rennet action is well known. This fact is taken ad- 

 vantage of in practical cheese-making by the addition of partially 

 soured starters to sweet milk in order to quicken the rate of co- 

 agulation. The effect of these starters is twofold. They have an 

 immediate effect due to the acid which has already been formed 

 and a later effect due to the increased rate at which the sugar of 

 the milk is changed to acid. 



3 Schirokich. Ann. Past., \'2,:i,oo. (1898.) 



* Chodat & Hoffman-Bang. Ann, Past., 15:37. (1901.) 



