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    Water Activity – Reliable Measurement for Product Stability

    When product quality, shelf life, and safety depend on moisture conditions, water activity (aw) becomes a critical parameter.

    It gives you insight into how water behaves inside your product – and how that affects stability, microbial growth, and overall performance.

    Understanding and controlling water activity allows you to make informed decisions about formulation, storage, and processing.

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    Water Activity Is Not the Same as Moisture Content

    Water activity is often confused with total moisture content. However, the two describe different things.

    Water activity expresses the amount of free water available for microbial growth, not the total amount of water in a product.

    This means water activity is only relevant for hygroscopic materials – products that can absorb or release moisture depending on their surroundings.

    In practice, this includes materials such as:

    • Food products (e.g. biscuits, dried fruit, meat products)
    • Pharmaceuticals and powders
    • Wood, paper, and building materials
    • Concrete and other porous materials

    By contrast, materials like metal or stone do not have water activity, as they do not interact with moisture in the same way.

    The Water Activity Scale – What the Values Mean

    Water activity is expressed as aw on a scale from 0.00 to 1.00.

    • 0.00 aw → completely dry
    • 1.00 aw → pure water

    In practical terms:

    • Values above 0.8 aw significantly increase the risk of microbial growth
    • Lower values are often targeted to improve shelf life, texture, and product stability

    The aw scale directly corresponds to 0–100% relative humidity. This reflects the equilibrium between the product and the surrounding air at a given temperature and pressure.

    In practice, this means that controlling water activity is closely linked to controlling humidity conditions.

    How Water Activity Is Measured in Practice

    Water activity is determined based on the vapour pressure equilibrium between a product and the surrounding air.

    In practical measurement:

    • The sample is placed in a sealed chamber with a small air volume
    • A humidity and temperature sensor measures the air above the sample
    • The moisture exchange between the product and the air continues until equilibrium is reached

    At equilibrium, the measured relative humidity in the chamber reflects the water activity of the product.

    Different materials reach this equilibrium at different speeds. This depends on their sorption behaviour – their ability to absorb or release moisture.

    In practice, this means measurement time can vary significantly between products.

    Faster Water Activity Measurement with AW Quick

    In standard water activity measurement, the result is only valid once equilibrium is reached between the sample and the surrounding air. Depending on the product, this process can take a significant amount of time.

    With Rotronic’s AW Quick function, this limitation is reduced.

    Instead of waiting for full stabilization, the system analyzes the initial part of the measurement curve and predicts the final water activity value using a mathematical model. This allows you to obtain reliable results much faster.

    In practice, measurements that would normally take up to an hour can often be completed in just a few minutes.

    This is particularly valuable in applications where:

    • Fast decisions are required in production
    • High sample throughput is needed in quality control
    • Measurement time is a bottleneck in laboratory workflows

    What Can Affect Your Measurement Accuracy

    Accurate water activity measurement depends on stable and controlled conditions.

    Temperature stability is critical

    The amount of water air can hold depends on temperature. Even small changes can affect the result:

    • A temperature change of 1 °C can introduce an error of approx. 0.03 aw at room conditions
    • Measurements must always be reported together with the measurement temperature

    To ensure reliable results, both the instrument and the sample must remain temperature-stable throughout the measurement.

    Sample handling matters

    To obtain representative results:

    • The sample must reflect the actual product condition
    • Exposure to ambient air before measurement should be minimized
    • Samples should be stored in sealed containers if not measured immediately

    Improper handling can change the moisture balance and lead to incorrect results.

    Equilibrium takes time

    Not all products stabilize at the same rate.

    If the measurement is read too early – before equilibrium is reached – the result will be inaccurate.

    In practice, this means the operator must ensure that the measurement has fully stabilized before recording the value.

    Solutions for Laboratory and Quality Control

    Rotronic offers dedicated solutions designed for accurate and repeatable water activity measurement:

    • Laboratory analyzers (e.g. HygroLab) for high-precision, multi-sample analysis
    • Advanced digital probes adapted to different sample types
    • Stable measurement chambers ensuring reliable equilibrium conditions
    • Integrated data handling for documentation and traceability

    These solutions support both routine quality control and advanced product development.

    Applications Where Water Activity Is Critical

    • Food and beverage – shelf life, texture, and microbial stability
    • Pharmaceuticals – product integrity and compliance
    • Cosmetics – formulation stability and preservation
    • Packaging development – moisture migration and barrier testing
    • Material science – hygroscopic behaviour and durability

    What This Means for Your Work

    With reliable water activity measurements, you can:

    • Make better decisions about product formulation and storage
    • Reduce the risk of microbial growth and product degradation
    • Improve consistency and quality across batches
    • Support documentation and compliance requirements
    • Work with confidence in your measurement data

    These products are included in the Water Activity portfolio

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