LACTIC FERMENTATION 269 



mixed in equal proportions the resulting solution has no 

 action on polarized light, one neutralizing the effect of 

 the other. The acid in such a solution is spoken of as 

 inactive lactic acid. Many kinds of the lactic bacteria 

 produce the inactive acid in milk and solutions 

 containing sugar and other carbohydrates. Formerly 

 this substance was spoken of as fermentative lactic acid. 

 Some organisms, however, are now known which produce 

 dextro-lactic acid, others the laevo-lactic variety. Perhaps 

 the majority produce the former, although the power 

 of a certain kind of bacterium of producing a definite 

 kind of lactic acid is dependent on the carbohydrate 

 fermented as well as on the character of the medium 

 and other factors. The acid produced under similar 

 conditions may be used as a diagnostic character to assist 

 in the discrimination of varieties or races of nearly allied 

 kinds. 



Over a hundred so - called species or kinds of 

 bacteria have been described which have the power of 

 producing lactic acid from various sugars and other 

 carbohydrates. Some of them have not been found in 

 milk, nor are they ever likely to occur in it. A very 

 large number, however, have been obtained from milk 

 and dairy products ; and bacteriologists, who have been 

 concerned with their investigation, have often bestowed 

 on them different specific names in such a way as to 

 create the impression that they were distinct and unre- 

 lated. There is little doubt, however, that many of them 

 are closely related but slightly modified forms of the 

 same organism, and that the number of fundamentally 

 different species is comparatively few. Indeed, among 

 organisms whose form, physiological power, and other 

 diagnostic characters are so readily modified by their 



