The D (laevo-) fructose, also known as laevulose or fruit sugar is found is many fruits and in honey. It occurs in combination with glucose in cane and beet sugars. The most important source of fructose is the polysaccharide, inulin (a polymer of fructose).
Commercially, fructose is obtained by the hydrolysis of inulin. It melts at 104°C, and sweetest of all sugars, but is seldom used in pure form. It exhibits mutarotation, the specific rotation values of α-,β- and equilibrium mixture are - 21°, 133.5° and -92.3° respectively. Fructose responds most of the usual reactions of the ketonic and hydroxyl groups. Two important properties of fructose are oxidation and reduction.
Oxidation : Fructose, although, has no aldehydic group, it reduces Fehling solution and Tollen's reagent. This reducing property of fructose is said to be due to the presence of an α-hydroxy ketonic group which is readily oxidised by Fehling solution and Tollen's reagent.
Reduction : Reduction of fructose with Na-Hg and water gives two alcohols, sorbitol and mannitol (difference from glucose which gives only sorbitol on reduction).
Fructose forms an insoluble calcium fructosate with lime water (difference forms soluble calcium glucosate). This fact is utilised in the separation of glucose and fructose forme by the hydrolysis of sucrose.
Configuration of fructose. Since the structure of fructose has 3 asymmetric carbon atoms, it ha eight (23 = 8) optically active isomeric forms; i.e. four pairs of enantiomers.
