Water Activity 101: Master the basics

What is water activity? Is water activity the same as moisture content? What is the measurement of water activity? Can you lower water activity?

These are some of the most fundamental questions when it comes to water activity. To answer them – and more – we've gathered insights from Dr. Brady Carter and Dr. Nate Olson – Decagon water activity experts – to give you a comprehensive overview of water activity basics. Their answers are transcribed from the Water Activity 101 webinar.

What is water activity?

Water activity is the ratio of the vapor pressure of water in a substance to the vapor pressure of pure water at a given temperature. On a scale of 0 to 1.0, it is a measure of how available or "free" that water is within a substance.

An aw of 0 means no free water, and an aw of 1.0 equals the vapor pressure of pure water. Water activity is not the amount of water in a substance – it is the energy state of the water. There's a key difference between the two, which we'll get into later.

Why do we care about water activity?

Water activity is important because water can drive numerous kinds of change in a product. The presence and availability of water can lead to:

• Microbial growth (bacteria, mold, yeast)
• Chemical changes (nonenzymatic browning)
• Physical changes (caking, flow, texture changes)

You can understand a lot about a product's stability and safety by knowing its water activity. Knowing water activity allows formulators and manufacturers to control and extend shelf life and ensure safety.

Measuring water activity

The most common instrument for measuring water activity is the chilled-mirror dew-point instrument (e.g. the AQUALAB 4TE).

A chilled-mirror dew-point instrument works by placing the sample in a sealed chamber where a small electric fan circulates the air. Inside the chamber is a mirror which is cooled until dew forms on its surface. At the moment condensation occurs, the instrument records the air temperature above the sample and the temperature of the mirror. These two values are used to calculate the vapor pressure, which is then used to determine water activity.

Other instruments for measuring water activity include electronic sensors and the isopiestic (or gravimetric) method. For the purpose of this introduction, we'll focus on the chilled-mirror dew-point instrument.

The ideal temperature for measuring water activity is 25°C, which is also the temperature at which the instrument performs its automatic temperature correction by default. Many AQUALAB water activity meters can provide a readout in about five minutes at +/- 0.003 aw accuracy.

Water activity vs moisture content

These two measurements are not the same thing. Here's how they differ:

Moisture content – the amount of water in a substance, expressed as a percentage of the total weight of a material (wet basis) or a percentage of the dry matter (dry basis).

Water activity – the energy status of the water in a substance, expressed as a ratio of vapor pressures.

There is no universal relationship between moisture content and water activity. This is why water activity is the better measure for predicting the behavior and stability of a product.

As an example, consider pure water and a saturated salt solution. Both have the same moisture content – 100% water – but their water activities are different. Pure water has a water activity of 1.0, while a saturated salt solution (even though it's still mostly water) has a water activity lower than 1.0 because the ions in the salt solution interfere with the free movement of water molecules.

There are some similarities. As moisture content increases in a product, water activity generally increases as well. But the relationship between the two is product-specific, not universal.

Sorption isotherms

In the world of water activity, a sorption isotherm is a graph of moisture content as a function of water activity at a constant temperature.

Sorption isotherms have two primary uses in food science:

1. Understanding how a product interacts with moisture (packaging selection, shelf life).
2. Understanding and predicting quality attributes – everything from texture to stability to shelf life.

Creating a sorption isotherm requires measuring a series of water activity – moisture content data points for a specific product and temperature and then graphing those data points. The shape of the sorption isotherm is product-specific.

Some products have S-shaped (sigmoid) isotherms. Others are more linear. The shape of the isotherm is related to the binding affinity of the product for water.

Sorption isotherms show the relationship between moisture content and water activity for a specific product – not the relationship between one product and another. If you have two products, each with a different sorption isotherm, and you blend them together, the final water activity of the blend will depend on the water activities of the individual components, not their moisture contents.

The effect of temperature

Water activity varies with temperature. As temperature increases, water activity increases. The relationship is product specific but in general, a 10°C increase in temperature corresponds to an increase in water activity of about 0.03 aw.

The effect of temperature on water activity is an important consideration during processing and storage. If a product is manufactured at a high temperature and then cooled, the water activity will decrease as the temperature drops. This can lead to condensation, which can cause caking, clumping, and other quality issues.

Water activity and microbial growth

Water activity has a direct relationship with microbial growth. Each type of microorganism has a minimum water activity requirement below which it cannot grow.

Common water activity thresholds for microbial growth:

• 0.97: bacteria like Clostridium botulinum E, Pseudomonas fluorescens
• 0.95: bacteria like Salmonella spp., E. coli
• 0.91: most bacteria, including Bacillus subtilis
• 0.87: Staphylococcus aureus (aerobic)
• 0.70–0.85: most molds
• 0.60: lower limit of all microbial growth

These values are the minimum requirements for microbial growth. If a product has a water activity below the minimum requirement for a given microorganism, that organism cannot grow.

Water activity in action

We mentioned earlier that water activity is relevant for microbial growth, chemical changes, and physical changes. Let's look at how water activity is used in practice to control these variables:

Microbial growth – By designing a product with a water activity below the minimum requirement for pathogens, formulators can create products that are shelf-stable without refrigeration. The FDA uses a water activity of 0.85 as the threshold below which a product is not required to be refrigerated for safety.

Chemical changes – Water activity affects the rate of nonenzymatic browning, lipid oxidation, and other chemical reactions. Understanding the relationship between water activity and chemical reactions helps formulators optimize shelf life and flavor stability.

Physical changes – Water activity affects physical properties like texture, caking, and powder flow. Understanding how water activity affects these properties helps manufacturers produce products with consistent physical characteristics.

Water activity is a key tool in the formulator's toolkit for designing products with predictable safety, stability, and quality. By understanding and controlling water activity, manufacturers can produce better products with longer shelf lives and fewer quality defects, contributing to reducing food waste and improving sustainability. Researchers also use water activity as the basis for predicting microbial growth, maintaining physical and chemical stability, formulating products, and for predicting shelf life.

Frequently asked questions

How is water activity measured in food?

Water activity is measured by sealing a small sample in a closed chamber and reading the equilibrium relative humidity of the headspace, which equals the water activity of the sample at equilibrium. The most accurate instruments use a chilled-mirror dew point sensor — like the AQUALAB 4TE — which returns a reading in about five minutes without requiring calibration against reference standards.

What water activity level indicates a shelf-stable food product?

The FDA uses aw 0.85 as the threshold below which a product is generally not required to be refrigerated for safety — products at or below this level are considered shelf-stable against most pathogens. Mold growth stops below aw 0.70, and no microbial growth of any kind occurs below aw 0.60, which is why low-moisture snacks and dried goods are formulated to stay in that zone.

Does temperature affect water activity readings?

Yes, water activity increases as temperature rises because warmer water molecules generate higher vapor pressure, which means the same product measured at 25 degrees C will show a higher water activity than at 15 degrees C. Measurements should always be reported with the measurement temperature, and AQUALAB meters apply an automatic temperature correction by default.

Can water activity predict chemical reactions like browning or lipid oxidation?

Yes — water activity directly governs the rate of nonenzymatic browning, lipid oxidation, vitamin degradation, and enzymatic reactions in food. Most chemical degradation rates are lowest at water activities between 0.2 and 0.4 and increase significantly above aw 0.6, making water activity a key variable in predicting flavor stability, color stability, and nutrient retention alongside microbial safety.

Is water activity regulated by the FDA?

Yes, the FDA uses water activity as a control parameter under 21 CFR Part 117 and FSMA Preventive Controls, with aw 0.85 as the threshold below which a product is generally not required to be refrigerated for safety. Manufacturers who use water activity as a preventive control must validate targets, define monitoring procedures, and document finished-product verification.

References

Labuza, T. P. "Sorption phenomena in foods." Food Technology 22, no. 3 (1968): 263-272.

Scott, W. J. "Water relations of Staphylococcus aureus at 30 degrees C." Australian Journal of Biological Sciences 6 (1953): 549-564.

Labuza, T. P., A. Kaanane, and J. Y. Chen. "Effect of temperature on the moisture sorption isotherms and water activity shift of two dehydrated foods." Journal of Food Science 50, no. 2 (1985): 385-392.

Troller, J. A., and J. H. B. Christian. Water activity and food. Academic Press, 1978.

Fontana, A. J. "Water activity: why it is important for food safety." In Proceedings of the First NSF International Conference on Food Safety, pp. 177-185. 1998.

Slade, L., and H. Levine. "Beyond water activity: recent advances based on an alternative approach to the assessment of food quality and safety." Critical Reviews in Food Science and Nutrition 30, no. 2-3 (1991): 115-360.

Chirife, J., and M. Del Pilar Buera. "Water activity, water glass dynamics, and the control of microbiological growth in foods." Critical Reviews in Food Science and Nutrition 36, no. 5 (1996): 465-513.

Rahman, M. S., and T. P. Labuza. "Water activity and food preservation." In Handbook of Food Preservation, edited by M. S. Rahman, pp. 339-382. CRC Press, 1999.

Ross, K. D. "Estimation of water activity in intermediate moisture foods." Food Technology 29, no. 3 (1975): 26-34.

Carr, J. M., T. Sufferling, and J. Poppe. "Water activity determination: a collaborative study of different methods." Journal of Food Science 41, no. 4 (1976): 910-917.

Scott, W. J. "Water relations of food spoilage microorganisms." In Advances in food research, vol. 7, pp. 83-127. Academic Press, 1957.

‍