Why has my honey fermented?

What actually happened

Honey does not spoil because its water activity is too low for microbes — most of its water is bound up with sugar and unavailable. Osmotolerant yeasts are present in honey as a matter of course and do nothing at all while it stays concentrated. Raise the water content and they begin to ferment the sugars, producing carbon dioxide and alcohol.

• Harvested before it was ripe — the bees had not yet reduced the water and capped the comb.
• Stored somewhere humid with the lid loose. Honey is hygroscopic and will draw water out of the air.
• Partially crystallised, which is the commonest cause. Crystals are nearly pure glucose, so the liquid left around them holds a higher share of the water and can cross the threshold locally even when the jar as a whole is fine.

How you know

Fine bubbles rising, a raised or domed lid, a smell of alcohol or cider, foam on the surface and a sharp or yeasty taste. That is different from the harmless white froth of trapped air after bottling, and different again from the pale bloom crystallising honey leaves on the glass.

Preventing it

Keep the lid on, store at ordinary room temperature and away from steam. Cold does not cause it — refrigeration speeds crystallisation, which is a texture change rather than spoilage. The underlying variable is water, and once honey is above the threshold nothing about storage will bring it back.