Occurrence of 2-hydroxy-5-methyl-3-hexanone and 3-hydroxy-5-methyl-2-hexanone as indicators of botanic origin in eucalyptus honeys

SPME GC–MS analysis of the volatile fraction from 22 Spanish eucalyptus honeys revealed two components which were present in all samples. From their mass spectra they were tentatively assigned as 2-hydroxy-5-methyl-3-hexanone and 3-hydroxy-5-methyl-2-hexanone. As no commercial standards were available for these compounds, they were enzymatically synthesized, respectively, from isovaleraldehyde and pyruvic acid, and from α-ketoisocaproic acid and acetaldehyde, using reactions catalyzed by yeast pyruvate decarboxylase. The prepared products presented retention indices and mass spectra identical to those of the compounds found in eucalyptus honeys. Since these compounds were not found in 165 other honey samples from 19 different floral origins, they appear to be good markers for eucalyptus honey.