THE CARBOHYDRATES 



193 



(ID) Fermentation. Glucose is fermented by yeast into alcohol 

 and carbon dioxide. If a little fresh yeast be rubbed up with some 

 glucose solution and a 

 test tube be filled with the 

 mixture and inverted in 

 warm water in a crucible 

 at 25, it will be seen that 

 after a short time bubbles 

 rise to the top and dis- 

 place the liquid. In about 

 24 hours most of the glu- 

 cose will have disappeared 

 and alcohol can be de- 

 tected in the liquid. 



(u) Molisch's Test. 

 On adding a drop of a- 

 n iphthol solution to about 

 5 c.c. of glucose solution 

 and running about 5 c.c. of 

 concentrated sulphuric acid below it, a purple ring appears at the surfaces 

 of contact, either at once or after a short time. The two liquids may 

 be mixed but the mixture must be kept cold by holding under running 

 water. The whole liquid becomes reddish- violet. An examination of the 

 coloured solution with a spectroscope will show an absorption band 

 >ct,veen D and E, whilst the violet end is totally absorbed (cf. p. 478). 

 u,-Hydroxymethyl furfural, which gives the pigment with a-naphthol, 

 H . c C . H 

 II I! 



CH 2 OH . C C . CHO 



FIG. 39. Glucosazone. 



is formed. 



This reaction is the most general one for all carbohydrates. 



(12) Rotation. Glucose in solution is dextrorotatory when 

 examined with a polarimeter, and shows mutarotation the initial high 

 rotatory power decreases and becomes constant in about 24 hours, or on 

 boiling, or on adding a drop of ammonia. 



(13) Hydrogen Cyanide. Glucose combines with hydrogen 

 cyanide forming a cyanohydrin, which yields an acid containing seven 

 carbon atoms on hydrolysis. This acid (or its 7-lactone or anhydride) 

 on reduction yields an heptose. Octoses and nonoses have been pre- 

 pared by continuing the addition of hydrogen cyanide to the heptose 

 and octose. 



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