Quantitative Estiniatiou of the Carbohydrates. 1 1 3 



becomes acid owing to tiie presence of free acetic acid which is 

 removed by the addition of dilute sodium carbonate solution until 

 the extract is faintly acid to litmus. 



Portions of the extract are then fermented with yeasts which 

 do not contain the enzyme maltase, namely, Sacchnromyces 

 iiiarxiauHS, S. anonmhis, S. exignus. Two other portions are 

 fermented with bakers' yeast. The yeast is allowed to incubate 

 for 21 to 28 days at 25"C, by which time fermentation is complete. 

 After addition of alumina cream and filtration, the cupric-reducing 

 power is measured. The differences between the cupric-reducing 

 power of the extracts fermented with maltase-free and maltase- 

 containing yeasts, must be due to the gtueose-i'esttl ting fro m the 

 !>y<i*olji^s^ maltose, and so the proportion of maltose in the extracts 

 may be calculated. 



(iv.) The pentoses are estimated by distillation with hydi'o- 

 chloric acid and weighing the furfural produced as phloroglucide. 



(v.) The reducing power of the maltose and pentose is 

 calculated, and from the reducing power of the original extract, the 

 reducing power of the hexoses can be calculated. The optical 

 activity of the sucrose and maltose is calculated, and that of the 

 pentoses on the assumption that they consist of /-xylose and 

 i-arabinose in equal proportions. By comparison of these data and 

 the optical activity of the original solution, the optical activity due 

 to the hexoses can be calculated. From this and their cupric- 

 reducing power, the quantities of glucose and fructose can be 

 calculated on the assumption that these are the only hexoses 

 present. 



It will be observed that the accuracy of the determinations of 

 glucose and fructose depend upon the accuiacy of the following 

 assumptions : — 



(i.) That sugars are the only cupric-reducing and optically active 

 substances in the purified extracts. 



(ii.) That the only sugars that can be present in the extracts 

 are sucrose, maltose, rf-glucose, rf-fructose and pentoses. 



(iii.) That the pentoses present are only /-arabinose and /-xylose, 

 and that these are present in equal quantities. 



In regard to this last assumption, Davis, Daish and Sawyer 

 show that the error involved is not very large (about 7%) if the 

 whole of the pentose is either /-arabinose or /-xylose. 



The accuracy of the glucose and fructose estimations also 

 depends upon the accuracy of the following operations :— 



(i.) The completeness of the extraction of sugars. 



