Many factors work in harmony to make honey a chemical masterpiece. To begin, honey is hygroscopic, meaning that honey absorbs water from anything around it. This is because honey is a super-saturated solution and the water content is low, at 16 to 18%. This is lower than the water content of most bacteria and fungi, meaning that the honey will dehydrate it via osmosis. In addition, honey has low water activity. Water activity is a measure on a scale of 0 to 1 that determines the water available to support microbial growth in a substance. Most moulds and bacteria are unable to grow in environments of water activity under 0.75. The water activity of honey is 0.6. Therefore, the combination of low water activity and osmotic action on bacteria and fungi make honey very resistant to spoilage; unless one adds water or leaves it uncovered in a humid room.
The next big factor that makes honey resistant to spoiling is acidity. Honeys' average pH is 4. This is because the action of bee enzymes on some of the glucose molecules produces gluconic acid, and there are also formic and citric acids in the honey. Many bacteria thrive in neutral pH conditions, so this acidity adds to the anti-microbial conditions provided by the low water content. Lastly, the production of gluconic acid also produces some hydrogen peroxide, which inhibits bacteria growth too.
These antibacterial properties are potent enough that honey will be effective as a wound dressing, so consider adding a sealed container of honey to your first aid kit!
Brunning, A., The Chemistry of Honey. Compound Interest, 2014.
Geiling, N., The Science Behind Honey’s Eternal Shelf Life. Smithsonian Magazine, August 22, 2013.