Why can't most microbes grow in honey?

Its very high sugar concentration and low water content stop them reproducing, for osmotic reasons

What makes an open jar of honey risky in a humid kitchen?

Honey absorbs moisture from the air, and once its water rises much above 25%, its natural yeast can ferment it

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Why doesn't honey go bad?

Honey is so full of sugar and so short of water that microbes trying to live in it get sucked dry.

Honey doesn't go bad because it is a terrible place for microbes to live: it holds too much sugar, too little water and too much acid. Bees evaporate nectar until only 15.5% to 18% of it is water. At that concentration, for osmotic reasons, microbes can't reproduce, and fermentation stops. Put simply, any bacterium or yeast landing in honey has its water pulled away by all that sugar around it. On top of that, bees add an enzyme, glucose oxidase, that produces gluconic acid and hydrogen peroxide, making honey acidic and helping to suppress bacteria. Its average pH is 3.9.

A wooden frame of honeycomb with cells capped in pale wax — Finished honey, capped with wax by the bees: sealed from contamination and further evaporation.Photo: Michaelb1983 · CC BY 3.0

The result is a food with an indefinite shelf life, as long as its water content doesn't creep much above 18%. That is the one catch. Honey absorbs moisture straight from the air, and because it naturally contains yeast, it can start fermenting if its water content rises much above 25%. So the only real rule is to keep the lid on.

You may have heard that archaeologists found edible honey in Egyptian tombs. It's a good story, but not true: all such cases turned out to be other substances or only chemical traces. Honey has, however, preserved itself and objects dipped in it for centuries.