THE CARBOHYDRATES 207 



(4) Ammonium Sulphate. 



Starch is precipitated from solution by adding an equal volume of 

 saturated ammonium sulphate solution, i.e. by half saturation with this 

 salt 



(5) Fehling's Solution. 



Starch solutions do not reduce Fehling's solution. 



(6) Hydrolysis. 



Starch is easily hydrolysed into glucose by boiling its solution with 

 dilute sulphuric acid for a few minutes. The presence of glucose can 

 be shown by neutralising with soda and testing with Fehling's solution. 



(7) Rotation. 



Solutions of soluble starch have a high dextrorotatory power. 



DEXTRINS. 



Dextrins are glucosans which are intermediate in complexity be- 

 tween starch and maltose. They have been found in plants, but are 

 usually obtained by the hydrolysis of starch by the diastase in malt 

 extract 



The existence of a large number of dextrins has been supposed, 

 but only two can be easily distinguished erythrodextrin, which gives 

 a reddish-brown colour with iodine, and achroodextrin, which gives no 

 colour with iodine. Erythrodextrin is probably a mixture of achroo- 

 dextrin with a small amount of starch (Ost). 



Baker 1 has described a dextrin, termed tf-amylodextrin, which 

 results from the action of the diastase of ungerminated barley upon 

 starch paste, or soluble starch. It gives a blue colour with iodine. 



Preparation. 



The dextrins are prepared by the action of malt extract at 55 or 

 by the diastase of ungerminated barley at 45-50 upon starch paste 

 or soluble starch. The starch is converted by the malt diastase in 2-3 

 hours into a mixture of 80 per cent maltose and 20 per cent dextrin, 

 by the barley diastase into a mixture of 60-65 percent, of maltose and 

 35-40 per cent of dextrin. In the former case the reaction can be 

 followed by testing portions of the solution at intervals with iodine ; 

 the blue coloration disappears passing through a stage at which a red- 

 dish-brown colour is observed. 



The prolonged action of malt extract slowly converts the dextrin 

 into maltose. Maquenne and Roux consider that the maltose is 

 derived from amylose and the dextrin from amylopectin. 



The solution containing the products of hydrolysis is concentrated 

 and the dextrin is precipitated by pouring it into alcohol. The precipi- 

 tate is dissolved in water and reprecipitated with alcohol. 



Commercial dextrin is prepared by heating starch at 180-200, 

 until it has a pale brown colour. If the starch be previously treated 

 with acid, it is heated at a lower temperature. 



