Fluorescence screening of indigenous New Zealand honeys

Abstract

Some honeys exhibit unique fluorescence characteristics which depend on the fluorescent components present in the honey matrix. Previous studies have shown that fluorescence spectroscopy can be used to discriminate between honeys of different floral types and geographical origins. This study examined fluorescence in a range of New Zealand indigenous honeys to determine if fluorescence spectroscopy could be extended estimate the floral purity of these honeys, as well as the non-peroxide antibacterial activity of manuka honey. This study found that New Zealand manuka and kanuka honeys exhibited unique fluorescence profiles that distinguished them from each other as well as from other New Zealand honey types. Two fluorescence markers were established for both manuka and kanuka honeys; the former honey at MM1 (275-365 nm excitation-emission) and MM2 (330-470 nm excitation-emission); the latter honey at KM1 (275-305 nm excitation-emission) and KM2 (445-525 nm excitation-emission). The floral dilution of manuka and kanuka honeys with the other honey types resulted in a proportional change in fluorescence intensity of the honeys, and therefore could be used to establish the floral purity of these honey types. The other New Zealand honeys did not exhibit unique fluorescence. The fluorescence intensity of manuka and kanuka honeys at their respective fluorescence marker wavelengths changed with age and heat treatment in an Arrhenius-like manner. It was also established that there was a strong correlation between fluorescence and dihydroxyacetone (DHA), the precursor molecule to methylglyoxal (MGO) which is responsible for the non-peroxide antibacterial bioactivity of manuka honey. There was a strong regional categorization of DHA potential based on fluorescence. Honey age and harvest year also appeared to have an effect on the fluorescence and DHA relationship. Two predictive models were developed in this study: the “year + region” model and the “regions combined” model. These models fitted the data well when employing a closed set of honeys and predicted the non-peroxide antibacterial bioactivity of manuka honeys with a fair degree of accuracy; however, it was observed that when a larger set of honey drum samples were examined the importance of regional analysis became apparent. It is probable that fluorescence detection can be utilised to determine the monoflorality status of manuka and kanuka honeys, and furthermore be used to predict the bioactive non-peroxide potential of manuka honeys.