Aroma Characterization, Odour-Active Markers Determination and Formation Mechanism Prediction of New Zealand Unifloral Honey

Abstract

New Zealand honey is world-renowned with its supreme quality, in particular Mānuka honey has the highest commercial value, reaching 116.5 NZD per kilogram for the premium UMF® 20+ in 2015. These high prices make New Zealand honey vulnerable to adulteration by counterfeiters claiming Mānuka origin and flavour profile investigation. Volatile compounds are ideal “anti-counterfeiting” molecular candidates to guarantee trust in source and product authenticity as they can be analysed cost effectively. By elucidating the formation pathways for specific volatile compounds, a chemometric sensory profile and an optimal “anti-counterfeiting” volatile signature is generated to promote both botanical origin monitoring and authenticity control. This study aims to profile the flavour characteristics of 11 predominant New Zealand unifloral honey species by identifying the key odour-active compounds and the sensory descriptors of the above honey species. Following this, attempt was conducted to screen out some volatile compounds as potential signature flavour markers for authentication of honey. In addition, the formation pathways of some key odour-active compounds were predicted to help prevent honey from adulteration. In this research, gas chromatography-mass spectrumetry coupled with olfactometry (GCO- MS) was used to detect the odour-active compounds in unifloral honey. All the volatile compounds were then identified and quantified, and were categorized according to their formation sequence. Sensory evaluation by olfactometry and aroma profile analysis were conducted to profile the aroma attribution of odour-active compounds. The results were represented in Wind-Rose diagram and flavour dilution (FD) chromatogram. Aroma extraction dilution analysis (AEDA) and odour activity values (OAVs) were applied to figure out the key odour-active compounds. Hierarchical cluster analysis (HCA) and principal component analysis (PCA) were then employed to conduct the multivariate data analysis. Based on the results obtained from these methods, the formation pathways of some key odour-active compounds and their precursors also were predicted. From the results, a total of 171 volatile compounds were detected from the 11 honey species. A total of 149 volatile compounds were reconfirmed on GC using 5MS column, 164 were identified by mass spectrumetry, and 159 compounds were detected at sniffing port and noted with their odour descriptions. The changes of pH within each honey species could reflect honey purity and quality, which, in turn, gave an insight to the blending of honey species. In Mānuka honey, pH decreased as the UMF® value increased. The HCA results of sensory scores from sniffing port was more representative than that of the volatiles concentration. All the 34 honey samples were appropriately grouped into three patterns: Mānuka (Group I), Kamahi (Group II) and other species (Group III). Pohutukawa, Rata, Tawari, and Thyme honey samples very closely associate, whereas Kanuka, Thyme, and Mānuka honey have more independence, while Rewarewa and Tawari samples are relatively more similar to the rest of the samples. Mānuka honey, exclusively, has a fine differentiation with other honey species, while has an intraspecific difference. In conclusion, some volatile compounds were successfully identified as key aroma contributors and as candidates to define the unifloral origin for product authenticity and “anticounterfeiting” measures. As a final note, this study has successfully identified aroma characteristics and determined odour-active markers of 11 unifloral honey species in New Zealand, as well as elucidated the possible formation mechanism of key aroma-compounds and their unique biomarkers. The research has achieved the goal of developing an effective way in profiling flavour compounds in New Zealand commercial unifloral honey, aiming to safe guard consumers’ right and interests, and provide source product authenticity and “anti-counterfeiting” measures.