Water activity measures the fraction of water in a food that's actually free to move and act as a solvent — the water available to a bacterium or a mold spore, as opposed to water that's chemically tied up elsewhere. It runs on a scale from 0 to 1, where 1.0 is a dish of pure water and 0 is none available at all. Honey holds about a fifth of its weight in water and keeps for years on a shelf. A fresh loaf of bread, at a similar moisture content, grows visible mold within a week. The difference is what that water is doing, not how much of it there is.
Why moisture content doesn't predict spoilage
A moisture-content test just weighs a sample, dries it in an oven, and weighs it again. It tells you how much water left, not what the water that's still there is doing. Water activity asks a different question — it's the ratio between the vapor pressure of water in the food and the vapor pressure of pure water at the same temperature. Sugar and salt molecules form hydrogen bonds with water molecules strong enough to hold them in a fixed cluster, so that water can't evaporate as freely, and it can't be pulled into a microbial cell either. It's still there on the scale. It just isn't free anymore.
That's the whole honey story. Its water sits down around 0.60 to 0.65 on the activity scale, deep in the range where dried fruit also lives, because the sugar concentration is so high that almost every water molecule is locked to a sucrose or fructose molecule instead of drifting loose. A fresh loaf's water, by contrast, sits high on the scale — in the same top bracket as raw milk and fresh fruit — because a starch gel doesn't trap water nearly as tightly as a sugar solution does.
Where the growth actually stops
Every organism that spoils food has a water activity floor below which it can't function at all, and those floors stack up in a fairly narrow band near the top of the scale.
| Water activity | What stops growing below it |
|---|---|
| 0.93 | Clostridium botulinum, the most water-activity-tolerant common pathogen |
| 0.90 | Most spoilage bacteria, including Salmonella |
| 0.86 | Staphylococcus aureus, growing with oxygen present |
| 0.80 | Most molds and yeasts |
| 0.61 | Everything else — even drought-tolerant xerophilic molds and osmophilic yeasts stop here |
The FDA's line for whether a canned or acidified food needs refrigeration sits at 0.85 — not at 0.93, where botulinum stops, and not at 0.90, where most other bacteria stop. It sits just under 0.86, where staph stops. Botulinum has the scarier name, but staph is the more water-activity-tolerant organism of the two, and the regulatory number is built around the tougher one, not the more famous one. Foods that land between roughly 0.60 and 0.84 are what food scientists call intermediate-moisture foods: shelf-stable without refrigeration, because nothing left can grow, but nowhere near bone-dry.
The same move, four different ways
Salt, sugar, drying, and freezing all lower water activity, and they do it through only two mechanisms. Salt and sugar bind water directly — dissolve enough of either into a food and there's less free water left over, even though nothing has evaporated. It takes roughly 12% salt by weight, or 55% sugar, to bring water activity down into the range where Salmonella and its relatives stop growing, and about 15% salt or a saturated 65% sugar syrup to push out most of the remaining yeasts as well. That's the arithmetic behind a strong brine, a jar of jam at the right sugar concentration, or a bottle of soy sauce sitting at room temperature without spoiling.
Drying removes water outright, but not evenly. The first water to leave a food is the loosely held water near the surface, and water activity barely moves while it goes. What's left toward the end of drying is water already bound to starch or protein, and pulling out that last fraction drops water activity fast relative to how little total weight is actually leaving. A food can go from soft to leathery with only a modest drop in water activity, then from leathery to shelf-stable with a much smaller further loss of weight.
Freezing is the odd one out, because it doesn't remove water at all. It converts liquid water into ice, and ice can't act as a solvent for anything. A frozen food's total moisture content is exactly what it was before it went into the freezer. The water activity collapses anyway, because none of that water is available to a cell that needs to pull it across a membrane to survive.