POLYSACCHAROSES 73 



( i ) Polysaccharoses. (a) Starch. The chief Polysac- 

 charose is starch, a carbohydrate having 1 the empirical 

 formula, C 6 H 10 O 5 , but really possessing a much 

 higher molecular weight. It is a reserve food very 

 commonly distributed in the cells of tubers, roots, seeds, 

 and other parts of plants, and occurs in the form of solid 

 grains built up of a layer of material arranged round a 

 more or less central nucleus or hilum. The substance of 

 the grain consists of starch or amylose, a compound 

 which is insoluble in cold water but swells up and 

 partially dissolves in hot water, forming " starch paste.'' 

 A solution of iodine in potassium iodide turns it a 

 characteristic deep blue colour. When boiled for some 

 time with dilute acids it is finally converted into grape- 

 sugar, intermediate products, such as maltose and gummy 

 substances being first produced. 



The Hydrolysis of Starch by Ferments. When thin 

 starch-paste (prepared by boiling about a gram of starch 

 in 50 c.c. of water) is mixed with an extract of malt and 

 kept warm, the liquid gradually loses its opalescent 

 appearance and finally becomes clear. If small portions 

 of the mixture are removed and tested at short intervals, 

 with a solution of iodine and potassium iodide, the first 

 portion containing unaltered starch is coloured blue, the 

 second purple, later a reddish brown colour is observed, 

 and finally the solution when tested remains colourless. 

 The changes going on in the starch-paste are due to the 

 action of the enzyme diastase (amylase) contained in the 

 malt extract. At a temperature of 50 to 60 the starch 

 is hydrolysed into maltose and dextrin, a gum-like 

 substance. 



3 C 6 H 10 5 + H 2 = C 12 H 22 O n + C 6 H 10 5 



starch maltose dextrin 



