546 ENZYMES DECOMPOSING SACCHARIDES. 



commercial analyses a blank experiment has been recommended, 

 in which the same quantity of yeast is employed with the liquid 

 under examination in the one case, and with an equal volume of 

 distilled water or nutrient solution free from sugar in the other. 

 If the sugar content be determined from the carbon dioxide 

 liberated, it has been thought that these precautions would 

 eliminate the influence of the autofermentation of the yeast. 

 The experiments of C. J. Lintner, however, have shown that the 

 saline matters present act differently on autofermentation ; and 

 therefore, as the full composition of the liquid under examination 

 is generally unknown, it is impracticable to carry out the blank 

 experiment in such a manner that the yeast acts in a liquid that 

 is identical with the one in question, except that sugar is absent. 

 In the blank experiment one is working with unknown quantities, 

 whose influence on autofermentation is still undetermined, so 

 that new sources of error are introduced to an indefinite extent. 

 It has been proposed to stop the fermentation experiment at the 

 point at which a preliminary test has shown that the sugar of the 

 counter experiment is completely fermented ; but, as was seen on 

 p. 173, vol. ii., the degradation of the glycogen begins at a time 

 when fermentable sugars are still present in the nutrient medium. 

 According to Jodlbauer, this danger does not exist when not 

 more than two parts of moist yeast (containing about 25 per cent. 

 of dry substance) are used to one part of sugar. The method 

 recommended by him for the physiological determination of sugar 

 is as follows : The nature of the sugar present in the samples is 

 first ascertained by qualitative tests; and then the reducing 

 power of the substance toward Fehling solution is determined 

 (after hydrolysis in the case of saccharose or raffinose ; this method 

 being inapplicable in the case of trehalose), and from the results 

 so obtained a calculation is made of the amount of substance that 

 contains 2 grms. of the sugar. This amount which in the case 

 of solid substances is dissolved in 25 c.c. of water, is treated with 

 i grm. of fresh beer yeast, that has been freed from water on an 

 unglazed porcelain or earthenware plate, i c.c. of Hayduck's 

 nutrient solution being added when the substance is low in 

 nutritive substances. Fermentation is allowed to proceed at 

 about 34 C., a weak current of hydrogen gas being drawn 

 through the liquid, and the escaping carbon dioxide collected in 

 an absorption apparatus. If the continued examination of a 

 parallel experiment justifies the assumption that all the sugar in 

 the main flask has just been consumed, then the main experiment 

 is stopped, and the whole of the carbon dioxide remaining in the 

 liquid and the free space of the flask is driven over into the 

 absorption apparatus by careful boiling and the continued passage 

 of a small current of hydrogen, the amount of the fermented 

 sugar being determined from the increased weight of the absorp- 

 tion apparatus. In spite of all the excellent work performed by 



