Water Activity (aw)
Water activity, written aw, measures how much of the water in a food is free to escape, move around, and be used by microbes or chemical reactions. It runs on a scale from 0 (bone dry) to 1.0 (pure water), and it predicts shelf life and safety better than moisture content alone.
The idea
Not all water in food is equal. Some is tightly bound to sugar, salt, proteins, or starch and cannot easily leave. The rest is free. Water activity compares the vapour pressure of water above a food with that of pure water at the same temperature. If a sealed jar of food reaches a humidity of 85%, its aw is 0.85.
Microbes need free water to grow. If the food holds its water tightly, a bacterium cannot pull enough across its own membrane, even when the food feels moist.
Typical values
| Food | Approx. aw | What it means |
|---|---|---|
| Fresh meat, fish, milk | 0.98–0.99 | Spoils quickly; refrigerate |
| Bread | 0.94–0.97 | Moulds within days |
| Aged cheese, salami | 0.85–0.90 | Bacteria slowed; some moulds grow |
| Jam, jelly | 0.75–0.85 | Moulds possible once opened |
| Honey | 0.5–0.6 | Very shelf-stable |
| Dried pasta, flour | 0.5–0.6 | Shelf-stable when dry |
| Crackers, chips | 0.2–0.3 | Crisp; goes stale if it climbs |
Growth thresholds
- About 0.91: most pathogenic bacteria stop growing
- About 0.86: limit for Staphylococcus aureus
- About 0.80: limit for most moulds
- About 0.60: almost no microbial growth
These are approximate. Acidity, temperature, and preservatives all interact, which is why cured meats combine salt, low aw, and sometimes nitrite.
How cooks lower water activity
Salt in curing, brining, and fermenting binds water and draws it from cells by osmosis.
Sugar does the same in jams, candied fruit, and syrups. A jam is roughly 65% sugar for a reason.
Drying removes water outright, as in jerky, dried fruit, and dehydrated herbs.
Freezing turns water into ice, which is unavailable to microbes. It does not kill most of them, though; they resume when thawed.
Texture effects
Water activity also governs texture. Crisp foods stay crisp only at low aw, so a cracker left out on a humid day absorbs moisture and softens. Soft foods dry out when stored next to drier ones, because water moves from high aw to low aw until they match.
Low water activity is not the same as sterile. Spores can survive in dry foods and wake up when water is added, so cooked rice and rehydrated foods still need prompt refrigeration.
Questions
What is the difference between water activity and moisture content?
Moisture content is how much water a food contains by weight; water activity is how much of that water is free to move and be used. Jam can be about 30% water yet have low water activity, because sugar binds most of it.
What water activity stops bacteria from growing?
Most food-poisoning bacteria need about 0.91 or higher. Staphylococcus aureus can grow down to about 0.86, many moulds to about 0.80, and almost nothing grows below 0.60.
Why doesn't honey spoil?
Honey has a water activity around 0.5–0.6 plus natural acidity, so microbes cannot pull enough water out of it to grow. If it absorbs moisture and its water activity rises, it can ferment.
How does water activity affect crispness?
Crackers and chips are crisp at very low water activity, around 0.3 or below. As they absorb moisture and climb toward 0.4–0.5, the starch network softens and they go stale and bendy.