METHOD OF PRODUCING LACTIC ACID. 75 



growth of bacteria to a considerable degree ceases, but the 

 production of lactic acid goes on for some time. It is also a 

 fact that the disappearance of milk sugar, out of which the 

 lactic acid is doubtless formed, does not proceed parallel with 

 the production of lactic acid nor with the growth of 

 bacteria (47). 



No.w, while it is not an absolutely necessary conclusion 

 that the production of la'ctic acid should be parallel with the 

 growth of bacteria if the phenomenon is the result of the 

 vital processes, still such would be the natural inference. The 

 fact that the two curves are not parallel is certainly more in 

 accordance with the belief that the lactic acid is the result 

 of an enzyme produced by the bacteria. If an enzyme is thus 

 produced it is easy to understand that for the first few hours 

 of bacteria growth not enough of the enzyme is secreted to 

 produce lactic acid, but that after the lactic bacteria have 

 grown several hours they develop an enzyme .which begins 

 to work at once upon the milk sugar, causing a rapid conver- 

 sion into lactic acid. Later the presence of lactic acid thus 

 formed checks the growth of bacteria, as will be seen in the 

 next chapter, but the enzyme which has been developed is 

 not so rapidly checked in its action, and continues to develop 

 lactic acid for some time even after the growth of the bac- 

 teria has stopped. These facts together, then, suggest that 

 the production of lactic acid by these microorganisms is not 

 the direct result of growth, but rather due to the action of 

 some enzyme that is secreted by the bacteria. 



ENZYME-FORMING BACTERIA. 



We have just noticed that the lactic bacteria may possibly 

 produce enzymes, although this has not as yet been demon- 

 strated. The group of bacteria that we are now to consider 

 certainly does produce enzymes. They are commonly not 

 capable of producing acid, although a few, as mentioned on 

 page 71, do produce a small amount. Many of them produce 



