What is water activity?
Water activity is the measurement of the equilibrium relative humidity (ERH) of a material, that is the humidity that a hygroscopic material generates when it comes into balance with the air surrounding it in a sealed headspace. This “Available Water” is the unbound water able to come and go from material by adsorption/desorption.
The water activity of the sample is equal to the relative humidity of the air surrounding the sample in a sealed measurement chamber.
Water Activity can be expressed as either:
Equilibrium relative humidity (ERH) scaled 0% – 100% ERH
Water activity (aW) scaled 0 – 1 aW (Most microbiologists tend to use aW units).
Air relative humidity is influenced by temperature so it follows that equilibrium relative humidity (water activity ) will be too. The higher the aW value, the greater the influence of temperature on the stability of water activity. Typically, a sudden change in the ambient temperature of 2°C can result in several ERH% shifts in the reading of a previously stabilised sample. Whilst this can re-equilibrate over time, it may take several hours to do so and an erroneous reading may have been taken in the meantime.
The aw of a product may be critical to ensure microbial stability and safety, it may even be a legal parameter, often measured as part of Critical Control Point (CCP) validation, monitoring or verification. Water activity can be used for microbiological growth control, shelf-life, the stability of product composition (moisture migration), general product quality (texture, taste, potency & colour). Some laboratories offer accredited testing for aw.
How is water activity measured?
Water activity is usually measured as an offline sample measurement in the laboratory or 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 that 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 over half an hour, depending on the hygroscopic nature of the sample. It is normally the samples with higher oil content that 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 that are used as preservatives (common examples are: glycerol and sorbitol) and can affect the readings of water activity if not filtered out.
Some typical food aw values:
|Raw meats||0.97 – 0.99|
|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|
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 achieving 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.
- 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.
What instruments are used to measure water activity
Novatron supply the full range of Novasina precision, high-quality water activity meters and are the sole UK agent and suppliers for Novasina Swiss-made precision instrumentation. The range includes portable battery-operated instruments, leading up to large multi-channel instruments. The Novasina water activity instruments feature a unique measuring sensor cell for very high accuracy measurement of water activity, with no hysteresis, low drift and resistance to a wide range of chemicals and solvents that would otherwise affect the results of other water activity sensors. Better than 0.003 aW repeatability is achievable from the instruments and resilience to powders, splashes and solvents are guaranteed by the wide range of protective filters for all applications. Used around the world in many industries from food and pharmaceuticals to oil and chemicals. The water activity instruments can be used in process/quality control on the production line or laboratory analysis and product development. All instruments can be serviced, maintained and calibrated by Novatron either on-site or in-house by our specialist team of highly trained engineers. Novatron is UKAS accredited to ISO 17025 & UKAS traceable calibration can be performed if required.
Novatron water activity instruments
A Novasina video on water activity
Service & Calibration
Regular service and calibration of all measuring instruments is essential to guarantee continued reliability of results, fast readings and certificates of compliance for your own quality system and audits.
Service & Maintenance
It’s important to perform regular maintenance and servicing on your
Defensor humidiﬁers to ensure they are working their best and keeping
the environment at the preferred humidity.
Novatron Scientiﬁc has a team of qualiﬁed, experienced engineers who
will visit your premises to carry out preventative maintenance and make
sure that your portable humidiﬁers remain safe, efﬁcient and reliable.