Manuka
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 also called "tea tree" from the practice of making a tea from the leaves. It should be noted however that there is no published research that shows a significant therapeutic benefit from ingesting manuka honey.
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
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.
Manuka as a Honeydew Source
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
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 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 in 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".
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". 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.
Glossary
This is a list of terms and acronyms that are, and have been, used in relation to manuka honey. Presenting them here should not be taken to mean that these terms carry any validity in the claims they may imply.
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
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.
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 (and other) honey.
Additional References
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, plus the presence of 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 when it is eaten (ingested).