Honey Water Activity: The Science of Why Honey Keeps So Long

What water activity means

Water content tells you how much water a food contains. Water activity tells you how much of that water is actually available. Sugar and salt hold on to water molecules so tightly that microbes cannot use them.

Scientists measure water activity on a scale from 0, completely dry, to 1, pure water. Food safety experts use it to predict which microbes could grow in a food.

Honey water activity compared with other foods

Honey is about 17 to 18 percent water, but its dense sugars bind that water tightly. The result is a very low water activity.

Why honey keeps for so long

Smithsonian Magazine's well-known article on the science behind honey's eternal shelf life explains that honey's low moisture, high acidity and natural hydrogen peroxide work together.

Low water activity draws moisture out of any microbe that lands in honey through osmosis. Acidity, explained in our honey pH level article, makes conditions even harsher. Small amounts of hydrogen peroxide in honey add another layer.

That is why our guide to whether honey goes bad explains that well-stored honey keeps its safety for a very long time, even if flavor and color slowly change.

When honey water activity rises

Honey is hygroscopic, which means it pulls moisture from the air. If a jar is left open in a humid kitchen, the top layer can absorb water. That raises water activity just enough for natural yeasts to wake up.

Those yeasts can ferment the honey, producing a sour, beery smell and sometimes foam. Our article on fermented honey explains how to spot it.

• Leaving the lid off for long periods
• Using a wet spoon
• Storing honey in a damp place
• Honey harvested before bees finished drying it

How beekeepers protect it

Bees cap honey with wax once it is ripe and dry enough. Good beekeepers harvest capped frames and may check moisture with a refractometer. US grading standards, published by the USDA, include moisture limits for top grades.

Raw honey that is harvested ripe and stored well rarely ferments.

Honey water activity and crystallization

When honey crystallizes, glucose forms solid crystals and the remaining liquid holds slightly more water. In some cases this can nudge honey water activity up in the liquid part, which is why beekeepers watch moisture carefully in honeys that set quickly.

Well-ripened honey usually handles crystallization without any problem. Our crystallized honey guide explains how to soften it gently.

If you see a thin layer of liquid on top of a set jar and notice a sour smell, that is a sign moisture has crept in. A sweet smell and firm crystals are normal.

Honey water activity at a glance

Here is a quick summary to keep in mind:

• Honey water activity is usually between about 0.50 and 0.65.
• Most bacteria need about 0.90 and most molds about 0.80 to grow.
• Sugar binds water, so honey can contain about 17 percent water and still stay stable.
• Moisture from the air or a wet spoon is the main risk at home.
• Low water activity does not destroy spores, so honey is never for babies under 12 months.

Why honey water activity matters for safety

Low honey water activity stops most microbes from growing, but it does not destroy every one. Bacterial spores, including those of Clostridium botulinum, can survive in honey in a dormant state.

Healthy older children and adults handle these spores without trouble. Babies under 12 months do not yet have a mature gut, so the CDC advises never giving honey to infants. Our guide on why babies can't have honey explains more.

Storing your jar

Keep honey at room temperature with the lid closed tightly, use a dry spoon and avoid the fridge, which speeds crystallization. Crystals are harmless and can be softened in warm water.

Every Al Safa honey comes in glass, which seals well and does not absorb odors. Our Pure Raw Honey is a pantry staple. Pick up a 3 or 6 jar pack and save, with free shipping on orders over $50, in our shop.