Glossary
An explanation of water activity (aw), the food science concept that explains why honey is one of the few foods that never spoils. Water activity measures the availability of water in a food for microbial growth, and honey's extremely low water activity is the primary reason for its indefinite shelf life.
Water activity (aw) is the ratio of vapor pressure of water in a food to the vapor pressure of pure water at the same temperature. Pure water has an aw of 1.00. Most bacteria require aw above 0.91 to grow, most yeasts above 0.88, and most molds above 0.80. Below these thresholds, microbial growth is impossible.
Honey has a water activity of approximately 0.55-0.62, far below the threshold for any harmful microorganism. At this water activity, even the most drought-tolerant bacteria, yeasts, and molds cannot grow. This is why properly stored honey has an effectively unlimited shelf life.
Honey's low water activity results from its extremely high sugar concentration (approximately 80 percent sugars by weight). The sugar molecules bind most of the water molecules through hydrogen bonding, making those water molecules unavailable for microbial use. This is the same preservation mechanism used in jams, candied fruits, and concentrated syrups.
This osmotic effect is also why honey draws moisture from wound surfaces (beneficial for wound care) and from microbial cells (killing bacteria through osmotic dehydration). The same property that preserves honey indefinitely also makes it an effective antimicrobial agent.
Properly stored honey with moisture content below 18 percent does not spoil. However, if honey absorbs moisture (from being stored uncovered in a humid environment) and its water activity rises above 0.60-0.62, osmotolerant yeasts can begin fermentation. This is not spoilage in the toxic sense but does change the product.
Researchers have found 3,000+ year-old honey in Egyptian tombs that was still edible. While this is frequently cited, it demonstrates long-term preservation under specific conditions (sealed, dark, temperature-stable tomb). Honey stored in less ideal conditions can still absorb moisture and ferment, so the claim is true but context-dependent.
Honey provides dual preservation: osmotic preservation (similar to sugar) plus enzymatic antimicrobial activity (glucose oxidase producing hydrogen peroxide) plus acidic pH (3.2-4.5). Sugar provides only osmotic preservation. Honey is therefore a more comprehensive preservative.
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