Manuka Honey

Manuka honey is produced in New Zealand from two closely related plants, both of which are commonly referred to as manuka.

The most common honey source of these is Leptospermum scoparium, Other names for this plant include kahikatoa, red tea tree, and red manuka.

The Leptospermums are a genus comprising around 50 species that is widely spread throughout the South West corner of the Pacific.

The other plant is Kunzea ericoides (reclassified from Leptospermum ericoides in 1983) and is called manuka and kanuka. Other names include white manuka, white tea tree and heath like manuka.

Both the manuka and kanuka plants have historically been used by Maori and early European settlers for medicinal purposes. These include use of the bark as a poultice, for colds, for flu, and stomach aches. Both plants are called "tea tree" from the practice of making a tea from the leaves.

The honey is dark coloured, (around 84mm average colour ± 11.8mm SD - Pfund scale), strongly flavoured, with a herbal, woody characteristic, and is often highly "thixotropic" (jellied) like European Heather honey (Calluna vullgaris). Another Leptospermum in Australia (L. polygalifolium) also derives its name (Jellybush) from the thixotropic nature of its honey.

Manuka is classified as an over represented pollen type and has a higher than normal conductivity (about 4 times that of normal flower honeys.)

Conductivity

Conductivity is an indirect measurement of the mineral content of a honey. Most flower honeys have low mineral content and a low conductivity. Manuka however has a conductivity that is approaching that of some honeydews. It has an average of 5.8 ± 1.54 SD. This may be due to manuka being a honeydew source, or it may be a feature of manuka honey.

Manuka as a Honeydew Source

Both manuka and kanuka are inhabited by a variety of scale insects, but particularly Eriococcus orariensis and Coelostomidia sp. These scale insects are producers of honeydew and the consequence of this is often seen as black sooty mould on the plants, and the plants exuding a sweet honeydew smell. Often this smell can be detected more than 200 metres away from the source. The sooty mould is seen as a blackness all over the plants but particularly on the branches and stems of the plants.

It is common for honeydew elements (fungal particles from the sooty mould) to be found in manuka honey. It is likely that some of the high conductivity for manuka honey is caused by it being a honeydew source.

Pollen Analysis

Pollen analysis of manuka honey is a reliable determinant of its floral origin in most cases. There are instances however where some other honey plants can provide a significant proportion of the nectar without contributing to the pollen spectrum. Two in particular are worth noting. These are Rewarewa and Beech honeydew. Both these honeys have a colour similar to manuka and both have stronger flavours that are not completely dissimilar to manuka.

In the case of Rewarewa, it has a low total pollen count. Any honey purporting to be manuka with a low total pollen count (less than 100,000 pollen grains per 10 grams) and with the presence of Rewarewa pollen, should be carefully examined, even if it has more than 70% manuka.

The same applies to blends of manuka and Beech honeydew. This particular blend can be very difficult to assess due to the high conductivity and presence of honeydew elements of manuka. Local knowledge of the production location is helpful here.

The pollen of both manuka and kanuka are indistinguishable from each other under a compound microscope. Any attempt to differentiate between the two honeys is thwarted by this and also the close proximity of both plants to each other, their close (often overlapping) time of flowering, and the fact that both plants are referred to by the common name "Manuka".

Antibacterial Activity in Manuka Honey - "Active" Manuka Honey

One area that is of particular interest regarding manuka honey is its antibacterial activity. Often this is just shortened to "Active" or "Active Manuka". Most honeys are in some way antibacterial (some quite highly so), but normally this antibacterial activity is almost exclusively derived from Hydrogen Peroxide (H2O2) and is referred to as Peroxide Activity or PA. This is created from the activity of the enzyme Glucose Oxidase in honey. Like many enzymes, Glucose Oxidase can become inactivated over time by by light and heat. The stronger the light and/or heat, the faster it is inactivated. Room temperature and low light, given enough time, will in theory also reduce the Glucose Oxidase activity.

Non Peroxide Activity

Manuka honey also has this varying degree of antibacterial activity due to H2O2, but has been found to have a further amount of antibacterial activity that is present after the H2O2 has been neutralized with Catalase. This activity is referred to as the Non Peroxide Activity (NPA). The letters UMF ("Unique Manuka Factor") have been privately trademarked in New Zealand (UMF®) to represent a standard of NPA antibacterial activity that is compared to the disinfectant phenol. The UMF® letters are usually appended with a number. This number refers to the percentage of phenol in water. e.g. UMF12 equals an NPA activity equal to or greater than a 12% solution (%w/v) of phenol/water. Unlike Glucose Oxidase (the source of PA), the NPA in Manuka is more stable to moderate heat, light and even gamma radiation. Until 2006 only a small part of the NPA had been accounted for with the discovery of a number of naturally occurring compounds in manuka honey.

MGO - Methylglyoxal

In 2006 Methylglyoxal was discovered to be the main substance in manuka honey responsible for NPA by professor Henle from Dresden University. This work was confirmed and elaborated by Waikato University in 2007. MGO is found in numerous food substances but only at low levels (usually less than 10 ppm) compared to high NPA manuka honey. MGO is a member of the dicarbonyl group (a group of toxic substances) and at the levels found in some manuka honeys, (up to 1,000 ppm) there is some concern regarding its food safety. MGO is the main precursor to Advanced Glycation End products (AGEs). AGEs are associated with a number of age related diseases including Alzheimer's disease, cardiovascular disease, stroke, eye cataracts, cancer and diabetes. The body has a specific enzyme system (the glyoxalase system) to detoxify this compound.

The main benefit of NPA/MGO active manuka is that it can be sterilized by irradiation for use as a wound dressing (this same irradiation would neutralize any glucose oxidase). As a topically applied wound dressing the MGO has little chance of entering the body to cause any significant negative effects. However, eating high NPA/UMF/MGO manuka honey is another story. Since manuka with MGO has no proven MGO related benefit once swallowed (see below for information on stomach ulcers), it should be noted that oral consumption of manuka with high NPA values may provide a significant health risk.

Further research at Waikato University in New Zealand has shown that MGO in manuka honey is derived from dihydroxyacetone (DHA) that can be found in the flowers of some manuka sub species. It is clear from this research (published April 2009) that this substance is found in differing quantities in various manuka sub species. Because of this variability it cannot therefore be used as a quantitative floral marker compound for manuka honey. i.e. the NPA activity of manuka is NOT an indicator of the purity of manuka honey, in contrast to some claims to the contrary. In fact honey with moderate levels (10-15 % phenol equivalent) of NPA activity may only have very small levels of manuka honey.

Not all manuka honey has PA and not all manuka has NPA. Some manuka honeys have both types of activity, and some have little or none. There is also a great deal of seasonal variation, with both types of activity being individually either present or absent in any particular honey season. To date, manuka has been tested in the laboratory against several strains of wound infecting bacteria and found to be effective in inhibiting the growth of most of them.

Glossary.

PA - Peroxide activity. The antibacterial activity that is derived from Hydrogen Peroxide found in most honey in varying amounts
Glucose Oxidase - An enzyme in honey mostly responsible for the formation of Hydrogen Peroxide.
NPA - Non Peroxide Activity. ANY antibacterial activity found in any honey once it has been treated with Catalase to remove any hydrogen peroxide. Measured by microbial assay.
Catalase - An enzyme that breaks down Hydrogen Peroxide
UMF® - A brand owned by the Active Manuka Honey Association (AMHA) that provides a measurement scale of the NPA in manuka honey relative to phenol. When used on honey products it is usually associated with a number being the equivalent of a percentage of phenol.
AMHA - Active Manuka Honey Association. http://www.umf.org.nz. Owner of the UMF brand.
MGO - Methylglyoxal the substance attributed with the NPA in some manuka honeys. Measured directly and levels usually reported in parts per million (same as milligrams per kilogram Mg/Kg)
DHA - Dihydroxyacetone, the manuka plant derived substance from which MGO is formed in manuka honey. Also is the key ingredient in most sunless tanning products - causes browning of the skin.
AGEs - Advanced Glycation End Products. Substances associated with age related diseases. MGO is the main precursor of AGEs.
Phenol - A chemical with antiseptic properties. It is used as a comparative standard for the measurement of antibacterial activity in microbiological assays of honey. Numbers usually refer to a percentage of Phenol in water. A higher number indicated a higher percentage of Phenol, thus higher antibacterial activity.

Additional References

Manuka Honey and Stomach Ulcers (Helicobacter pylori)

"Active" Manuka honey has also been shown in vitro (in the test tube) to inhibit the growth of Helicobacter pylori - the bacteria considered the main cause of stomach ulcers. However clinical trials in New Zealand with manuka (and repeated by clinical trials in the UK) failed to show manuka to be effective against Helicobacter pylori in the stomach. Further research on this, particularly the ideal delivery system,including dosage rates, is needed before claims of a cure for stomach ulcers can be made. IMPORTANT! Anyone contemplating using manuka honey as a treatment for stomach ulcers should only do so under the guidance of a health professional.

© 1999 Airborne Honey Ltd