Hydroxymethylfurfural (HMF)

Background

Hydroxymethylfurfural (HMF) is one of the small molecules produced when sugars are heated, stored long-term, or exposed to acidic conditions — the broader chemistry sometimes referred to as the Maillard reaction family. HMF turns up in foods that have been baked, caramelised, or cooked at length, including breakfast cereals, jams, dried fruit, coffee, and many baked goods, generally at concentrations far higher than what is permitted in honey.

In honey, HMF is interesting for a different reason. Fresh honey, properly handled, contains very little HMF. As honey is heated or stored over time, HMF accumulates predictably. That predictable accumulation makes HMF a useful quality marker: a low HMF reading is consistent with careful handling and a fresh product, while an elevated HMF reading indicates that the honey has been overheated, stored too long, or both. International honey standards have used HMF in this role for decades, and Manuka honey grading systems have followed suit.

The relevance to Manuka honey specifically is that some of the antibacterial and enzymatic components of honey are heat-sensitive. Even though methylglyoxal (MGO) itself is comparatively heat-stable, sustained heat exposure can degrade glucose oxidase — the enzyme responsible for hydrogen-peroxide-based activity in honey generally — and can compromise other quality attributes. HMF therefore functions as a freshness signal that complements the more direct potency and authenticity markers.

How it works

The chemistry is straightforward. Fructose, which makes up roughly 38–40% of honey by weight, dehydrates under heat or extended storage to form HMF — losing water and rearranging into the characteristic furfural ring structure. The reaction is acid-catalysed and accelerates with temperature. Honey held cold develops HMF very slowly; honey held at room temperature develops HMF gradually over months and years; honey heated above typical processing temperatures develops HMF much more rapidly.

Other Maillard-type reactions can also contribute to HMF formation, particularly involving fructose and amino acids in honey. The shared signal across these pathways is the same: HMF rises with heat and time, in a way that is predictable enough to be used as a quantitative marker rather than just a qualitative flag.

The reason this matters for Manuka honey specifically is interlocking with the DHA-to-MGO conversion: warm storage accelerates MGO development, but it also accelerates HMF formation. A producer cannot simply hold a young, high-DHA batch hot to drive up MGO without also driving HMF towards its certification cap. The practical art of ripening Manuka honey is, in part, balancing those two reactions so that MGO can develop fully without HMF exceeding 40 mg/kg.

What the evidence shows

HMF as a quality marker in honey is uncontroversial — it is established in international standards (including the Codex Alimentarius standard for honey, which sets a 40 mg/kg cap for honey from temperate regions and 80 mg/kg for tropical sources) and is routinely measured in laboratory testing of commercial honey. UMF certification adopted the 40 mg/kg cap as part of its quality requirements, alongside thresholds for MGO, leptosperin, and DHA.

Studies of honey storage have characterised the temperature- and time-dependence of HMF accumulation in detail, with the practical conclusion that careful storage (cool, dark, sealed) keeps HMF low for years and that heat exposure during processing or in retail and home storage is the main driver of elevated HMF. Cooking or baking honey — for example into a hot drink, sauce, or baked good — produces measurable HMF rises, though typically still well below levels of safety concern in food.

What the evidence does not support is using HMF in isolation as either a potency or authenticity marker. HMF tells you about how a honey has been handled; it does not on its own tell you whether the honey is genuine L. scoparium (that requires leptosperin testing) or how potent its non-peroxide antibacterial activity is (that requires MGO testing). The four-marker panel — DHA, MGO, HMF, and leptosperin — is the way these questions are answered together.

Practical use

For consumers, HMF is the quality assurance check behind the UMF licence: a UMF-certified jar has been tested and confirmed below the 40 mg/kg cap. For most retail purchases, the practical signal is the UMF licence number and the laboratory testing it represents, rather than the HMF figure itself.

For people storing Manuka honey at home, the relevant practice is to keep it cool, dark, and sealed. Sustained heat exposure — for example storing honey above a stove, in a sunny window, or in a hot pantry — will accelerate HMF accumulation and reduce shelf life. Brief warming, such as stirring honey into a warm drink for consumption, is fine and is not the kind of exposure that drives HMF outside acceptable limits.

For people working in the supply chain, HMF is a routine analytical marker that complements MGO, DHA, and leptosperin in batch testing and certification. Storage and processing conditions are managed to keep HMF well below the 40 mg/kg cap throughout the product's life cycle, not just at the point of certification.

The honest summary is that HMF is the unglamorous but useful part of the Manuka honey marker panel: it does not tell you what the honey is or how potent it is, but it tells you a great deal about whether the honey has been treated properly along the way. The UMF and MGO grading primer walks through how the markers fit together in certification.