With one bound it was free: Rotronic defines the science of water activity

Bassersdorf, Switzerland: – Rotronic AG, one of the world’s leading measurement solutions specialists, has released an easy to read ‘Water 101’ presentation on the science of water activity (aw) and why it matters for pharmaceutical, fine chemicals and other industries.

The briefing comes in the form of a series of slides that explain the fundamental relationships between aw, moisture, humidity and substances.

For those needing more detailed and technical information, Rotronic has also authored a White Paper on water activity that also serves as a briefing on the various solutions available for aw measurement.

The paper is now available for free download from the Rotronic website, along with informative videos on how and why to measure water activity.

Bound or free?

The briefing explains that while water affects most biological and chemical processes, water activity (also called Equilibrium Relative Humidity) is of particular concern to industry as it directly impacts on product characteristics, including spoilage, shelf life, viability, taste and overall quality. Technically, this is because water activity works directly on the physical properties of compounds such as tensile strength, adherence, yield pressure, compaction and elastic recovery and determining behaviors such as flow and caking.

There are two key forms of water activity within products: bound water and free water.

Bound water is so called because it is chemically bound within the substance, which means it is not available to most organisms, does not act as a solvent on salts or sugars and generates no vapor pressure on heating. In contrast, free water is only physically bound into the substance which means it t can be extracted easily, is available for interaction with organisms and will generate vapor when heated.

Defining water activity

Water activity is distinct from moisture content, which is defined as total water content in a product, both bound and free and can be measured by infra-red or drying loss.

Water activity, on the other hand, is defined as the free water content and can be measured by quantifying the vapor pressure above a product that is in stable equilibrium with its environment, both in temperature and inherent water vapor pressure.

The distinction is important for the pharma industry because it is this ‘active’ moisture content that directly affects the microbial, chemical and enzymatic stability of perishable products, the cohesion and flow of powders, coating adherence, and caking and compaction properties of particles.

Qualitative measurement

“Water activity is a useful parameter for many industries,” commented Dr. Jeremy Wingate of Rotronic UK, who helped prepare the briefings.

“Measuring water activity takes only a few minutes so can provide results quickly, ideal for critical control points,” said Dr. Wingate.

“However water activity is a qualitative measure therefore interpreting results can be complex,” he cautioned.

Water activity alone does not define product physical properties, shelf life or growth of organisms within product, which are also affected by actual product parameters.

However, measuring aw does provide a useful fast track for assessing product changes, isolating probable causes and making predictions on shelf life, etc. Moreover, aw testing is much quicker and less costly than total moisture testing. This is particularly true when dealing with ‘known’ products such as grains, cheese and processed meats whose basic parameters and aw limits have already been defined by regulatory organisations.

About Rotronic

Founded in 1965, Rotronic AG has become one of the world’s leading names in humidity and temperature measurement and is renowned for the high quality of its Swiss-made meters, instrumentation, sensors, data loggers and other measuring equipment.

As the leading brand for Process Sensing Technologies, Rotronic specialist measuring solutions are in daily use worldwide in pharmaceutical, chemical, foodstuffs and many other industrial sectors.

Rotronic’s lines of business include products for measuring humidity, temperature, CO2 and differential pressure. As a leader in the field of humidity and temperature measurement, Rotronic is also increasingly making a name for itself as a world leader in the measurement of other parameters including low dew point, CO2, differential pressure, process pressure and airflow. As Rotronic is a brand of the PST Group (Process Sensing Technologies), our customers are benefitting from a wider product range and distribution network.