🌡️ Water Activity (aw) in Food and Animal Feeding Stuffs

Food Chemistry PN-ISO 21807

Overview

STATUS (as of 02.11.2026). PN-ISO 21807:2005 — Polish version, “Microbiology of food and animal feeding stuffs — Determination of water activity”, published on 05.09.2005, implements ISO 21807:2004 [IDT]; the PKN catalogue card has no withdrawal note (read on 02.11.2026). At the international level ISO 21807:2004 (published on 13.09.2004) was withdrawn on 01.11.2018 and replaced by ISO 18787:2017 “Foodstuffs — Determination of water activity” (first edition, November 2017); there is no Polish adoption of ISO 18787 in the PKN catalogue. The description follows ISO 18787:2017 (iTeh preview with clauses 1–9); the requirements of ISO 21807:2004 are given where they differ.

SCOPE (ISO 18787, clause 1). Principles and requirements for methods of determining the water activity (aw) of food intended for human consumption and of animal feeding stuffs, in the measuring range from 0 to 1. The method does not apply to products stored below freezing point (appearance of ice crystals in the product), water-in-fat emulsions or crystalline products such as sugars, salt and minerals; for products containing volatile substances, for example alcohols, the apparatus may need to be adapted. According to ISO 21807, water activity makes it possible to predict the growth of microorganisms and the microbiological stability of a product and to assess its shelf life.

DEFINITION (clause 3). aw = pF(T) / PS(T) — the ratio of the partial pressure of water vapour in equilibrium with the product under test at temperature T to the saturation pressure of water vapour over pure water at the same temperature. It is a dimensionless quantity from 0 (completely anhydrous sample) to 1 (pure water without salts); the equilibrium relative humidity is 100 times aw. The FDA inspection guide No. 39 gives the same: aw = ERH/100. According to ISO 21807, the water activity of most foodstuffs lies between about 0.992 (raw meat) and about 0.800 (salted and dried products).

MEASURING PRINCIPLES (clause 4). Dew-point measurement: the sample is placed in a sealed chamber with a cooled mirror (Peltier module) until condensate appears on its surface; at equilibrium the relative humidity in the chamber equals the water activity of the sample, and aw is determined from the dew-point temperature and the sample surface temperature. Sensors that change the conductivity of an electrolyte or the electrical permittivity of a polymer measure the relative humidity of the air above the sample; it equals the water activity only when the sensor and the sample are at the same temperature. Other principles (change in the length of a thread, freezing point in an open system) are alternative methods that still require validation. ISO 21807 listed eight measuring principles, from manometric pressure measurement to the freezing point.

APPARATUS AND REAGENTS (clauses 5–6). Apparatus for measuring water activity: linear response over the calibration range, a chamber suited to the measuring principle, temperature control ensuring 25 °C ± 1 °C, internal resolution of at least 0.000 1 aw, display of at least 0.001 aw, end of measurement on reaching a plateau with an amplitude of not more than 0.000 3 (three successive readings or stability for 1 min), if necessary a system eliminating interference from volatile substances (e.g. filters). In addition, a class A volumetric flask, a balance weighing to the nearest 1 mg (reading 0.1 mg), homogenizing tools and measuring cups — preferably new for each sample. Salts of more than 99 % purity, water according to ISO 3696, saturated salt reference solutions prepared in the laboratory or commercial ones — with a known uncertainty — and optionally activated carbon. ISO 21807 (Annex A) gives the water activity of saturated salt solutions at 25 °C, among others: MgCl₂ 0.328, K₂CO₃ 0.432, Mg(NO₃)₂ 0.529, NaCl 0.753, KCl 0.843, BaCl₂ 0.902, K₂SO₄ 0.973.

SAMPLE AND MEASUREMENT (clauses 7–8). The sample is stored in sealed containers so that its aw does not change (change in moisture, freezing, crystallization of salts or sugars). A sample with non-uniform water activity is homogenized without increasing or decreasing its moisture (e.g. by heating during too intensive grinding); a homogeneous sample needs no preparation. Depending on the purpose, selected zones of the product (e.g. the centre and the outer part of ripened meat products) or separate components of composite products are tested, and this is stated in the test report. The test portion is taken quickly, according to the manufacturer’s instructions; after preparation it is measured within 4 h at the latest. Measurement is carried out at 25 °C; before each series the apparatus is checked with a reference with an activity close to that of the samples and adjusted if the mean deviation exceeds 0.005; during the series its stability is checked with reference solutions. Carry-over between samples is limited by waiting about 15 min after opening the cover, by measuring granular activated carbon between samples, and by measuring in order from the lowest to the highest aw. With electrolytic and polymer sensors the chamber is ventilated before the next measurement (hysteresis). Calibration — with at least two references covering the sample values, a check at one point with a maximum permissible error of ±0.005, after adjustment a check at an intermediate point with the same error.

RESULT (clause 9). The result is reported to two decimal places. ISO 21807 required the method to measure aw from 0.999 to 0.600 with a repeatability corresponding to a standard deviation of 0.002, and the apparatus to be calibrated at least once a day at no fewer than three points.

REGULATIONS. Under Regulation (EC) No 2073/2005 (Annex I, Chapter 1, category 1.3, footnote 8), ready-to-eat products with pH ≤ 4.4 or aw ≤ 0.92, products with pH ≤ 5.0 and aw ≤ 0.94, and products with a shelf life of less than 5 days are automatically considered to belong to the category of foods in which growth of L. monocytogenes cannot occur. In US regulations a water activity of 0.85 is the point that defines whether a low-acid or acidified canned food is subject to 21 CFR Parts 108, 113 and 114 (FDA guide No. 39).

WHAT WE DO NOT GIVE HERE. The table of water activities of saturated salt solutions from ISO 18787 (Annex A), the precision data (clause 10) or the results of interlaboratory studies (Annex B) — these parts of the standard are not in the preview; nor do we give aw limits for the growth of individual microorganisms, which the standards do not contain.

Method principle

In a closed measuring chamber the sample reaches equilibrium with the surrounding air; the equilibrium relative humidity divided by 100 is the water activity of the sample (aw = pF / PS). ISO 18787 allows two measuring principles: dew point (cooled mirror, aw from the dew-point temperature and the sample surface temperature) and sensors that change the conductivity of an electrolyte or the permittivity of a polymer, which are reliable only when the sensor and the sample are at the same temperature.

Measurement is carried out at 25 °C ± 1 °C; the apparatus is calibrated with at least two saturated salt solutions covering the sample values and checked with a maximum permissible error of ±0.005. The result is reported to two decimal places.

Applications

• Food intended for human consumption and animal feeding stuffs — water activity from 0 to 1 (ISO 18787, clause 1)
• Prediction of microbial growth, microbiological stability and shelf life of a product (ISO 21807, introduction and clause 1)
• Assigning ready-to-eat food to category 1.3 of Regulation (EC) No 2073/2005 — among others aw ≤ 0.92, or pH ≤ 5.0 and aw ≤ 0.94
• Meat products with a water activity gradient after drying — measurement in selected zones of the cross-section (ISO 18787, 8.2)

Key parameters

Standard

Standard number

PN-ISO 21807:2005

Title (PL)

Mikrobiologia żywności i pasz — Określanie aktywności wody

Title (EN)

Microbiology of food and animal feeding stuffs — Determination of water activity

Step-by-step procedure

1. Storage and preparation of the sample

   Sealed container; homogenization only of a sample with non-uniform aw, without changing its moisture; product zones or components — stated in the test report (8.1–8.2).

2. Test portion

   Quick transfer into the cup according to the manufacturer’s instructions; measurement within 4 h of preparation at the latest, test portion at a temperature close to 25 °C (8.3–8.4).

3. Check of the apparatus

   Before the series, a reference with aw close to the samples; mean deviation greater than 0.005 — adjustment (8.5–8.6).

4. Measurement

   At 25 °C, until a plateau with an amplitude of not more than 0.000 3; limiting carry-over: a pause of about 15 min after opening the cover, activated carbon between samples, order from the lowest to the highest aw (5.1, 8.5).

5. Calibration

   At least two references covering the sample values; check at one point ±0.005, after adjustment at an intermediate point ±0.005 (8.6).

6. Result

   aw to two decimal places (9).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• The standard does not address all safety problems associated with its use — safety measures are established by the user (warning in ISO 18787)
• Barium chloride used for a reference solution is toxic if swallowed — follow the safety data sheet

Frequently asked questions