This question originally appeared on Quora.
By Jonas M Luster, http://jluster.org/
For something to spoil there has to be something to spoil it. Honey is almost unique among organic compounds in that it constitutes a "perfect storm" of attributes against spoilage:
Most honey is a supersaturated, the rest is a saturated, solution of sugar. Sugar acts hygroscopic, which means it attracts water. Bacteria and some other microorganisms that come in contact with this solution are being desiccated (water is drawn from them into the solution) and explode (ok, ok, they kind of just shrivel, but I like the idea of them blowing up) and die.
This supersaturation of sugar also inhibits the growth of yeast and other fungal spores.
Its pH is 3.26 to 4.48, a killing field for bacteria. Combined with the above-mentioned supersaturation, you have both a pH that weakens bacterial walls and a hygroscopic environment. Them bacters don't stand no chance.
And if all that isn't enough, bees process honey by means of an enzyme called glucose oxidase, which modifies sugar into gluconic acid ( D-glucono-δ-lactone, a contributor to the above-mentioned pH) and hydrogen peroxide. You might know glucose oxidase from something else: it used to be called "Penicillin A" and is now known as Notatin. Poor bacteria, eh?
This is, by the way, why you should never leave a jar of honey standing open. The supersaturated sugar solution will absorb moisture from the air and gradually become weaker, losing its anti-bacterial properties.
One last warning: honey is, as we discover above, rather safe. It does, however, sometimes contain inactive spores of Clostridium botulinum, the bacterium responsible for botulism. Healthy humans don't get sick from that, but infants whose intestinal tract dilutes the honey without digesting it quickly can get sick from it. There is honey that has been radiated with gamma rays to kill those spores dead for good that can be purchased for lots of money. Just wait until the kid is a year old or so and you'll be safe.