Characterisation of the aroma profiles of New Zealand monofloral honeys
Permanent link to Research Commons versionhttps://hdl.handle.net/10289/15742
New Zealand hosts a variety of monofloral honeys with distinct aromas and flavours. These sensory properties are determined by the volatile organic compounds present in the honey, of which over 600 have been identified. The volatile profile of honey is primarily influenced by botanical origin, geographic origin, storage and processing conditions. Not all compounds in the volatile fraction are represented in the aroma profile of the honey, due to the varying odour thresholds at which compounds can be detected by the consumer. Volatile compounds in New Zealand honeys have previously been studied for authentication purposes, but no New Zealand studies to date have linked the volatile composition with honey aroma. Thus, the aim of this research was to develop and optimise a method for the extraction and analysis of volatile compounds in New Zealand monofloral honeys, to determine the chemical differences responsible for the unique aromas of honeys of different botanical origins. Various methods were trialled for the extraction of the volatiles, which are present in very low concentrations, from the complex honey matrix. Solvent extraction using dichloromethane was chosen, and samples were analysed by gas chromatography-mass spectrometry in selected ion monitoring mode. This demonstrated excellent sensitivity at the low part-per-billion level and allowed for identification of compounds not seen in total ion chromatograms. The method showed excellent precision and linearity over wide concentration ranges. Eighteen compounds were semi-quantified using this method, with full quantification achieved for four of these: nonanal, cineole, benzyl alcohol and guaiacol. Additional volatiles were screened using total ion chromatogram. The optimised method was used for the analysis of 44 New Zealand monofloral honey samples spanning 11 floral origins. All 18 compounds were detected in all honey types, but not in all samples. ᴅ limonene, β-ionone and cineole are reported for the first time in any New Zealand honey, and phenylacetaldehyde and nonanal are reported for the first time in kānuka (Kunzea ericoides) honey. Results were comparable with the literature for these honey types, with mānuka honey characterised by o-methoxyacetophenone and kāmahi honey by 4-oxoisophorone. New Zealand clover honey contained low levels of volatiles which has also been reported for international clover honeys. The volatile profiles were also compared with odour thresholds to suggest possible aroma contributors for each honey type. Nonanal was determined to be aroma-active in all honeys analysed, and cineole in all but ling, lotus/blackberry and rātā. Chemometric methods including principal component analysis, classification and regression trees and linear discriminant analysis were used to identify similarities in samples based on their volatile profiles. Samples were classified by honey type using linear discriminant analysis with 82.9% accuracy, with the most accurate classification achieved for kāmahi and kānuka honeys.
The University of Waikato
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