CARBOHYDRATES AND RELATED BODIES. 31 



J-FRUCTOSE, fruit sugar, levulose, is a ketohexose similar 

 to rf-glucose in some respects but very different in others. It 

 occurs in honey and sweet fruits, but is not easily separated 

 in a pure state because it is very soluble and does not readily 

 crystallize. The preparation of glucose from ordinary starch 

 has been referred to above. In like manner fructose may be 

 obtained from certain less common starches, especially from 

 inulin by hydrolysis with very weak acid. In pure condition 

 this sugar has no technical importance. 



The various reduction and fermentation reactions are 

 shown by fructose as well as by glucose, but the quantitative 

 relation between copper hydroxide and fructose is not quite 

 the same as with glucose. As they yield the same osazone 

 the phenyl hydrazine test can not be employed to distinguish 

 between them. The most characteristic property of fructose 

 is found in its optical behavior. While the specific rotation 

 of J-glucose is about 53 to the right, that of d-fructose is, 

 at 20 C., and for a strength of 20 per cent, about 93 to 

 the left. Because of this behavior the sugar is commonly 

 called levulose. Another reaction which may be applied is 

 this: 



Ex. Dissolve resorcin in 20 per cent hydrochloric acid and heat a lit- 

 tle of this solution with the levulose solution to be tested. A red color 

 results. At the same time a precipitate forms which may be dissolved in 

 alcohol with a red color. One-tenth gram of resorcin to 5 cc. of the acid 

 is sufficient. 



This is the third hexose of importance, but 

 it is not a natural substance. In the inversion of milk sugar 

 by weak acids galactose is formed along with glucose, and it 

 results also from the action of acids on several gums. The 

 sugar is readily soluble in water, fermentable and dextro- 

 rotatory like glucose. It forms a characteristic osazone. It 

 reduces Fehling's solution but not in the same proportion as 

 glucose. On reduction it yields dulcitol, which shows its 

 chemical relations most characteristically. On oxidation it 

 yields galactonic and mucic acids. 



