SELECTED PAPERS 



ever meet an organic chemist who is so ambitious that he will endea- 

 vour to synthesize a French 'vin de chateau' or one of the many dairy 

 products which owe their merits to their characteristic flavour, such 

 as Roquefort cheese. Here, there will always be a task for the micro- 

 biologist, namely to direct microbial life into the required paths. 



However, still more important than the preparation of these com- 

 plex luxuries will be the conscious preparation of certain valuable 

 products of microbial metabolism, of such particular structure that 

 their purely chemical synthesis cannot be achieved, at least not eco- 

 nomically. Various factors may be responsible for the latter restriction. 

 In some cases this condition may already be fulfilled, although the 

 required compound has a simple configuration. This holds, for in- 

 stance, when one is dealing with compounds containing an asymmet- 

 rical carbon atom ; synthesis will always produce the two stereoisomers 

 in equal quantities, whilst owing to the asymmetrical character of the 

 catalysts active in the microbial cell one of the isomers is often formed 

 either preferentially or exclusively. This difference may be of decisive 

 importance. The purely chemical preparation of lactic acid has made 

 much progress in recent years. In as far as the lactic acid is prepared 

 as a constituent of products for human consumption there is every reason 

 to demand that the acid should be the physiological form, i.e., 1( + ) 

 lactic acid. The product of organic synthesis will never meet this 

 requirement; on the other hand, the industrial microbiologist will be 

 able to prepare the right form by a deliberate selection of the bacterial 

 strain to be applied in the fermentation (Table IX). 



TABLE IX 



Stereoisomers of lactic acid produced by representatives of different 

 genera of lactic acid bacteria 



Thermo bacterium d ( — ) or i Micro bacterium 1 ( + ) 



Streptobacterium 1 (-(-) or i Streptococcus 1 ( + ) 



Betabacterium i Betacoccus 1 ( + ) or i 

 Pediococcus i 



A somewhat similar case occurs in the preparation of a compound 

 like 2,3-butanediol which is known to exist in three configurations: 

 laevo-, dextro-, meso-. 



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