74 THE TYPES OF MILK BACTERIA. 



Method of Producing Lactic Acid. From the very outset 

 the question of the exact method by which bacteria produce 

 lactic acid from milk has been much discussed. That it is 

 produced from the milk sugar seems certain, but a primary 

 question arises, whether the lactic acid is produced directly 

 by the growth of the microorganisms, or indirectly by the 

 production of a chemical ferment or enzyme. These two 

 views as to the possible action of the lactic organisms have 

 been held from the very beginning of their study. In recent 

 years there has been a tendency on the part of biologists to 

 refer a large number of the bacterial fermentations to the 

 direct action of enzymes. Bacteria certainly do secrete 

 enzymes as the results of their life processes. This has been 

 abundantly proved in regard to some bacteria, and a similar 

 fact has been shown to explain the alcoholic fermentation by 

 yeast. Whether the lactic bacteria produce a similar enzyme 

 is not yet known, for no lactic enzyme has been yet separated 

 from them. 



There are, however, some reasons for believing that the 

 production of an enzyme by lactic bacteria is the simplest 

 explanation of the lactic fermentation. If the lactic acid was 

 produced by the bacteria seizing certain ingredients in the 

 milk sugar for their nutritive processes and thus producing 

 a destruction of the milk sugar molecule and its conversion 

 into lactic acid, it would naturally follow that the amount of 

 lactic acid produced would be directly proportional to the 

 growth of the bacteria. Now, while there is a general par- 

 allel between the growth of these microorganisms and the 

 production of lactic acid, the parallel is by no means a close 

 one. For a considerable time after milk is inoculated with 

 lactic bacteria there is no increase in the amount of lactic 

 acid, although the bacteria are themselves multiplying 

 rapidly and becoming very numerous. Then there is a 

 rapid increase in the development of lactic acid, accompany- 

 ing a still further growth in the bacteria. Finally, the 



