Abstract
Honey is a naturally sweet, carbohydrate-rich, viscous fluid consisting of mainly fructose and glucose produced by honeybees (Apis mellifera) mainly from the floral nectars of plants. New Zealand honey is some of the most desired in the world with substantial export value to New Zealand producers. With this growing industry, a greater understanding of the volatile organic compounds (VOC’s) in New Zealand honey could be valuable in developing and promoting the benefits in regards to flavour and quality. The aim of this research was to characterise the VOC composition of three varieties of New Zealand unifloral honeys using three different analytical methods, namely gas chromatography mass spectrometry (GC-MS), GC-olfactometry (GC-O) and proton transfer reaction mass spectrometry (PTR-MS).
Eighty-seven volatile compounds were identified (out of 124 peaks detected) in 11 samples of New Zealand honey from three floral origins (thyme, rata and manuka) with analysis by headspace solid phase micro-extraction gas chromatography mass spectrometry (HS-SPME-GC-MS), of which thirty three were common across all three types of honey. However, the level that some of these compounds were present in each honey varied significantly.
Examination of this data by principal components analysis (PCA) explained 57% of the variation in the data set for the first two principal components (33% PC1 and 24% PC2). Manuka honey was found to have the greatest variety of VOC’s while rata honey was found to have the least. A range of compounds were found to occur in all three honey types, but unique compounds were also identified, with 11 found in manuka (e.g. dimethyl disulfide and pinocarveol), 3 in thyme (2-heptanone, lilac alcohol D, dihydrojasmone lactone) and 1 in rata (p-cresol). Some honeys showed discrimination by compound class with the VOC’s in thyme honey being mainly acids and terpenes, whereas the majority of VOC’s in manuka honey were terpenes, alcohols and ketones.
Gas chromatography mass spectrometry and olfactometry (GC-MS-O) was used to identify odour active compounds using a detection frequency method with a panel of six assessors. Thyme honey was found to have the most odour active compounds with 48, followed by manuka with 40, and rata honey had the least with 29. Some odour active compounds were found in all three honeys, such as linalool and phenylethyl alcohol, whereas other odour compounds were distinct, being only identified in specific samples, such as hexanoic acid in thyme, (Z)-linalool oxide in rata and acetoin in manuka.
The third objective of this study was to evaluate discrimination between 19 honey samples from four floral origins (thyme, rata, manuka and kanuka) by means of proton transfer reaction mass spectrometry (PTR-MS). Using a rapid headspace method, 130 mass ions (m/z) were found to have significant differences between the samples. It was found that the first two principal components (PCs) explained 60% of the variation in the data set (PC 1 38%; PC 2 22%). The honey samples of thyme and rata were clustered together in easily defined groups with thyme showing the greatest discrimination. However, there was no clear grouping for manuka or kanuka and the samples for both of these honeys tended to cluster together with considerable overlap, so that they could not be discriminated from each other in a robust manner.
Results suggested that the analysis of volatile organic compounds is an effective method for the characterisation of unifloral New Zealand honeys. The results obtained using GC-MS were better than PTR-MS for discrimination of honey samples. However, PTR-MS might still prove useful as a fast, online screening method. GC-O provided complementary information specifically on the odour active compounds to identify compounds that contribute to the flavour character. These findings could be useful for improving the export value of New Zealand honey by providing a greater understanding of how odour compounds and their sensory aspects relate to honey quality. This information could also be useful for monitoring changes in flavour during processing and storage for quality assessment and shelf life prediction.