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 (warrior wood), red tea tree, and red manuka.
The other plant is Kunzea ericoides (reclassified from Leptospermum ericoides in 1983) and is called manuka and kanuka. Other names include white manuka, makahikatoa (white warrior wood), white tea tree and heath like manuka, (ericoides means heath like). A botanical review "A revision of the New Zealand Kunzea ericoides (Myrtaceae) complex" published in 2014 splits this one species into 10 with K. ericoides now confined to the North West of the South Island. K. robusta is now the most common Kunzea in New Zealand.
The Leptospermums are a genus comprising over 80 species that is widely spread throughout the South West corner of the Pacific. Hybridizing and cross breeding occurs naturally between some of the species and this is also used as a tool by plant breeders to create ornamental varieties with a high variability of leaf and flower colours and petal variations.
For a further detailed discussion on "manuka" and common names, click here.
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 physical identification between the two plants is often difficult even for experienced observers. In some cases there are easily identifiable characteristics e.g. the flowers and seed capsules for manuka can be up to twice the size of those of kanuka and mature kanuka is usually significantly larger than any manuka, but with the absence of these clues, a more detailed knowledge of the taxonomic differences is required to differentiate the two. One notable difference is that the leaves feel spikier on L. scoparium than K. ericoides, but without prior experience, this may be difficult to interpret.
Colour - Physical Characteristics
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 vulgaris). Another Leptospermum in Australia (L. polygalifolium) also derives its name (Jellybush) from the thixotropic nature of its honey.
Manuka honey has an average glucose of 29.7%, fructose of 37.9%, maltose of 1.2% and sucrose of 0.5% (HPLC method, 775 samples)
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 higher than normal conductivity (about 4 times that of normal flower honeys.) approaching that of some honeydews. It has an average of 0.58 ± 0.154 standard deviation (SD). This may be due to manuka being a honeydew source, or it may be a feature of manuka honey.
Both manuka and kanuka are inhabited by a variety of scale insects, but particularly Eriococcus sp and Coelostomidia sp. These scale insects are producers of honeydew and the consequence of this is often observed as a black sooty mould on manuka and kanuka, 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 possible that some of the high conductivity for manuka honey is caused by it being a honeydew source.
Anecdotally there are observations from beekeepers reporting a significant amount of honey production (>20kg per hive) after the manuka flowering had finished with the collected honey looking and tasting like manuka. It is possible that this is a cause of low manuka pollen in honeys exhibiting some of the characteristics of manuka, but little or no formal study has been done in this area.
Pollen analysis of manuka honey is a reliable determinant of its floral origin in most cases. Manuka is classified as an over represented pollen type requiring over 70% manuka pollen to classify as a monofloral honey. There are instances however where some other honey sources can provide a significant proportion of the nectar without contributing to the pollen spectrum so a level of 70% manuka pollen or more can overstate the contribution of manuka nectar. 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 200,000 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 but in extreme cases, the sugar spectrum will be biased more to the honeydew side. Local knowledge of the production location is also helpful here.
Because of manuka honey's thixotropic nature, the honey extraction process requires a mechanical loosening or "pricking" prior to the frames being extracted. Some producers scrape the combs back to the mid rib rather than use a honey loosener. Being a valuable honey, producers may also take honey from around or close to the hive's brood nest where there is a high occurrence of stored pollen. Both this and the extraction techniques peculiar to manuka honey may result in high total pollen counts of pollen species collected by bees foraging on pollen and not derived from the nectar source. For pollen analysis purposes these species are extraneous and cause the percentage of manuka pollen to be lower. Producers need to adjust their management to minimize this effect and buyers need to be aware of this effect when interpreting pollen data.
There is in an effort to discredit the 70% manuka pollen level required for a monofloral classification by some saying that L. scoparium produces little pollen and bees aren't observed gathering much of it. But it is nectar gatherers that produce honey, not pollen gatherers (the two tasks are separate) and as this macro photo of a manuka nectar gatherer clearly shows (click on this link or the image for a close-up), the source of pollen in manuka honey (which is invisible to the naked eye) is the pollen falling into the nectar from the anthers of the plant and then picked up by the bees collecting that nectar. It is understandable when one looks at the image at the small size here (already larger than life) that one might wonder about where the pollen is coming from. Once you see the close-up, all is revealed.
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".
One area that is of particular interest regarding manuka honey is its antibacterial activity. Often this is just shortened to "Active" or "Active Manuka". Considerable research has been carried out in this area, particularly testing against various species of bacteria in the laboratory and clinical trials for topical applications such as wound dressings. However it is important to note that there has been no clinical research that shows a benefit from this activity once ingested.
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 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. It is claimed by some sellers that they pack their product in dark coloured jars to protect this enzyme from light. Another reason perhaps is their desire to hide the variable nature of the contents from the discerning consumer.
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®) and originally represented a standard of NPA antibacterial activity that was compared to the disinfectant phenol. The UMF® letters are usually appended with a number. This number referred to the percentage of phenol in water. E.g. UMF12 equaled an NPA activity equal to or greater than a 12% solution (%w/v) of phenol/water. With the advent of labeling regulations preventing unproven therapeutic claims, this numbering system is no longer allowed to be promoted as an indicator of antibacterial activity. 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.
Methylglyoxal - MGO
In 2006 Methylglyoxal (MGO) (Wikipedia Link) 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 on 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, (over 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. This enzyme system has been found in the simplest life forms on earth as well as in mammals, indicating that its detoxification of MGO is universally important to most life on earth. Animal studies have shown MGO to cause cancer in mice and heart disease in cattle at levels functionally close to some high levels found in manuka honey.
The main benefit of NPA / MGO active manuka is that it can be sterilized by irradiation for use as a wound dressing. Theoretically this same irradiation would neutralize any glucose oxidase due to the large molecule size and fragility of glucose oxidase but this is not borne out by this study.
It was thought that as a topically applied wound dressing the MGO has little chance of entering the body to cause any significant negative effects but recent research showed manuka honey was significantly worse for diabetic patients in a wound dressing trial than patients without diabetes. And eating high NPA / UMF® / MGO manuka honey should present more concerns. 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 MGO values may provide a significant health risk. MGO's main effect is to destroy proteins and particularly the action of enzymes, which are responsible for the basic functioning of cellular metabolism. A side effect of this toxicity is that it kills bacteria.
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 L. scoparium sub species. (DHA is a readily available pharmaceutical chemical that is used as the key ingredient in sunless tanning products - it causes browning of the skin). It is clear from this research (published April 2009) that this substance is found in differing quantities in various L. scoparium sub species.
To quote from the research:"All the manuka nectars contained dihydroxyacetone but in varying amounts"
".... there is variation in the amount of dihydroxyacetone in the nectar and that certain manuka trees have the potential to produce honeys with high nonperoxide antibacterial activity, whereas others do not."
Because of this variability it cannot therefore be used as a quantitative floral marker compound for manuka honey. I.e.the level of 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% phenol equivalent) of NPA activity can be shown to have less than 20% manuka honey. Additionally, MGO increases over time as the DHA decays into MGO as can be seen here. If the MGO content is changing all the time, it cannot represent the proportion of manuka nectar. Muddying the waters even more, 2011 research in Australia (home of the Leptospermums) found DHA/MGO in honey from four Leptospermum species. This line of research is in its infancy and It seems that many more of the over 80 species of Leptospermums will contain DHA/MGO.
And recent 2014 research has shown that the addition of DHA to a clover honey produced over 1,600 ppm of MGO in 83 days raising the spectre of MGO being "created" in honeys by the addition of DHA, a further reason the use of MGO as an indicator of manuka content is problematic.
Not all manuka honey has PA and not all manuka honey 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. It should be noted that action recorded in a petrie dish does not automatically translate into the same action with topical application or particularly ingestion as has been demonstrated with stomach ulcers (see below).
Active - A contraction of "antibacterial activity" meaning the property of inhibiting the growth of bacteria. Not allowed as a claim on labelling in New Zealand.
PA - Peroxide Activity. The antibacterial activity that is derived from Hydrogen Peroxide found in most honey in varying amounts. Not allowed as a claim on labelling in New Zealand
TPA - Total Peroxide Activity. Same as PA above.
TAA - Total Antibacterial Activity - a measurement of all antibacterial activity. Sometimes also called TA. Not allowed as a claim on labelling in New Zealand
Glucose Oxidase - An enzyme in honey mostly responsible for the formation of Hydrogen Peroxide and also producing much of honey's acidity.
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 against a standard antiseptic (see Phenol below). Not allowed as a claim on labelling in New Zealand
Catalase - An enzyme that breaks down Hydrogen Peroxide.
UMF® - A brand owned by the UMF Honey Association (UMFHA)
AMHA - Active Manuka Honey Association. http://www.umf.org.nz. Now changed to UMFHA - see below.
UMFHA - UMF Honey Association. See AMHA above. The owner of the UMF® brand
MGO - Methylglyoxal - a toxic substance attributed with most of the NPA in some manuka honeys. Measured directly and levels usually reported in parts per million (same as milligrams per kilogram mg/kg).
MG - Same as MGO - see above.
Hydrogen Peroxide - Formed in most honeys (including manuka) from the action of the enzyme Glucose Oxidase (see above). There is a large degree of variability between honeys. A highly effective antibacterial agent. Used by white blood cells as a mechanism for killing bacteria.
DHA - Dihydroxyacetone, the L. scoparium 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 causing age related diseases. MGO is the main precursor of AGEs
Phenol - Also known as Carbolic Acid. A chemical with antiseptic properties. It is used as a comparative standard for the measurement of antibacterial activity in microbiological assays of some honey. An old and outdated method, but Numbers quoted usually refer to a percentage of Phenol in water. A higher number indicates a higher percentage of Phenol, thus theoretically higher antibacterial activity. In practice the use of this methodology and scale is problematic and has been the cause of much uncertainty and debate over activity ratings for manuka honey.
Manuka Honey and Stomach Ulcers (Helicobacter pylori)
NPA Active Manuka honey has 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 NPA manuka (and repeated by clinical trials in the UK) failed to show manuka to be effective against Helicobacter pylori in the stomach.
The failure of these trials, along with the 15 year absence of further trials using manuka honey for stomach ulcers, and the glyoxalase enzyme system to detoxify the active ingredient in manuka honey (methylglyoxal) is compelling evidence that manuka honey is unlikely to have any benefit beyond a placebo effect.