BIOGRAPHICAL MEMORANDA 



can be interconverted through their common enol form. Nevertheless, 

 the reports indicating that some glucose-fermenting yeasts did not fer- 

 ment mannose could not be dismissed offhand; and if such claims 

 could be corroborated it would, of course, have become possible to 

 determine mannose separately by appropriate fermentation tests.* 

 This provided an additional incentive for a thorough re-examination 

 of the yeasts for which a differential behaviour towards glucose and 

 mannose had been observed. The results showed that the previous 

 claims could not be upheld ; all these strains fermented glucose as well 

 as mannose. But the new experiments indicated why negative results 

 had sometimes been recorded ; this was usually the case when the fer- 

 mentation rate was low, and the tests were conducted in an apparatus 

 in which the inoculated sugar solution was in contact with air. Owing 

 to the relatively high solubility of carbon dioxide in aqueous solutions, 

 coupled with a rapid diffusion of this gas into the atmosphere, an ac- 

 cumulation of sufficient magnitude to cause its appearance in the 

 form of a detectible gas phase might thus be prevented. A corollary 

 of this situation is that the amount of yeast used as inoculum and its 

 rate of growth, hence of fermentation, in the medium supplied may 

 yield spurious results unless a completely closed system is used. Since 

 the Van Iterson-Kluyver apparatus represents such a system, with 

 mercury as the movable barrier, carbon dioxide cannot escape, and 

 will appear as a gas phase as soon as the aqueous solution has become 

 saturated. Under atmospheric pressure this happens when about i ml 

 of carbon dioxide has been formed per ml of solution; this requires the 

 fermentation of approximately 4 mg of sugar. If the concentration of 

 the fermentable sugar is above 0.4 per cent, the result of a fermentation 

 test can never be obscured by a low fermentation rate ; eventually gas 

 production will become observable. The test can be made even more 

 sensitive by examining the result under reduced pressure. 



The empirical rule concerning the fermentation of sucrose and raf- 

 finose also seemed to be challenged by the results of other investiga- 

 tors. But in this case Kluyver's attitude was perhaps more critical. 

 Raffinose, as a galactose-glucose-fructose trisaccharide, possesses a 

 sucrose configuration, which made the regularity he had observed 

 with his own collection of yeasts readily intelligible if not downright 

 predictable. Abandoning what thus appeared like a logical inference 

 * See also p. 109. 



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