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NEW ZEALAND BEEKEEPER, DECEMBER 2016
Table 2. Summary of DHA in honey and nectar.
Analysis of nectar and normalisation of results Nectar is primarily made up of glucose (~40%), fructose (~40%), sucrose (~2%) and water (~20%). There are also minor compounds present (e.g., amino acids and phenolic compounds). Samples are analysed using High Performance Liquid Chromatography (HPLC) to detect DHA, Leptosperin, fructose and glucose. The three methods for nectar collection dilute samples to various volumes (i.e., no dilution, 0.1 mL total volume or 1.5 mL total volume); hence results from different sampling methods cannot be directly compared without normalising the DHA and Leptosperin to the sugar concentration. Honey is made up of approximately 80% sugar (800 g sugar per 1 kg). Therefore the DHA and Leptosperin are reported per 800 g of sugar; this normalised result gives an approximate level of mg/kg that would be expected in a honey that was created entirely from the one sample, allowing samples to be compared. Historically DHA results were been compared to 80° Brix. This is a measure of the sugar concentration of a solution, which is equivalent to 800 g of sugar per 1 kg. The importance of chilling samples Once the nectar is collected, it is necessary to chill or freeze it due to the high sugar content which can cause fermentation (depletion of sugars). Alternatively, samples collected using the Wash or 10x10 methods can be preserved using a 10% alcohol solution. To illustrate the effect on the normalised result, a pure nectar sample was divided in two: one half was diluted with water and the other with 10% methanol to preserve the sample and prevent fermentation. The samples were stored at room temperature and analysed over four days to simulate samples sitting in the field and during postage. The DHA and Leptosperin concentrations did not change over this period; however, the sugar concentration in the sample diluted with water decreased by ~20% during this time due to fermentation. If we say that the original sample had 100 mg/L DHA and 20 g/L sugar, the normalised DHA would be 4,000 mg DHA/800 g sugar. However, if the sugar concentration dropped by 20%, then the normalised result would be 5,000 mg DHA/800 g sugar, making the tree appear to have a higher DHA content that could cause it to be wrongly selected.
DHA in honey
Normalised DHA in nectar
(mg/kg)
(mg/800g sugar)
Average
981
3,887
Median
817
2,940
Maximum
5,330
27,070
Total # samples
3,661
1,309
A tree which produces high levels of DHA, but does not produce many flowers or large volumes of nectar may not be as good as a tree with slightly less DHA, heavy floral density and good nectar flow.
Interpreting results to select good trees The normalised results for DHA in nectar can be very high (results over 20,000 mg DHA/800 g sugar have been reported). It is important to note that the concentration of honey will not be this high due to the dilution from other flowers. A set of 1,300 nectars and non-related honey samples (3,661 samples with < 4 mg/ kg HMF and >100 mg/kg DHA) analysed at Analytica Laboratories showed the nectar results were about four times higher than the honey samples (Table 2). In addition, a paper published in 2009 (Adams, Manley-Harris and Molan) reported the DHA in nectar was more than double the concentration found in honey. When selecting trees for planting, a number of factors need to be taken into consideration, aside from the DHA concentration. Physical properties of the tree are important, such as floral density, and volume of nectar and resilience of the tree to the environment also need to be taken into consideration. A tree which produces high levels of DHA, but does not produce many flowers or large volumes of nectar may not be as good as a tree with slightly less DHA, heavy floral density and good nectar flow. The flowering period of the species of mānuka tree needs to line up with the latitude of planting—plants that flower early in the season will perform poorly if planted too far south because it will not be warm enough for them to produce nectar at the time of flowering. Kauri Park (2016) has collated data on a number of mānuka varieties which includes their natural flowering time. For example, trees in the Far North flower in weeks 39 to 44, while Hawke’s Bay varieties flower in weeks
49 to 1. Further south, varieties fromWestport flower in weeks 51 to 3. Therefore, if the Far North variety was planted too far south, it is unlikely to provide the bees a sufficient nectar source. Obtaining the most information from a site For beekeepers and landowners wanting to understand the variability over a hive site, the way a site is sampled may differ depending on whether it is a naturally planted site or if the trees are from a nursery. The higher the number of plants analysed from one site, the greater the amount of information that will be gained. Samples could be collected, then combined, to get an average concentration for a site, but this comes with the risk of masking high-producing plants. For example, 10 trees were sampled from a naturally planted site. The results were analysed as individual samples and a portion of each were combined to created one sample. Eight of the 10 samples were below detection limit, and the remaining two samples had high DHA (8,243 and 27,070 mg/800 g sugar). When the concentration of individual samples were averaged, the result was 3,531 mg/800 g sugar; however, the composite sample was below the detection limit of the method (see Figure 2 on page 15). In comparison, samples collected from 10 trees on a site that had been planted with nursery-supplied trees had only a 2% difference between the average results and the composite result. The popularity of nectar testing is growing due to the information that can be learned about a floral variety or a hive site. The normalised results are used to compare sites
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