1 62 THE CARBOHYDRATES 



starches which do not contain this substance. The ratio 

 amylose : amylopectin though approximately 2 : i in most 

 cases is not quite constant, and for this reason, in addition to 

 the variation in the activity of the barley diastase, a control 

 is carried out upon pure potato starch, and from the deter- 

 mination of the maltose as a percentage of dry starch the 

 amylose : maltose ratio can be deduced. If this ratio has 

 been estabhshed for one set of conditions, and the same 

 conditions are applied to a cereal starch, it is possible to 

 determine the amylose : maltose ratio for that cereal. The 

 method, for the details of which the original paper should be 

 consulted, gives the true starch content exclusive of hemi- 

 cellulose, and the results are sHghtly lower than those given 

 by malt diastase. 



The following method depending on the hydrolysis of 

 starch by hydrochloric acid and the subsequent estimation of 

 the glucose produced, is only rehable if there are no pentosanes 

 or other substances present which on hydrolysis would yield 

 reducing sugars. 



About 3 grams of the substance in as fine a state of 

 division as possible are covered with 50 c.c. of cold water 

 and shaken at frequent intervals ; after an hour the insol- 

 uble portion is filtered off and washed with water until 

 the total filtrate measures 250 c.c. ; the addition of a little 

 alumina shaken up with water will frequently facilitate clear 

 filtration. The soluble carbohydrates contained in the fil- 

 trate may if desired be determined both before and after 

 inversion. 



The residue remaining on the filter paper is then transferred 

 to a flask with a 250 c.c. graduation mark and heated for two 

 and a half hours under a reflux condenser with 200 c.c. of 

 water and 20 c.c. of hydrochloric acid (sp. gr. 1-125). After 

 cooling, the solution is neutralized with caustic soda and 

 made up to 250 c.c, whereupon it is filtered and the amount 

 of glucose contained in an aliquot portion of the filtrate is 

 estimated by Fehling's or Benedict's solution. The amount 

 of glucose found when multiplied by O-p gives the weight of 

 starch. 



