THE CARBOHYDRATES AXD THEIR METABOLISM 239 



galactoso on hydrolysis. Galactose is present here in a polymeric form 

 called galactans. Galactans are also found in certain gums and pectins. 

 The pectins are found in apples, pears, beets, carrots, flax, etc., and these, 

 on mild hydrolysis, are converted to pectic acids, the calcium salts of which 

 cause fruit juices to jell. On hydrolysis with acids they yield d-galactose 

 and 1-arabinose. 



It is usually prepared from lactose by heating with two per cent 

 sulphuric acid, precipitating the sulphuric acid with barium carbonate 

 and concentrating the filtrate to a syrup from which d-galactose slowly 

 crystallizes in large prisms with one molecule of water of crystallization. 

 The hydrated form melts at 118-120 C. From alcoholic solution it 

 crystallizes in leaflets which melt at about 165 C. It is sweet, easily 

 soluble in water, but practically insoluble in absolute alcohol and in ether. 

 It behaves somewhat like d-glucose; it exhibits mutarotation, both a- and 

 (5-forms having been prepared and when treated with sodium amalgam, 

 it is reduced to the alcohol dulcitol, whicji occurs naturally in Madagascar 

 manna. 



On oxidation with nitric acid mucic acid is formed. Mucic acid is 

 a very characteristic oxidation product of galactose (and lactose), with a 

 melting point of 212-215 C., quite insoluble in water (about 0.3 per 

 cent at 15 C.), and therefore is used frequently as a means of identify- 

 ing galactose. It is optically inactive. 



Fructose. d-Fructose (levulose) was discovered by Dubnmfaut in 

 1847 in the hydrolysis products of cane sugar. It occurs in the juices of 

 many plants and fruits with glucose, especially in tomatoes, certain man- 

 na and mangoes. In young sugar cane it occurs in equal amount with 

 glucose and sucrose. As the cane grows older, the proportion of fructose 

 to the two other sugars decreases to about 15 per cent and in the mature 

 plant to about. 1.5 per cent of the total amount of the three sugars present. 

 In honey, glucose and fructose are found in nearly equal proportions, to- 

 gether with a little sucrose and dextrine. 



d-Fructose also occurs combined with other sugars, as in sucrose (glu- 

 cose and fructose) ; raffinose (glucose, galactose and fructose) ; etc. It 

 is a constituent of certain glucosides and saponins. The polysaccharide 

 iuulin. which is obtained in quantity from the tubers of the dahlia, sun- 

 flower and other members of the same family, is a fructosan^ and hence 

 yields only fructose on hydrolysis. This is, *in fact, the simplest way to 

 obtain fructose, as from 7 to 17 per cent of inulin is found in the roots 

 of the dahlia. It is purified by recrystallization from water at 60-70 C. 



Fructose forms anhydrous rhombic crystals, tastes almost, as sweet as 

 cane sugar and melts between 95 to 105 C. It is very soluble in water 

 and hot alcohol, but only slightly soluble in cold alcohol. Its aqueous solu- 

 tions exhibit the property of mutarotation and exist in solution, presum- 

 ably as an equilibrated mixture of stereoisomeric forms, but the two forms 



