Characterization of 3,4-dihydroxy-3-hexen-2,5-dione as the first open-chain caramel-like smelling flavor compound

3,4-Dihydroxy-3-hexen-2,5-dione (DHHD) was identified in a thermally treated mixture of cysteamine and fructose. GC/MS and ¹³C-NMR measurements revealed that its flavor activity is present only in the open chain form, while the cyclic species was shown to be completely odorless. The flavor quality of DHHD was similar to that of 4-hydroxy-2,5-dimethyl-3(2H)-furanone (furaneol) and its odor threshold was determined to be 10 μg/l in sunflower oil. NMR measurements, however, showed that the odor-active open chain tautomer is present only in aprotic solvents, such as oils, while the cyclic, odorless tautomer (diacetylformoin) is immediately formed in aqueous solution.