🌡️ Water activity (aw)
Measurement of water activity (aw) in food — ratio of water vapor pressure over sample to vapor pressure over pure water. Critical parameter for food stability and safety.
Overview
Water activity (aw) is a measure of water availability in a product for chemical, enzymatic reactions and microbial growth. It is defined as the ratio of water vapor pressure over the sample (p) to vapor pressure over pure water (p₀) at the same temperature: aw = p/p₀. The scale ranges from 0 (absolutely dry) to 1.0 (pure water).
Water activity is a critical parameter in food safety and stability assessment. Pathogenic bacteria (Salmonella, Listeria, E. coli) do not grow at aw < 0.90; yeasts at aw < 0.88; molds at aw < 0.70; below aw = 0.60 no microbial growth occurs. Therefore, FDA and EU regulations use aw as a criterion for food classification by microbiological risk.
Water activity measurement is performed by physical methods described in ISO 21807 and ISO 18787. Most popular methods are: (1) chilled mirror dew point measurement — most accurate, measurement time 5 min, accuracy ±0.003 aw; (2) measurement of electrolyte conductivity change or capacitive sensor capacity — time 30–60 min, accuracy ±0.01 aw.
Water activity differs from moisture content — two products with the same moisture may have different aw, depending on how tightly water is bound to the matrix (hydrogen, osmotic, capillary bonds). For example, honey with 18% moisture has aw ≈ 0.60, while fresh meat with 75% moisture has aw ≈ 0.99. Therefore, aw is a better predictor of microbiological stability than moisture alone.
Method principle
Water activity measurement is based on achieving thermodynamic equilibrium between the sample and the closed gas space above it. At equilibrium, the relative humidity of air above the sample (ERH — Equilibrium Relative Humidity) equals water activity: aw = ERH/100. Chilled mirror (dew point) method involves cooling a mirror to the temperature at which water vapor condenses — from temperature difference, vapor pressure and aw are calculated. Sensor method (capacitive/resistive) measures the change in electrical properties of a polymeric sensor absorbing moisture from the equilibrium space above the sample.
Applications
- Microbiological stability assessment of food (FDA/EU criterion)
- Quality control of dried products (meat, fruits, herbs)
- Testing of confectionery products (chocolate, caramel, candy)
- Control of cereal products (flour, grain, bread)
- Cosmetic and pharmaceutical analysis (stability)
- Shelf-life design and minimum durability date
- Food classification by microbiological risk (HACCP)
- Control of drying, freeze-drying and salting processes
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 0–1.000 aw |
| Accuracy (dew point) | ±0.003 aw |
| Accuracy (sensor) | ±0.01 aw |
| Measurement time | 5 min (dew point), 30–60 min (sensor) |
| Measurement temperature | 25.0 ± 0.5°C (standard) |
| Microbiological thresholds | < 0.60 — no growth; < 0.90 — no pathogenic bacteria |
Standard
- Standard number
- PN-EN ISO 21807:2004 / ISO 18787:2017
- Title (PL)
- Mikrobiologia żywności i pasz — Oznaczanie aktywności wody
- Title (EN)
- Microbiology of food and animal feeding stuffs — Determination of water activity
Step-by-step procedure
1. Meter calibration
Calibrate aw meter with saturated salt standards in the range appropriate for samples (e.g., NaCl 0.753 and BaCl₂ 0.903).
2. Sample preparation
Homogenize sample (if solid — cut into small pieces). Fill sample cup to ¾ volume. Sample must be at 25 ± 2°C.
3. Placement in chamber
Place cup in meter measurement chamber. Close chamber tightly.
4. Water activity measurement
Start measurement. Wait for equilibrium (automatic signal). Dew point: ~5 min. Sensor: 30–60 min.
5. Result reading
Record aw value to 0.001 accuracy. Record measurement temperature. Perform at least 2 replicates.
6. Control verification
Every 10 samples measure aw standard (saturated NaCl = 0.753). Deviation max ±0.003 (dew point) or ±0.01 (sensor).
7. Cleaning
Clean measurement chamber with dry cloth. Do not use water or solvents in sensor chamber.
8. Reporting
Report result as average of replicates to 0.01 aw accuracy. Report measurement temperature.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Water activity meter (dew point) | METER AquaLab 4TE, Novasina LabMaster-aw neo | 25 000–55 000 PLN |
| Water activity meter (sensor) | Rotronic HygroLab C1, Testo 650 with aw probe | 8 000–25 000 PLN |
| Disposable sample cups | METER/Novasina cups Ø 40 mm, pack of 100 pcs. | 300–600 PLN |
| Water activity standards (saturated salts) | Salt set: LiCl (0.113), MgCl₂ (0.328), NaCl (0.753), BaCl₂ (0.903) | 500–1 500 PLN |
| Thermostat / bath (option) | For temperature stabilization of samples before measurement | 3 000–8 000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Lithium chloride LiCl (aw standard 0.113) | 7447-41-8 | Saturated solution for calibration in low aw range |
| Magnesium chloride MgCl₂ (aw standard 0.328) | 7786-30-3 | Saturated solution for calibration in medium aw range |
| Sodium chloride NaCl (aw standard 0.753) | 7647-14-5 | Saturated solution — most commonly used calibration standard |
| Barium chloride BaCl₂ (aw standard 0.903) | 10361-37-2 | Saturated solution for calibration in high aw range |
Health and safety (OHS)
- BaCl₂ — toxic (H301, H332). Use gloves and goggles. Do not swallow!
- LiCl — irritating (H302, H315, H319). Avoid skin and eye contact.
- Food samples may contain pathogens — use gloves with raw meat samples.
- Personal protection — gloves, lab coat.