Manuka (Leptospermum scoparium) is a shrub or small tree with tiny white flowers, native to New Zealand and south-eastern Australia. Its honey is thick, amber and strongly flavoured. What made it famous – and expensive – was something laboratories found: an unusually strong and stable ability to stop bacteria growing in a test tube.

The puzzle of non-peroxide activity

In laboratory tests most honeys act against bacteria mainly through hydrogen peroxide, produced by the enzyme glucose oxidase. That effect is easily destroyed by heat or light. In the 1980s the biochemist Peter Molan at the University of Waikato found that manuka honey kept its activity even when the peroxide was removed. The property became known as non-peroxide activity, and the UMF mark (Unique Manuka Factor) was created to grade it. For years, nobody knew what caused it.

The answer came from Germany in 2008. Thomas Henle's group at the Technical University of Dresden identified the main active compound as methylglyoxal (MGO). Six samples of manuka honey contained between 38 and 761 milligrams per kilogram – up to a hundred times more than ordinary honeys.

From nectar to jar

Manuka nectar, however, contains no methylglyoxal. It contains a different compound, dihydroxyacetone (DHA). In 2009 New Zealand researchers showed that fresh manuka honey is high in DHA and low in MGO; stored at 37 °C, the DHA fell while the methylglyoxal rose. When they added DHA to clover honey, methylglyoxal appeared in similar amounts. The conversion is slow and not one-to-one, and the DHA content of nectar varies with the plants, the region and the year.

What the numbers mean

The figure after 'MGO' is the methylglyoxal content in milligrams per kilogram. UMF is run by the UMF Honey Association in New Zealand: licensed laboratories measure methylglyoxal, leptosperin (a compound characteristic of manuka that confirms authenticity), dihydroxyacetone and hydroxymethylfurfural (HMF), which reveals heating or long storage. The grades correspond roughly as follows: UMF 5+ to about MGO 83, UMF 10+ to about MGO 263, UMF 15+ to about MGO 514 and UMF 20+ to about MGO 829.

Government rules and a fight over the name

Since 5 February 2018 any honey exported from New Zealand as manuka must pass a test at a laboratory recognised by the Ministry for Primary Industries (MPI). The test looks for four compounds from the nectar (3-phenyllactic acid, 2′-methoxyacetophenone, 2-methoxybenzoic acid and 4-hydroxyphenyllactic acid) and for manuka DNA from pollen. Monofloral manuka must meet all five criteria at set levels; otherwise it may still qualify as multifloral. Methylglyoxal is not part of the definition: the test establishes authenticity, not 'strength'. The word itself is disputed with Australia, where honey from related Leptospermum species is also produced. In 2021 the UK Intellectual Property Office refused to register 'manuka honey' as a certification mark reserved for New Zealand, finding that the public also associates the name with Australia.

Sources

  1. Mavric et al. (2008): Identification and quantification of methylglyoxal as the dominant antibacterial constituent of Manuka honeys
  2. Adams, Manley-Harris, Molan (2009): The origin of methylglyoxal in New Zealand manuka honey
  3. MPI: science-based definition for mānuka honey and export requirements
  4. UMF Honey Association: UMF grading – the four factors
  5. Charles Russell Speechlys: opposition to the 'Manuka Honey' certification mark in the UK