METHODS OF ESTIMATING CARBOHYDRATES 45 



tures, they state that this method is totally inapplicable to 

 the analysis of plant extracts, owing partly to this destruc- 

 tion of glucose, but chiefly to the far more serious loss of 

 fructose which takes place. 



Their maltose estimations were carried out, therefore, as 

 follows: The extract, clarified as usual, is de-leaded by 

 sodium carbonate and hydrogen sulphide in succession, 

 though the former affords a sufficiently complete removal 

 of lead for ordinary chemical methods. The excess of 

 hydrogen sulphide is removed by bubbling air through 

 the solution, and finally by addition of ferric hydroxide. 

 Sterilized portions are then fermented with pure cultures 

 of the maltase-free yeasts, Saccharomyces marxianus, S. 

 anomalus and S. exiguus, and in duplicate with baker's 

 yeast. Fermentation is complete at the end of three to 

 four weeks at 25. The reducing-power of the solution is 

 then determined after clarifying with alumina cream and 

 boiling to remove alcohol. 



The fermentation in the first three cases leaves the 

 maltose untouched, the two with baker's yeast are carried 

 out to remove the maltose also, and thus to enable a small 

 correction to be applied for the reducing-power of the un- 

 fermentable pentoses present. 



Of these d-ribose has been shown by Levene and Jacobs 

 (1909) to be an essential constituent of the nucleus, and 

 arabinose, xylose, and methyl-pentoses, may also occur in 

 small quantities. On the whole the methods of Davis and 

 Daish leave little to be desired on the score of accuracy, 

 but are very laborious. It cannot, however, be expected 

 that such complicated mixtures will ever be analyzed with 

 both rapidity and accuracy, and in research work the latter 

 is essential. More recently Daish, Davis, and Sawyer, 

 (1914, 2 and 3) have studied the reducing-power of pentoses 

 and their estimation in plant extracts. 



