HEXOSES ror 



FRUCTOSE OR LEVULOSE. 

 Occurrence. 



Fructose occurs in most sweet fruits and in honey, together 

 with both cane sugar and dextrose, but usually in exoess of 

 the latter two. It is formed in equal quantity with dextrose 

 by the hydrolysis of cane sugar, and the resulting mixture, 

 known as invert sugar, may occur in sucrose-producing plants, 

 such as sugar beet and sugar cane, if kept for some time after 

 gathering. 



Much discussion has centred around the origin of levulose 

 in the actively assimilating leaf. It is often considered to be 

 chiefly employed in building up new tissue whilst the glucose 

 is consumed in respiration (see Vol. II.). It may be more 

 abundant than glucose as in Galanthus nivalis * and in oat 

 straw, a subject which is considered on page no. 



Preparation. 



The separation of pure levulose from invert sugar on a 

 small scale is not easy to carry out, but the operation is per- 

 formed on a large scale by making use of the fact that on 

 treating invert sugar with milk of lime the levulose is converted 

 into an insoluble calcium compound, which may be filtered 

 off and purified, while the glucose remains in solution. 



The easiest means of preparing levulose in the laboratory is 

 to hydrolyse inulin by boiling i part of this substance with 

 5 parts of -5 per cent sulphuric acid f for one hour ; the acid is 

 then removed by means of barium carbonate, and the solution, 

 after being treated with animal charcoal and filtered, is eva- 

 porated at a low temperature to a thin syrup. The latter is 

 then crystallized from alcohol after sowing with a crystal of 

 pure levulose. A modification of this method is employed 

 for the manufacture of pure levulose. | 



* Parkin : " Biochem. Journ.," 1912, 6, i. 



t Diill (" Chem. Zeit.," 1895, 19, 216) recommends the use of oxalic 

 acid ; see also Wiechmann : " Z. d. Vereins Deut. Zuckerind.," 1891, 41, 

 331- 



+ Cf. Stein : " Proc. Internat. Confer. Sugar lud.," April, 1908. 



