What is Water Activity in Simple Terms?

January 12, 2026 •

4 min read

Water activity is a fundamental concept in food science, first correlated to microbial growth in 1953 by W.J. Scott, establishing it as a more reliable predictor of food spoilage than simple moisture content. Unlike total water, it measures the unbound, available water that supports microbial and chemical reactions.

Quick Summary

Water activity (aw) quantifies the unbound water in a substance, indicating how available that water is for microbial growth and chemical reactions. This value, measured on a scale from 0 to 1, is a key determinant of a product's shelf life, safety, and quality.

Key Points

Availability, not Amount: Water activity ($$a_w$$) measures the available or unbound water in a product, not the total quantity of water.
Scale from 0 to 1: The $$a_w$$ scale ranges from 0.0 (completely dry) to 1.0 (pure water), with most foods falling somewhere in between.
The Key to Preservation: Lowering water activity is a primary method for preserving food, as it inhibits the growth of bacteria, yeasts, and molds.
Predicts Shelf Life: Water activity is a superior indicator of product stability and shelf life compared to moisture content, as it predicts microbial growth and chemical reactions.
Controls Moisture Movement: Moisture migrates from areas of higher $$a_w$$ to areas of lower $$a_w$$. This principle helps prevent issues like a crunchy cracker becoming soggy in a mix with a soft ingredient.
Impacts Texture and Quality: Beyond safety, $$a_w$$ influences a product's texture, flavor, and physical properties, such as caking and clumping.

Water Activity vs. Moisture Content: The Key Difference

To truly grasp what water activity is, it’s essential to distinguish it from moisture content. While related, these two measurements describe very different properties of a product.

Moisture Content

Moisture content is a quantitative measure, indicating the total amount of water present in a material, usually expressed as a percentage. This includes all water—both free water and chemically bound water that is unavailable for biological or chemical processes. While useful for determining product yield or texture, it can be misleading for predicting stability. For example, a slice of fresh salami and cooked beef might have similar moisture percentages, yet the salami has a much lower water activity due to the salt and curing process, making it resistant to spoilage.

Water Activity ($$a_w$$)

Water activity ($$a_w$$), on the other hand, is a qualitative measure of the energy state of water in a system. It reflects the availability of unbound water to support microbial growth and chemical reactions. It is measured on a scale from 0.0 (completely dry) to 1.0 (pure water). A higher $$a_w$$ value means more free water is available, which provides a more hospitable environment for microbes like bacteria, yeasts, and molds.

A Simple Analogy: Water in a Sponge

Imagine a dry sponge represents a completely dry product with a water activity of 0.0. Now, imagine a bucket of pure water represents a water activity of 1.0. If you soak the sponge in the water and then lift it out, it contains a lot of water (high moisture content). However, some of that water is tightly held within the sponge's fibers (bound water). The water that drips freely from the sponge is the “available” water, or high water activity, which can be used by microorganisms. The water bound tightly within the sponge's fibers, however, is not available to support microbial life, even though it contributes to the total moisture content. This is why a moist cake can spoil, but honey, with a higher percentage of water, will not, because sugar binds the water, lowering its water activity.

The Role of Water Activity in Food Safety and Preservation

Water activity is arguably the most crucial factor in determining a product's shelf life and stability. Food manufacturers and pharmaceutical companies rely on this measurement to predict how a product will behave over time.

Controlling Microbial Growth: By controlling $$a_w$$, manufacturers can prevent the growth of harmful microorganisms. Most bacteria cannot grow below an $$a_w$$ of 0.91, while most molds are inhibited below 0.80. This is why traditional preservation methods like salting meat or making jams with sugar are so effective—they lower the available water.
Preventing Chemical Reactions: Water activity affects various chemical reactions that impact a product's quality, including lipid oxidation and non-enzymatic browning. Lowering the $$a_w$$ can slow or prevent these reactions, preserving a product's color, flavor, and nutritional value.
Maintaining Texture: For products like crackers and cookies, maintaining a low water activity is critical for keeping them crispy. In mixed products like a snack cake with both cake and frosting, matching the water activity of the different components prevents moisture migration that would cause the cake to become soggy and the frosting to dry out.

How Water Activity is Controlled

Producers use several methods to manage water activity during processing:

Drying or Dehydration: Physically removing water to lower the overall $$a_w$$.
Adding Solutes: Incorporating salts or sugars, which bind water molecules and reduce their availability.
Freezing: Converting free water into ice, making it unavailable to microbes.
Hurdle Technology: Combining multiple preservation techniques, such as low $$a_w$$, low pH, and temperature control, to achieve a synergistic effect.

Water Activity vs. Moisture Content: A Comparison


Feature	Water Activity ($$a_w$$)	Moisture Content (MC)
What it Measures	The availability of unbound water.	The total amount of water present.
Scale	0 to 1 (pure water is 1.0).	Percentage, from 0% to 100%.
Application	Predicts microbial growth, shelf life, and chemical stability.	Used for calculating product yield or determining texture.
Key Insight	The driving force for reactions and moisture migration.	A quantitative amount that doesn't predict behavior.
Example	Salami ($$a_w$$ ≈ 0.82) is safe due to low available water, despite high total moisture.	Cooked beef (MC ≈ 62%) spoils quickly because it has high available water.

Conclusion

In simple terms, water activity measures the "free" or available water in a product, acting as a crucial indicator for stability and safety. By controlling this parameter, industries from food production to pharmaceuticals can predict and prevent problems related to microbial spoilage, undesirable chemical reactions, and changes in texture. This critical measurement provides far more predictive power than simply knowing the total moisture content, enabling manufacturers to create products that are safe, consistent, and have a longer shelf life.

Frequently Asked Questions

Moisture content measures the total amount of water in a product, both bound and unbound. Water activity, however, specifically measures only the free, unbound water that is available for microbial growth and chemical reactions. For food safety, water activity is the more critical measurement.

Microorganisms like bacteria, yeasts, and molds require available water to grow. By lowering a food's water activity below a certain threshold (typically below 0.85 for most bacteria), food producers can prevent or slow down microbial growth, significantly extending the product's shelf life and ensuring it is safe to eat.

Water activity is typically measured using specialized electronic instruments, such as chilled mirror dew point hygrometers or capacitive sensors. These devices place a sample in a sealed chamber and measure the relative humidity of the air after it has come to equilibrium with the sample, with the final value derived from this reading.

Yes, absolutely. For instance, salami and cooked beef may have similar total water percentages, but the salt in salami binds water, lowering its water activity and making it far more stable than the cooked beef.

Yes, water activity influences many factors beyond microbiology, including chemical stability, enzymatic reactions, lipid oxidation, and physical properties like texture and color. Controlling it is key to maintaining a consistent and high-quality product.

Several common techniques are used, including drying or dehydration to physically remove water, adding solutes like salt or sugar, and freezing to lock water into an unusable state.

Generally, most microbial growth is inhibited below a water activity of 0.60. Below this point, no known microorganism can grow.