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Measuring Water Activity

Some typical food aw values:

Distilled Water 1.0
Tap water 0.99
Raw meats 0.97 – 0.99
Milk 0.97
Juice 0.97
Cooked bacon 0.85
Saturated NaCl solution 0.75
Cakes 0.6 – 0.8
Honey & jam 0.5 0.7
Dried fruit 0.5 – 0.6
Biscuits & cereals 0.3
Pharmaceutical tablets (as low as) 0.04

Water activity is usually measured as an offline sample measurement in the laboratory or in the production area.  Either bench-top or portable instruments may be used but often a sample temperature controlled meter will be required where high aw values are expected for micro-growth related applications.

Most water activity meters are bench-top instruments which rely on taking a small sample, putting it into a plastic measuring cup which is placed into the sample testing chamber. This will then remain sealed whilst the sample equilibrates with the air headspace above it, which can take anything from a few minutes to in excess of half an hour, depending on the hygroscopic nature of the sample.  It is normally the samples with higher oil content which take longer to equilibrate but most materials will take no more than 15 mins to reach a reliable reading.  For samples above 0.9 aw (90% erh) it is recommended that the instrument used employs sample temperature control to max +/- 0.2°C.  The internationally specified temperature for testing aW is 25°C for the food industry.

Since it is only the water vapour in the air gap (relative humidity) above the sample which is to be measured, there needs to be some consideration given to the other non-aqueous volatiles which may be present in the air as well. These can be solvents which are used as preservatives (common examples are: glycerol and sorbitol) and can affect the readings of water activity if not filtered out.

Oily, high fat samples (emulsions, creams, butter etc.) at the surface can impede the ability of the sample to equilibrate with the air space above the sample.

Ultra hygroscopic products (normally below 0.1 aw) such as powdery samples can create dust build-up in some instruments with fan-assisted air stirring measurement chambers.

Production processes may rely upon close aw control, sometimes to within only 2% erh. Careful sample preparation, regular calibration, stable temperatures and correct use of the aw meter are all essential to achieve the most accurate readings. A small change in aw may mean a significant implication for product stability & shelf-life. Certain micro-organisms may only grow above specific levels of water activity so it is important to record the most reliable readings possible.

Sample preparation:

Consider the actual component of the product being measured for aw, for example the core may be different to the shell.

Homogenise the sample well beforehand.

Store samples in well-sealed containers to prevent moisture loss / ingress.

Take care with chilled or hot samples – consider condensation! Allow the sample to reach near room temperature before testing.