GIFSQ Tools · Module 2 — Food Chemistry & Physicochemical Testing · Tool 29 of 500
Log a water activity (aw) reading with its temperature and check it against the microbial growth thresholds — 0.60, 0.85 and 0.90.
Try sample values — fills the form with the worked example below.
Reading log
Saved on this device only. Use "Add current reading to log" after a calculation.
| Date / time | aw | Temp (°C) | Category | Spec max | Within spec |
|---|
–
Water activity (aw)
–
Growth category
–
Specification
Equation: Water activity aw = p ÷ p0, where p is the vapour pressure of water in the sample and p0 the vapour pressure of pure water at the same temperature. This tool compares the reading with the growth thresholds; it does not calculate aw from other measurements.
Method: AOAC 978.18 (water activity, chilled-mirror dew point). Growth thresholds from the International Commission on Microbiological Specifications for Foods (ICMSF) and the FDA Food Code: below 0.60 no microbial growth; 0.60–0.85 moulds and yeasts can grow; 0.85–0.90 some bacteria (including Staphylococcus aureus) can grow; at and above 0.90 most bacteria, including pathogens, can grow. An aw of 0.85 is the time/temperature-control threshold in the FDA Food Code.
Assumptions: the sample is equilibrated at the stated temperature; the meter is calibrated with standard salt solutions.
Limitations: water activity alone does not prove safety — pH, temperature, time and preservatives matter too. The reading shifts with temperature, so always record it.
Rounding: water activity to 3 decimal places.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Method: AOAC 978.18 (water activity, chilled-mirror dew point). Growth thresholds from the International Commission on Microbiological Specifications for Foods (ICMSF) and the FDA Food Code: below 0.60 no microbial growth; 0.60–0.85 moulds and yeasts can grow; 0.85–0.90 some bacteria (including Staphylococcus aureus) can grow; at and above 0.90 most bacteria, including pathogens, can grow. An aw of 0.85 is the time/temperature-control threshold in the FDA Food Code.
Assumptions: the sample is equilibrated at the stated temperature; the meter is calibrated with standard salt solutions.
Limitations: water activity alone does not prove safety — pH, temperature, time and preservatives matter too. The reading shifts with temperature, so always record it.
Rounding: water activity to 3 decimal places.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: A dried fruit sample reads aw 0.850 at 25 °C against a specification maximum of 0.850. The reading sits on the 0.85 threshold: moulds and yeasts can grow, and some bacteria can grow, so it meets the specification exactly but has no margin.
Enter values above to see results.
Typical aw: fresh meat 0.98–0.99, bread 0.94–0.97, jam 0.80–0.85, dried fruit 0.60–0.75, milk powder 0.20–0.30, crackers 0.10–0.20. Below 0.60 a food is shelf-stable against microbial growth. Tool 29 of 500 — GIFSQ
