CH. XIll] DEXTRINS. 263 



The appearance of the yeast (which forms a layer at 

 the bottom of the liquid when fermentation is complete) 

 and absence of frothing on the surface will easily shew 

 when the change is completed. 



Further examination of aqueous extract (pen- 

 toses), dextrins, (inulin), glucoses, cane-sugar, maltose, 

 (mannite). 



Qualitative. 



Concentrate a portion of the aqueous solution to a 

 small volume — a few c.c. — on the water bath, add a large 

 excess of strong alcohol and allow it to stand till the pre- 

 cipitate settles : if the addition of more alcohol causes a 

 further precipitabe allow it to settle again. The alcoholic 

 solution can, in most cases, be completely decanted from 

 the precipitate. If filtration is necessary, asbestos should 

 be used. 



[The precipitate is generally dextrins and need not be 

 further examined, except for inulin, which would be pre- 

 cipitated with the dextrins. To test for inulin dissolve a 

 portion of the precipitate in water, add a few drops of 

 strong hydrochloric acid, boil, cool, and add a few drops of 

 alcoholic solution of phloroglucin. Yellow-brown colour 

 indicates inulin'. 



If this reaction gives a positive result, dissolve the 

 rest of the precipitate in water and add baryta-water as 

 long as it causes a precipitate : collect and wash the pre- 

 cipitate, suspend it in water and decompose with a current 



1 Green, Annals ofBot., Vol. i. p. 233. 



