REACTIONS OF STARCH loi 



from the former a crystalline disaccharide (C,.Hj,pj„ and from 

 the latter a crystalline trisaccharide (C.jHi^OJa. All these 

 four compounds have a sweet taste and, according to the 

 authors, they are representatives of a new class of crystalline 

 polysaccharides which they term amyloses.* The substances 

 are accordingly named di-, tri-, tetra-, and hexa-amylose. 



Reactions. 



1. The appearance of the grains under the microscope 

 and their action on polarized light in the presence of water 

 are well known. 



2. The most characteristic reaction of starch is the blue 

 colour produced with iodine. The composition of this blue 

 substance varies; it contains, on an average, about i8 per cent 

 iodine, and cannot be formed unless a small quantity of hydri- 

 odicacid, which is always present in small amounts inordinary 

 solutions of iodine, be present. The blue colour is discharged 

 on heating the solution, but reappears on cooling. The dried 

 substance may, however, be heated to ioo° without under- 

 going alteration. It is stated that those parts of the grain 

 which are particularly rich in granulose are the most affected 

 by the iodine. 



If the starch grains are very small, or relatively so few in 

 number that they might easily be overlooked, Meyer's pro- 

 cedure may be followed. A section of the material to be 

 examined is cut, and is first treated with a fairly dilute solu- 

 tion of iodine in potassium iodide, the excess of the reagent is 

 then removed, and the section is irrigated with a concentrated 

 aqueous solution of chloral hydrate. This causes the starch 

 grains to swell, and at the same time the other cell contents 

 are dissolved, as are also the starch grains in time. 



3. For microscopic work, the action of dilute aqueous 

 solutions of gentian violet and of safranin is sometimes useful, 

 as they stain the colloidal parts more deeply. 



4. Starch grains are insoluble in cold water, but in hot 

 water they swell up and form an opalescent solution which, if 

 strong enough, will on cooling eventually form a paste. 



* The choice of this term is unfortunate in view of the various uses to which 

 it has already been put by other authors, such as Meyer, Maquenne and Roux, 

 etc. (see pp. 96, 97). 



