Water Activity Reference

Water activity (aw) is the fraction of water in a food that is available to microbes — not how wet it feels, but how much of that water they can actually use. It runs from 0 (bone dry) to 1.0 (pure water). For cured, salted, and dried foods, aw is the hurdle that does the preserving, the same way pH is the hurdle for an acid ferment. This page collects the thresholds that matter and where common products land.

For how aw works alongside salt, acid, and temperature, see preservation-hurdles. For the drying methods that lower it, see method-drying-dehydration.

The thresholds that matter

aw threshold	What it gates	Source
≤ 0.85	FDA shelf-stable / regulatory line. At or below this, a low-acid or acidified food is not subject to the low-acid/acidified canned-food regulations (21 CFR 108/113/114) — aw alone provides the safety margin against C. botulinum.	FDA: Water Activity (aw) in Foods
below 0.93	Clostridium botulinum cannot grow. Above this, botulinum growth is possible if other hurdles (pH, refrigeration) are absent.	FDA: Water Activity (aw) in Foods
below 0.91	Most spoilage bacteria stop (general bacterial growth limit).	FDA: Water Activity (aw) in Foods
below 0.88	Staphylococcus aureus toxin production stops (growth can continue lower under aerobic conditions, but toxin formation does not).	FDA: Water Activity (aw) in Foods
below 0.80	Most molds and yeasts stop; below ~0.70 even osmophilic/xerophilic spoilers are largely halted.	FDA: Water Activity (aw) in Foods

Caution

aw ≤ 0.85 is the regulatory shelf-stable line; aw below 0.93 is the botulism line. An acidified food is defined as one with a finished equilibrium pH at or below 4.6 and aw above 0.85 — meaning products held below pH 4.6 or at aw ≤ 0.85 control C. botulinum. Drying and curing that depend on aw require validated processes (cure salts, salt level, drying schedule, monitoring). We do not publish cure schedules; for low-acid cured/dried meats refer out to NCHFP, USDA/FSIS, and FDA. Taste and texture are never a safety test for aw. See safety-and-botulism.

Where common products land

Product	Typical aw	Shelf-stable on aw alone (≤0.85)?	Notes
Fresh vegetables / fruit	0.97–0.99	No	High free water; preserved by acid, salt, cold, or drying — not aw.
Fresh lacto ferment (sauerkraut, kimchi)	0.95–0.98	No	Safe via pH (≤4.6), not aw. Keep cold once finished.
Fresh bread	0.95–0.97	No	Stales and molds quickly; aw is high.
Soft cheese	0.95–0.99	No	Relies on refrigeration and/or acid.
Cured ham (country/dry-cured)	0.88–0.92	No (controls botulism only below 0.93)	Salt + nitrite + drying; below 0.93 stops botulinum but is not yet ≤0.85 shelf-stable.
Hard/dry sausage (salami)	0.83–0.90	At the line — depends on final aw	Fermented then dried; only shelf-stable when validated to land at aw ≤0.85.
Aged hard cheese (Parmesan)	0.68–0.85	Yes (at the dry end)	Low moisture + salt; very long keeping.
Jam / jelly	0.80–0.85	Yes (well-made)	High sugar binds water; near the line.
Maple syrup, honey	0.55–0.75	Yes	High sugar; below mold growth.
Dried fruit (raisins, apricots)	0.55–0.75	Yes	Sugar + low moisture.
Jerky (properly dried)	0.60–0.80	Yes (when validated)	Salt + heat + drying; must hit target aw with a tested process.
Dried herbs / spices	0.20–0.50	Yes	Effectively no available water.
Crackers, dry crisps	0.10–0.30	Yes	Bone dry; only fail by picking moisture back up.

Note

Equilibrium matters. aw is measured on the finished, equilibrated product — a salami’s surface and core, a ham after its full dry, a jar after its contents settle. A product can read safe on the surface while the center is still above the line. When aw is your preserving hurdle, the number that counts is the highest aw anywhere in the food at equilibrium.

Using aw as a hurdle

• aw is one hurdle of several. Most cured/dried foods combine lowered aw with salt, nitrite, acid, smoke, and/or cold. Lowering aw raises the bar for every microbe at once, but it is rarely the only control — see preservation-hurdles for how the hurdles stack.
• Salt and sugar lower aw by binding water. This is why salt percentage and sugar concentration show up as preservation levers: they make existing water unavailable rather than removing it.
• Drying lowers aw by removing water. method-drying-dehydration covers schedules and how far to take a product.
• Below 0.85 you leave the canned-food regulations, but only with a process validated to land there reliably — not an eyeballed “it seems dry enough.”

Tip

You cannot judge aw by feel. Two products at the same “dryness” to the touch can sit on opposite sides of the 0.85 line depending on salt and sugar content. If shelf stability depends on aw, it depends on a measured aw from a validated process — a meter or a tested recipe, never a hunch.

See also

• preservation-hurdles — how aw stacks with salt, acid, and temperature
• method-drying-dehydration — lowering aw by removing water
• safety-and-botulism — the botulism hurdle and when to refer out
• salt-percentage — salt as a water-binding (aw-lowering) lever
• ref-ph-targets — the parallel pH hurdle for acid foods

Sources: FDA — Water Activity (aw) in Foods; FDA — Acidified & Low-Acid Canned Foods Guidance.