188 STAECH 



C,;H n 5 - O - C 6 H 10 4 - O - C tf H n 5 

 Fructose. Glucose. Galactose. 



Sucrose. Melibiose. 



The Tetrasaccharoses, C 24 H 42 O 21 . The only example which need 

 be given is Stachyose or manneotetrose which occurs in Stachys tu- 

 berifera and in manna. By hydrolysis it splits up into fructose, 

 glucose and two molecules of galactose. 



The Polysaccharoses, (C 6 H 10 C>5) n . Starch or Amylum. This 

 substance is very widely distributed in plants and serves as a reserve 

 material for the nourishment of the growing portions. It exists in the 

 form of granules, which vary greatly in size and form in different 

 plants. 



The starch from any one source may show considerable differences 

 in the size of its granules, but their form and general appearance is 

 always the same and may be readily recognised under the microscope. 



Of the common starches, that from potatoes has the largest 

 granules, varying from 07 to -03 millimetre in diameter, while wheat 

 starch varies from -045 to 003 millimetre, and rice starch from '0075 

 to '0050 millimetre in diameter. 



The granule is made up of a cell wall of starch cellulose and an 

 interior mass of granulose. So long as the cell wall is uninjured, 

 starch will not dissolve in cold water, but by bruising the cell wall, or 

 better, by causing the granules to swell up and burst, the contents, 

 granulose, escape and yield with the water a kind of viscid solution 

 known as starch paste. 



The temperature at which this swelling up of the granule occurs 

 varies with different starches ; it usually commences about 50 and is 

 completed about 70 C. 



Starch is converted by free iodine, in the presence of water, into a 

 blue or violet-coloured substance. Starch paste is coloured deep blue 

 by iodine, the colour being deeper the lower the temperature ; even 

 below the boiling-point of water the colour disappears entirely, but re- 

 appears on cooling. The blue substance is said to have the composi- 

 tion [(C t} H 10 O 5 ) 4 I] 4 .HI and the presence of hydriodic acid or an iodide 

 is said to be essential to its formation. 1 



Starch unites with the alkaline earths to form definite compounds 

 which are insoluble in dilute alcohol. In the case of barium the pre- 

 cipitate has the composition BaO.(C H 10 O 5 ) 4 . A volumetric method 

 of estimating starch has been based upon this reaction. 2 



When starch is heated for half an hour in glycerine to 190 it is 

 converted into soluble starch, which can be precipitated from aqueous 

 solution by the addition of alcohol. The white powder so formed is 

 soluble in water, and according to Brown and Morris 3 has a molecu- 

 lar weight of 32,400, i.e., it has the formula (C H 10 O 5 ) 200 . 



, Ber. 20, 688; Jour. Chem. Soc., 1887, Abstracts, 568. 



2 Asboth, Chem. Zeit., 11, 785; Jour. Chem. Soc., 1887, Abstracts, 868. 



3 Jour. Chem. Soc., 1889, Trans., 449. 



