Water Activity vs Moisture Content: What's the Difference and Why It Matters
A clear explanation of water activity (aw) and moisture content, the microbial growth limits that depend on aw, typical values for common foods and how aw is measured in the lab.
Two foods with 20 % moisture can be completely different. One may be a shelf-stable jam; another could spoil in days. The difference is how tightly the water is held, and that is what water activity measures.
Definitions
Moisture content is a quantity: grams of water per 100 g of food. It is measured by drying, distillation or titration (see how to calculate moisture content).
Water activity is an energy state:
where p is the water vapour pressure above the food and p0 is the vapour pressure of pure water at the same temperature. Pure water has an aw of 1.00. In practice it equals the equilibrium relative humidity (ERH) of the air around the food, divided by 100.
Water is "bound" or made less available by dissolved solutes (sugar, salt), by interactions with surfaces of proteins and starches, and by capillary effects in porous structures. That is why adding salt or sugar lowers aw even when moisture stays the same.
Why water activity matters
Microbial growth
Each microorganism has a minimum aw below which it cannot grow. The figures below are widely cited approximate minimums under otherwise favourable conditions:
| Minimum aw | Organisms that can still grow |
|---|---|
| 0.95 and above | Most Gram-negative bacteria (e.g. Pseudomonas, Salmonella, E. coli); Clostridium botulinum type A and proteolytic B grow down to about 0.94 |
| about 0.91 | Most spoilage bacteria |
| about 0.88 | Most spoilage yeasts |
| about 0.86 | Staphylococcus aureus (growth, aerobic) |
| about 0.80 | Most spoilage moulds |
| about 0.75 | Halophilic (salt-loving) bacteria |
| about 0.61 | Xerophilic moulds and osmophilic yeasts |
| below 0.60 | No microbial growth (but survivors can persist) |
US low-acid canned food regulations use aw 0.85 as one boundary: foods with aw at or below 0.85 are not regulated as low-acid canned foods regardless of pH. FDA's Food Code also uses water activity, together with pH, to decide which foods require time and temperature control for safety.
Chemical and physical changes
Water activity also controls:
- Lipid oxidation, which is often slowest at intermediate-low aw and faster at very low aw
- Non-enzymatic (Maillard) browning, which typically peaks at intermediate aw
- Texture: crackers go soft and chewy above a certain aw; dried fruit hardens when aw falls
- Moisture migration between components, such as a dry biscuit layer and a moist filling. Water moves from high aw to low aw, not from high moisture to low moisture
Typical water activity values
| Food | Approximate aw |
|---|---|
| Fresh meat, fish, milk, most fruit and vegetables | 0.97–0.99+ |
| Bread | about 0.95 |
| Cured hams, some aged cheese | 0.85–0.95 |
| Jams and jellies | 0.75–0.85 |
| Dried fruit | 0.60–0.75 |
| Honey | about 0.6 |
| Dry pasta, flour | 0.4–0.6 |
| Crackers, biscuits, crisps | 0.2–0.4 |
| Milk powder, instant coffee | 0.2 or below |
Values vary by recipe and process; measure your own products.
The sorption isotherm
A moisture sorption isotherm plots moisture content against water activity at a fixed temperature. It is the bridge between the two numbers. With an isotherm for your product, you can:
- Set a moisture specification that guarantees a target aw
- Predict how much moisture a dry product can pick up before it goes soft
- Choose packaging with the right water vapour barrier
Isotherms often show hysteresis: the curve during drying (desorption) sits above the curve during wetting (adsorption). Measure in the direction relevant to your process.
How water activity is measured
- Chilled-mirror dew point instruments cool a mirror until dew forms, giving the dew point of the headspace above the sample. They are fast and accurate and are widely used in QC labs.
- Capacitance (resistive or capacitive humidity) sensors measure the relative humidity of the headspace. They are often cheaper and handle some volatile-rich samples better, but generally need longer equilibration.
Good practice:
- Verify daily with salt standards that bracket your samples (saturated or unsaturated salt solutions supplied by the manufacturer)
- Control temperature; aw changes with temperature
- Fill the sample cup to the recommended level, without contaminating the rim
- Measure samples promptly and keep them sealed, since dry samples gain moisture from room air
Putting it together
Use moisture content for yield, labelling, legal compositional standards and drying calculations. Use water activity for safety, microbial stability, texture and shelf life. In many products, aw combines with pH, preservatives and temperature as part of a multi-barrier system; see hurdle technology.
Sources
- Fennema, O. R., Damodaran, S. and Parkin, K. L. (eds.). Fennema's Food Chemistry, 5th edition. CRC Press, 2017. Chapter on water and ice.
- Barbosa-Cánovas, G. V., Fontana, A. J., Schmidt, S. J. and Labuza, T. P. (eds.). Water Activity in Foods: Fundamentals and Applications, 2nd edition. Wiley-Blackwell / IFT Press, 2020.
- U.S. Food and Drug Administration. 21 CFR 113.3, definitions for thermally processed low-acid foods packaged in hermetically sealed containers.
- U.S. Food and Drug Administration. Food Code, 2022 edition. Definitions and tables for time/temperature control for safety (TCS) food.
This guide is educational. Safety-critical limits (thermal processes, pH limits, allergen claims) must be confirmed with your process authority, certification body or regulator for your product and market. Spotted an error? Tell the editors.