PHYSIOLOGICAL CHEMISTRY 289 



thought to be composed of a fine interlacement of acicular crystals, 

 the density of the interlacement varying in different portions of the 

 granule, so as to give the appearance of concentric markings. The 

 outside of the granule has densely packed crystals, so that ferments 

 slowly attack it. By boiling, the crystals absorb water and swell 

 up, so that, after cooling, ferments more readily penetrate and digest 

 the granule. 



The exact shape of starch grains varies according to the plant from 

 which they are obtained. In this connection they may be divided 

 into two groups : (1) a group in which the contour of the grains is 

 even, such as wheat, barley, arrowroot, potato ; (2) a group in which 

 the contour is marked by facets, either completely, as in oats and 

 rice, or only partially, as in tapioca and sago. 



EXPERIMENT I. Examine some wheat flour, a scraping of potato, 

 and some ground rice under the microscope. To do this, mix 

 the flour, etc., with a drop of water on a slide, and examine under 

 a cover slip. 



Starch, like most other polysaccharides, is insoluble in cold water, 

 but it swells up in hot water, an opalescent mixture being formed. 

 This is not a true solution, for it does not depress the freezing point 

 of water (e.g. has no osmotic pressure, etc.). It is a colloidal solution. 

 Starch does not pass through a dialyser. 



EXPERIMENT II. Place some powdered starch in a test tube, and 

 half fill up with cold water no solution occurs now boil, when an 

 opalescent mixture will be produced, and, if of sufficient concentration, 

 this will gelatinise on cooling. Try Trommer's test no reduction 

 occurs. 



The standard test for starch is with iodine solution. 



EXPERIMENT III. To an opalescent cold solution of starch add a 

 drop or two of a very dilute solution of iodine in potassium iodide : 

 a blue colour results, which disappears on gradual heating and returns 

 again on cooling. Excessive heat must be avoided, since the iodine 

 is volatile. 



Starch granules also give this reaction under the microscope. The 

 cut surface of a potato gives it. 



Hydrolysis can be effected by boiling with a weak acid or by the 

 action of ferments such as ptyalin, amylopsin, and malt diastase. 



EXPERIMENT IV. Place some starch solution in a flask, add to it a 

 few drops of 25 % sulphuric acid and boil for about a quarter of an 

 hour. Neutralise and apply the iodine test and note that, instead of 

 a blue, a reddish brown colour is produced (due to dextrine). Apply 

 Trommer's or Fehling's test, and note that reduction occurs. 



