Water activity

Definition

Water activity describes how much of a food's water is actually available to microorganisms, on a scale where pure water is 1.0. Honey contains 15-20% water by weight, but almost all of it is bound to sugar molecules, giving a water activity of around 0.5 to 0.6. Most bacteria need about 0.9 and most moulds around 0.7, so honey denies them the water they need regardless of how much is nominally present. This, together with acidity, osmotic pressure and enzymatically generated hydrogen peroxide, is why honey resists spoiling.

Where the word gets used loosely

Water content and water activity are not the same thing, and confusing them produces the common misreading that honey is 'too dry' for microbes. It is not dry; the water in it is simply not available. This also explains the failure mode: dilute honey even modestly and every one of those defences weakens at once.

Guide records that discuss this

• Why honey lasts so longFour separate mechanisms make honey hostile to microbes. Understanding them also shows exactly where the famous claim that honey never spoils stops being true.
• What is honey?Honey is nectar that bees have concentrated and chemically altered, then sealed in wax. Every property people notice about it — the sweetness, the keeping quality, the setting — follows from that process.
• Why stingless bee honey is chemically unusualMore water, more acid, a sugar nobody expected, and antimicrobial protection that comes partly from tree resin rather than from the honey itself.