In the UK and the European Union, food production does not follow a single “water activity law” like a specific FDA section. Instead, water activity is managed through a combination of mandatory microbiological safety regulations and risk-management frameworks like HACCP.
For manufacturers and quality assurance teams, understanding and controlling water activity is essential. With modern instruments such as the Schaller humimeter RH2 water activity meter, it is now possible to measure this critical parameter quickly, accurately, and directly within production environments.
What Is Water Activity?
Water activity (aw) measures the amount of freely available water in a product, that which can support microbial growth, chemical reactions, and physical changes.
Unlike total moisture content, water activity focuses on how water behaves rather than how much is present. This makes it a far more reliable indicator of product safety and shelf life.
Water activity is expressed on a scale from 0 to 1, where 0 is completely dry and 1 is pure water.
Most bacteria require an aw above 0.91 to grow, while yeasts and moulds can grow at lower levels. Controlling aw is therefore key to preventing spoilage and ensuring food safety.
Water Activity vs Moisture Content
Moisture content and water activity measure entirely different properties.
Moisture content indicates the total amount of water in a product, usually expressed as a percentage. It does not reveal how tightly that water is bound within the product structure.
Water activity, as mentioned above, measures the availability of water for biological and chemical processes.
Two foods can have the same moisture content, but their water activity (aw) can vary significantly, directly affecting their shelf life. A good example is fresh bread and jam. Both can contain a similar overall amount of moisture, but their water activity is very different. In fresh bread, much of the water is relatively available, giving it a higher water activity that can support mould growth and limits shelf life. In contrast, jam contains large amounts of sugar, which binds the water tightly and lowers its water activity. As a result, even though jam is moist, the water is not readily available for microorganisms, allowing it to remain stable for much longer.
This is why water activity is the more critical parameter for assessing product stability.
Compared to traditional moisture analysis methods which often involve drying or chemical testing, water activity measurement offers several advantages:
- Faster results
- Non-destructive testing
- Higher relevance to product safety
- Suitable for real-time quality control
Why Water Activity Is Important in Food and Pharmaceuticals
In addition to food safety, water activity affects the product in terms of taste and texture, processability (e.g. frying and baking characteristics), shelf life, and choice of packaging materials:
Food Safety and Microbial Control
Water activity directly influences the growth of bacteria, yeast, and mould. By controlling aw levels, manufacturers can minimise the risk of contamination from pathogens.
Shelf Life Extension
Lower water activity slows down microbial growth and degradation processes, helping to extend product shelf life. This is especially important for packaged foods and pharmaceutical products.
Product Quality and Texture
Water activity affects texture, crispness, and mouthfeel. Products like biscuits, powders, and snack foods rely on precise aw levels to maintain their intended quality.
Chemical Stability
Many chemical reactions, including oxidation and enzymatic browning, are influenced by water activity. Controlling aw helps preserve flavour, colour, and nutritional value.
Packaging Optimisation
Understanding water activity helps manufacturers select appropriate packaging materials to prevent moisture migration and maintain product integrity.
Ideal water activity
The ideal water activity (aw) range is critical for maximising product safety, quality, and shelf life, making it a key processing specification in food and pharmaceutical production. Achieving this target requires a balance of formulation, processing, and regular monitoring. While drying is commonly used to reduce water activity, it can also lead to weight loss and reduced revenue, as many products are sold by weight. An effective alternative is optimising formulations by adding humectants such as sugar, salt, or glycerin, which lower water activity without removing moisture. Continuous water activity measurement using tools like the Schaller humimeter RH2 water activity meter, helps maintain this balance, preventing over-drying, reducing energy consumption, and preserving product weight. By defining a clear water activity specification and monitoring it consistently, manufacturers can ensure safe, high-quality products while maximising efficiency and profitability.
How Water Activity Is Measured
Measuring water activity with a portable device such as the Schaller humimeter RH2 water activity meter involves a simple but precise process. A sample is placed in a sealed measuring chamber, where it is allowed to reach equilibrium with the air inside. During this time, moisture naturally moves between the sample and the surrounding air until a stable balance is achieved.
Once equilibrium is reached, the instrument measures the relative humidity and temperature within the chamber using high-accuracy sensors. This value represents how much moisture the air is holding compared to the maximum it could hold at that temperature. The water activity is then calculated by converting this equilibrium relative humidity into an aw value (for example, 75% relative humidity corresponds to 0.75 aw).
Water Activity Measurement Implementation
Many manufacturers integrate at-line measurement.
Measurements are taken per batch or at regular intervals (e.g. hourly), also when raw materials or conditions change.
- To allow immediate verification at critical control points
- To Support batch release decisions
- Reduce delays associated with laboratory testing
- Improve overall process control
Trend analysis over time can also help optimise processes and improve consistency.
Final product testing is essential for regulatory compliance and safety documentation.
- Quality Assurance: Readings are typically taken at the end of each batch and again during packaging to confirm consistency and shelf-life predictions.
- Safety Benchmarks: For many “shelf-stable” foods, the final water activity must be below specific levels.
Why Choose the RH2 for Water Activity Measurement?
The Schaller humimeter RH2 water activity meter is designed for accurate, reliable, and user-friendly water activity measurement across different stages of production, from raw materials to finished goods.
Key capabilities include:
- High accuracy measurement ±0.01 aw over the range 0.1 – 0.8 aw, and ±0.04 aw at the extremes of the range.
- Rapid measurement – features a built-in fan to accelerate equilibration time.
- Long-term stable sensors for consistent accuracy
- Choice of two measurement chambers for different sample volumes.
- Portable design for use in laboratory and production environments
- Integrated data storage for quality documentation – automatic data logger for up to 10,000 logs, with optional USB data interface and LogMemorizer software for analysis
By using advanced tools like the Schaller humimeter RH2 water activity meter and combining portable measurement with in-process monitoring, manufacturers can achieve greater control, reduce risk, and maintain consistent product quality.
Video showing ease of operation of the RH2 water activity meter