FERMENTATION OF DEXTRIN BY YEAST. 539 



they subjected to the action of commercial, but starch-free, 

 pressed yeast. For example, 40 grms. of starch sugar were 

 dissolved in 250 c.c. of water containing nutrient salts, the 

 glucose being eliminated by fermentation. The resulting solution, 

 corrected to the original volume, gave a polarimeter reading of 

 + 7.65 in the first experiment, and +6.90 in the second. 

 After pitching the filtered solution with 5 grms. of pressed 

 yeast, the polarimeter readings at the end of 6 days receded to 

 3.35 and 2.75 respectively; and after a further sojourn 

 of 5 days at 30 C. the reading had fallen to 1.9 in both cases, 

 an additional diminution ensuing after the addition of more 

 yeast. In other experiments the same workers succeeded in 

 eliminating the dextro-rotatory substances completely ; but the 

 process cannot be regarded as a true fermentation of dextrin in 

 our sense of the term, neither carbon dioxide nor alcohol being 

 produced. 



E. VON RAUMEII (I.) also, in the course of examining certain 

 honeys suspected of containing added starch suger, endeavoured to 

 ascertain how far dextrins can be fermented by yeasts. His 

 results showed that pressed yeast, beer yeast and wine yeast 

 behave differently, the first named attacking the dextrins power- 

 fully, the latter not at all. 



FRESENIUS (III.) was able to confirm the reports of Medicus 

 and Immerheiser. According to his researches the so-called 

 unfermentable dextro-rotatory substances are also fermentable by 

 pressed yeast, though not affected by beer yeast. Fresenius, 

 moreover, drew attention to the circumstance that film yeasts 

 also eliminate dextrins. 



Hence, it is important for the analyst who has to examine 

 honey, or wine for added starch sugar, to know that he should 

 never employ commercial pressed yeast. The " Convention," 

 VEBEINBARUNGEN (II.), prescribes that 25 grins, of honey be 

 dissolved in 200 c.c. of a nutrient solution and sterilised, and 

 after cooling should be pitched with 5 c.c. of fluid, vigorous and, 

 preferably, pure-culture wine yeast, not pressed yeast, fermentation 

 being conducted at 2o-25 C. The liquid is then made up to 

 250 c.c. with water, after being clarified with alumina, or 3 c.c. 

 of lead acetate and 2 c.c. of sodium sulphate solution, and is 

 examimed in the polarimeter at 20 C. If the fermentation 

 residue exhibits any considerable dextro-rotation, it should be 

 examined for dextrin by precipitation with alcohol. 



In the examination of wine for the presence of impure starch 

 sugar, K. WINDISCH (VI.) recommends that 2 10 c.c. of the wine be 

 concentrated to one-third by evaporation, and then diluted with 

 water until the liquid contains not more than 15 percent, of sugar. 

 This solution is pitched with about 5 grins, of vigorous beer 

 yeast that is free from optically active constituents, and fermen- 

 tation is carried out to completion at a temperature of 2O-25 C. 



