Water Activity

Beyond Moisture Content

While moisture content tells you the total percentage of water in honey, water activity (aw) measures something more specific: how much of that water is available for chemical reactions and microbial growth. Water bound to sugar molecules is not available to support bacteria or yeast. Only "free" water, molecules not bound to the honey's sugar matrix, can participate in microbial growth.

Honey's water activity is typically 0.5 to 0.65. For context, most bacteria require aw above 0.9 to grow. Most molds need aw above 0.8. The only organisms that can survive at honey's low water activity are osmophilic (sugar-tolerant) yeasts, and even these remain dormant below aw of 0.6. This is why honey is effectively self-preserving: its sugar concentration binds so much water that nothing can grow in it.

The Fermentation Threshold

When honey's moisture content rises above 20% (corresponding to aw above roughly 0.65), osmophilic yeasts can activate and begin fermenting the sugars, producing alcohol, carbon dioxide, and off-flavors. This is why beekeepers test moisture content before bottling and why crystallized honey left in humid environments (where it can absorb atmospheric moisture through the crystal surface) is more susceptible to fermentation than liquid honey in a sealed jar.

Practical Implications

For our operation, water activity explains why our honey needs no preservatives, no refrigeration, and has no practical expiration date when stored properly. The sugar concentration does the preserving. However, it also means proper sealing matters: honey left in an open container in Florida humidity will gradually absorb atmospheric moisture, eventually raising the water activity to a point where fermentation becomes possible.