CH. V.] STARCH. 119 



It is possible to regard it as being formed by the condensa- 

 tion of forty molecules of glucose with the elimination of 

 thirty-nine molecules of water. On this assumption its 

 formula would be 39 (C 6 H 10 O 5 ) C 6 H 12 O 6 . Its reducing 

 power is slight, and has [ a ] D about + 195. At the stage 

 of apparent equilibrium in the action of amylases on 

 starch, about 85 per cent, of the initial weight is in the form 

 of maltose, and about 19 per cent, of stable dextrin is 

 formed. The increase in weight is due to the addition of 

 the elements of water in the hydrolysis. The following 

 equation has been suggested by Brown and Millar. 



100 (C 6 H 10 O 5 ) + 8 1 H 2 O = 80 C 12 H 22 O U + (C 6 H 10 O 5 ) 39 C 6 H 12 O 6 



Starch. Maltose. Stable dextrin. 



Motto-dextrin is the name given to an achroo-dextrin 

 obtained by the action of malt diastase in starch. It is 

 rapidly and completely hydrolysed to maltose by the 

 amylases. 



The exact relationships between these various dextrins 

 to one another and to the constituents of the starch grain 

 are so imperfectly understood at present that it is not 

 considered advisable to give a scheme of the stages of 

 hydrolysis. 



132. Place a small amount of dry potato-starch on a slide, add 

 a drop of water, cover with a slip, and examine under the microscope. 

 Note, the characteristic oval starch grains, the concentric markings 

 and the hilum, usually eccentric. Make a drawing of the grains. 

 Run a drop of iodine under the slip ; note that the grains take on a 

 blue colour. 



133. Shake a small amount of potato starch with cold water. 

 The starch does not dissolve. Filter, and add a drop of iodine 

 solution to the nitrate. The characteristic blue reaction is not 

 obtained. 



134. Shake some dry starch with a little sodium carbonate 

 solution. No change is effected. Repeat, with a little sodium 

 hydoxide. The starch is immediately gelatinised. Add a few drops 



