EXAMINATION OF STARCH 



35 



Starch exists in plants in the form of granules, which vary in 

 shape and size according to the source. Potato-starch granules 

 are largest in size (-07 to -02 mm. diameter), while amongst the 

 smallest-sized granules are those of rice starch (-097 to -005 mm.). 

 Wheat starch is circular in outline, while maize starch is angular, 

 and from a microscopical examination of a sample of starch it 

 is possible to infer the plant from which it was obtained (see 

 Fig. 9)- 



Percentages of Pure Starch in various Plant Products 

 {Dry matter). 



Potatoes 

 Rice . 

 Wheat 



80 

 78 

 67 



Oats . 

 Wheat bran 

 Straw, less than . 



55 

 8 



I 



20. Preparation and Qualitative Examination of Starch. — 

 One or two cleaned potatoes are grated to a pulp, the 

 latter tied in a cloth, dipped in water, and squeezed 

 into a large beaker of water, the cloth and its contents 

 being wetted from time to time. The starch granules 

 pass through the cloth, and render the water in the 

 beaker milky. The starch is allowed to settle to the 

 bottom, and is then washed once or twice by decanta- 

 tion. It is then placed on a porous tile and dried in 

 the steam oven. 



The white substance so obtained consists of a number 

 of starch granules, which are made up of a cell wall of 

 starch cellulose (insoluble in water) enclosing a mass of 

 starch granulose or soluble starch (soluble in water). 



The following reactions should be performed with 

 starch : — 



(i.) A little starch is rubbed into a paste with water, 

 and a few cubic centimetres of the milky liquid poured 

 into a large bulk of water and boiled. The opalescent 

 liquid obtained after boiling consists of a sticky solution 

 of starch granulose containing in suspension the in- 



