Why honey lasts so long

Four defences, working together

Honey resists spoilage through a combination of properties, none of which would be sufficient alone. It is the overlap that makes it so unusually stable.

• Low water activity. Honey holds around 17% water, but almost all of it is bound up with sugar molecules rather than available. Water activity is typically around 0.5-0.6, and most bacteria need above 0.9. There is simply not enough free water for microbes to live in.
• Acidity. Most honey's pH sits around 3.5-4.5 (nearer 5 in chestnut and honeydew honeys), largely from gluconic acid produced by the bee-added enzyme glucose oxidase. Most spoilage bacteria are uncomfortable below pH 4.5.
• Hydrogen peroxide. Glucose oxidase continues working slowly in stored honey, generating small quantities of hydrogen peroxide over time. This is honey's main antibacterial mechanism, and it is why the manuka case — which does not depend on peroxide — was considered notable enough to research.
• Osmotic pressure. A concentrated sugar solution actively draws water out of any cell that lands in it, which is lethal to most microbes before they can establish.

Where it stops being true

Honey does not spoil the way milk spoils. It can still be ruined, and there are three specific ways.

The second way is dilution. The defences above are all consequences of concentration, and a wet spoon left in the jar introduces both water and microbes. The third is quality loss rather than spoilage: over years, aroma compounds fade, colour darkens, and hydroxymethylfurfural rises. Such honey is safe and increasingly dull. That is the honest reading of a best-before date on a honey jar — a quality date, not a safety one.

The one genuine safety exception

About the honey found in Egyptian tombs

The claim that archaeologists found 3,000-year-old honey in Egyptian tombs and ate it is repeated constantly and is worth handling carefully. Honey has genuinely been recovered from ancient contexts, and given the chemistry above it is entirely plausible that a well-sealed vessel of honey would remain chemically recognisable for a very long time. What is much harder to find is a documented case of anyone analysing such a sample and confirming it was still edible honey rather than a crystallised, degraded residue.