Flavonoids in Australian Melaleuca, Guioa, Lophostemon, Banksia and Helianthus honeys and their potential for floral authentication

Abstract

Flavonoids in Australian honeys from five botanical species ( Melaleuca, Guioa, Lophostemon, Banksia and Helianthus) have been analyzed in relation to their floral origins. Tea tree ( Melaleuca quinquenervia) and heath ( Banksia ericifolia) honeys show a common flavonoid profile comprising myricetin (3,5,7,3 ′,4 ′,5 ′-hexahydroxyflavone), tricetin (5,7,3 ′,4 ′,5 ′-pentahydroxyflavone), quercetin (3,5,7,3 ′,4 ′-pentahydroxyflavone) and luteolin (5,7,3 ′,4 ′-tetrahydroxyflavone), which was previously suggested as a floral marker for an Australian Eucalyptus honey (bloodwood or Eucalyptus intermedia honey). These honeys of various floral species can be differentiated by their levels of total flavonoids, being 2.12 mg/100 g for heath honey and 6.35 m/100 g for tea tree honey. In brush box ( Lophostemon conferta) honey, the flavonoid profile comprising mainly tricetin, luteolin and quercetin is similar to that of another Eucalyptus honey (yellow box or Eucalyptus melliodora honey). These results indicate that the flavonoid profiles in some of the Australian non- Eucalyptus honeys may contain more or less certain flavonoids from Eucalyptus floral sources because of the diversity and extensive availability of Eucalyptus nectars for honeybee foraging yearly around or a possible cross contamination of the monofloral honeys during collection, transportation and/or storage. Further analyses are required to differentiate and/or verify the botanical sources of the flavonoids that contribute to the flavonoid profiles of these honeys, by restricting honey sampling areas and procedures, employing other complementary analytical methods (e.g. pollen analysis, sugar profile) and using materials (e.g. nectar) directly sourced from the flowering plant for comparative studies. In Australian crow ash ( Guioa semiglauca) honey, myricetin, tricetin, quercetin, luteolin and an unknown flavonoid have been found to be the main flavonoids, which is characteristic only to this type of honey, and could thus be used as the floral marker, while in Australian sunflower ( Helianthus annuus) honey, the content of total flavonoids is the smallest amount comparing to those in the other honeys analysed in this study. However, the flavonoid quercetin and the flavonoid profile mainly consisting of quercetin, quercetin 3,3 ′-dimethyl ether (5,7,4 ′-trihydroxy-3,3 ′-dimethoxyflavone), myricetin and luteolin are characteristic only to this sunflower honey and could thus be used for the authentication.