The Slow Chemistry of Manuka

The number you see on the label was not measured at harvest

When a jar of manuka honey is stamped MGO 514+, that figure was almost certainly produced months after the honey was extracted from the comb. The bees do not put methylglyoxal into the honey. They put in dihydroxyacetone. The conversion is slow, non-enzymatic, and continues for the life of the jar.

This matters because two jars from the same hive, harvested the same week, can land at very different MGO ratings depending on how long they sat before testing.

DHA: the precursor in the nectar

Manuka (Leptospermum scoparium) nectar is unusually rich in dihydroxyacetone, a small three-carbon sugar derivative. The amount varies tree to tree and season to season. Adams and colleagues, working in 2008 and 2009, were among the first to characterize the relationship: high DHA at harvest predicts high MGO after maturation, but the conversion is not instant. DHA on its own has no meaningful antibacterial activity. It is the chemical raw material.

The conversion

DHA dehydrates non-enzymatically into MGO, no enzyme required. The reaction proceeds in the jar, on the shelf, in the warehouse. Grainger and colleagues at the University of Waikato published a series of kinetic studies between 2016 and 2018 that mapped the conversion rate against temperature and time. Two findings from that work stand out for the consumer:

1. The reaction takes 12 to 24 months to approach equilibrium. A jar tested three months after harvest is not done converting.
2. The DHA-to-MGO ratio settles around 2:1 in mature honey. So a honey that ends up at MGO 500 needed roughly 1000 mg/kg of DHA at the start.

Fresh manuka can show DHA-to-MGO ratios closer to 4:1 or higher. That is not deception. It just has not finished maturing.

Storage temperature: the consumer-facing variable

Grainger's group also looked at storage conditions. Higher temperatures speed the rate of MGO formation initially but lower the final yield. Cooler storage is slower but more efficient: more of the original DHA ends up as MGO rather than degrading into other byproducts. The practical implication is unintuitive. Storing manuka in a cool pantry produces, over time, a more potent jar than the same honey held in a warm shelf above a stove.

This is also why responsible producers store unpackaged honey under controlled conditions for 12 to 24 months before bottling. The jar you buy at MGO 514+ has already done most of its conversion in a temperature-controlled warehouse, not in your kitchen.

Two implications for buyers

First, a low-MGO jar bought fresh and held for a year will gain potency. Not dramatically, but measurably. If the producer has not aged the honey before bottling, time on the shelf is doing some of the work.

Second, the MGO number on a label reflects a single test point. Reputable producers re-test mature honey before label claims. Less reputable producers may be quoting a measurement from the original production run, which can drift in either direction.

What this does not explain

Nothing in DHA chemistry tells you whether the honey came from manuka. MGO can be raised in any honey by adding DHA syrup as a precursor and waiting for it to convert. The chemistry of the conversion is not species-specific. That is the whole reason a single MGO test is insufficient as a proof of authenticity. Other markers, leptosperin in particular, fill that gap. We will get to those.

For now, the takeaway is narrower: the MGO rating on a manuka jar is not a fixed property of the honey at harvest. It is the result of a slow chemical reaction that runs on the producer's clock and continues, mildly, on yours.