THE CARBOHYDRATES 213 



Inulin. 



Inulin occurs in the sap of a number of plants and is most abundant in 

 the tubers of the dahlia (10-12 per cent.) and artichoke. 



Inulin is prepared from dahlia tubers by crushing and pressing out the 

 juice ; the residue yields more inulin if boiled up with water and chalk. 

 The two solutions are combined, boiled with chalk to neutralise acids, 

 filtered and treated with lead acetate as long as a precipitate is formed. The 

 filtered solution is treated with hydrogen sulphide, filtered from lead sulphide 

 and evaporated to half its volume. An equal volume of alcohol is added 

 and the precipitate of inulin filtered off after 1-2 days. It may be purified 

 by dissolving in water, warming the solution with animal charcoal, filtering 

 and reprecipitating with alcohol. The precipitate is washed with alcohol and 

 ether and dried in a desiccator over sulphuric acid. 



Inulin forms a white powder with a sphaerocrystalline appearance. It has 

 no taste. It swells up and dissolves in hot water giving a clear solution. 



Reactions. 



(1) Action of Alcohol. Inulin is insoluble in alcohol and is precipitated 

 from solution by adding an equal volume of alcohol. 



(2) Action of Iodine. Solutions of inulin give a brownish coloration with 

 iodine. The iodine solution used must be very weak and it is advisable to 

 carry out a control test, i.e. adding the same amount of iodine to an equal 

 volume of water. 



(3) Basic Lead Acetate. Inulin solutions are precipitated by basic lead 

 acetate. 



(4) Fehlings Solution. Inulin does not reduce Fehling's solution. 



(5) Hydrolysis. Inulin is very easily hydrolyse : by mineral acids and 

 converted into fructose. The hydrolysed solution, after neutralisation, gives 

 the reactions for fructose. 



(6) Rotation. Inulin has laevorotation. 



Mannans. Galactans. Hemicellulose, etc. 



Polysaccharides different from those previously described occur in the 

 seeds of numerous plants. They have resemblances to cellulose, but differ 

 from cellulose in dissolving in dilute alkali, in being hydrolysed by dilute 

 mineral acids and in yielding other monosaccharides as well as glucose. They 

 are soluble in Schweitzer's reagent after treatment for a short time with dilute 

 hydrochloric acid. They form a very indefinite group of substances and re- 

 quire further investigation. 



Gums. Pectins. Mucilages. 



The gums, pectins and mucilages are complex polysaccharides containing 

 both hexose and pentose units. The gums appear to be carbohydrates com- 

 bined with acids ; some are completely soluble, others are partially soluble in 

 water and others only swell up with water. 



Mucilages are very widely distributed in plants and form a slimy liquid 

 with water. 



Pectins are contained in fruits, turnips, etc. The gelatinisation of boiled 

 fruit extracts is probably due to the presence of pectin. 



Schryver and Haynes x showed that these plant materials contained the 

 acid substance, pectinogen, Which is soluble in water. Pectinogen is readily 

 changed into another acid substance, pectin, by dilute alkali. Mineral acids 

 precipitate pectin as a gel from the alkaline solution ; calcium chloride gives a 

 gelatinous precipitate of the calcium salt. Pectin has the composition C 17 H 24 Oi6 

 and contains a pentose group. Pectinogen is extracted from the pressed resi- 

 due of the plants by warm 0-5 per cent, ammonium oxalate solution. 

 1 Biochem. J., 1916, 10, 539. 



