Water activity (\(a_w\)) is a fundamental measure representing the energy status of water within a product. It quantifies the amount of unbound water available to participate in chemical reactions or support the growth of microorganisms like bacteria, yeasts, and molds. This measurement is paramount for maintaining product quality and safety, particularly in food, pharmaceutical, and cosmetic manufacturing. Understanding how to measure \(a_w\) is a prerequisite for predicting product degradation and ensuring regulatory compliance.
What Water Activity Is and Is Not
Water activity is measured on a scale from 0.0 (completely dry) to 1.0 (pure water), describing how intensely water is bound to the non-aqueous components of a system. This measure differs fundamentally from moisture content, which is simply the total quantity of water present, usually expressed as a percentage of the product’s total weight. Two products can have the same moisture content but vastly different water activities, resulting in different stability profiles.
For example, fresh meat and cured salami might contain a similar amount of water by weight. However, the salt and solutes in the salami bind the water molecules, lowering the \(a_w\) and making that water unavailable for microbial life. The meat has a high \(a_w\), meaning its water is readily available for spoilage organisms. Because only free water can be utilized by microbes or act as a solvent for chemical degradation, \(a_w\) is a more reliable predictor of product shelf life and microbial risk than total moisture content.
The Fundamental Relationship to Relative Humidity
The concept of water activity is rooted in thermodynamics, defined mathematically as the ratio of the water vapor pressure of the substance (\(P\)) to the water vapor pressure of pure water (\(P_0\)) at the same temperature: \(a_w = P/P_0\). This ratio indicates the escaping tendency of water from the sample relative to pure water.
This vapor pressure ratio has a direct relationship with the air surrounding the sample. When a sample is placed in a sealed chamber, the water in the product equilibrates with the water vapor in the headspace. At equilibrium, the water activity of the sample equals the relative humidity (RH) of the air in the chamber, known as the Equilibrium Relative Humidity (ERH). Therefore, \(a_w\) can be calculated directly from the ERH using the conversion \(a_w = ERH / 100\), since ERH is expressed as a percentage.
Methods for Measuring Water Activity
The practical measurement of water activity relies on instruments that determine the relative humidity of the air surrounding the sample at equilibrium. The most accurate and widely accepted method in industry is the Chilled Mirror Dew Point technique.
This method involves placing a sample in a sealed chamber containing a mirror and a temperature sensor. The instrument precisely cools the mirror until a thin film of condensation, or dew, begins to form. The temperature at which this occurs is the dew point temperature, which is directly related to the partial vapor pressure of water in the headspace. By measuring the sample’s temperature and the mirror’s dew point temperature, the instrument accurately computes the \(a_w\) based on thermodynamic principles.
Another common approach uses capacitance or resistive sensors, categorized as electric hygrometers. These sensors contain a layer of polymer or an electrolytic material that absorbs or desorbs water vapor from the headspace. As the sensor equilibrates, the change in its electrical properties (such as capacitance or resistance) is measured and correlated to the relative humidity. While often faster, these sensors typically offer lower accuracy compared to the chilled mirror method.
Critical Applications in Preservation
Measuring water activity informs preservation strategies across various industries. In food safety, \(a_w\) values predict the growth of pathogenic and spoilage organisms, which require a minimum amount of available water to thrive. The threshold of \(a_w\) 0.85 is a regulatory benchmark used by the FDA for certain low-acid canned and acidified foods.
Keeping \(a_w\) below 0.85 is effective at inhibiting the growth of most common foodborne bacteria, including Salmonella. For stability, a water activity value below 0.60 prevents the proliferation of all known microorganisms, including the most tolerant molds and yeasts. Beyond microbial control, \(a_w\) measurement is used to predict chemical degradation, such as lipid oxidation and non-enzymatic browning reactions, and to control the texture and flow properties of powders, preventing caking or clumping during storage.