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It is one of the most important parameters when determining the shelf life of foods. It’s what explains why your pie crust turns soggy over time. And it sounds more complicated than it is: water activity. For any food scientist, water activity plays an important role in developing and improving food products.
But water activity may be a somewhat abstract concept to some. Some may think that the water activity represents the amount of water in food. And whereas the two are related, they are not the same! We’ll be diving into the theory of water activity to explain what it really is.
Note: This is a general introduction to water activity. We have separate articles discussing the importance of water activity for the growth of microorganisms and the impact of water activity on moisture migration.
What is water activity (aw)?
Knowing how much water your food contains is relatively simple. If you don’t know what it is, can determine it by drying the food. The weight loss during drying represents how much water was present. If you know the water content of the ingredients that make up the product, you can calculate the water content of the overall mixture of these ingredients. You might need to take into account some evaporation, but generally, basic math will suffice.
However, knowing how much water is in your food isn’t always sufficient. Instead, for certain applications, you need to know how much water is ‘available’. Some of the water may be bound by other ingredients in your product for instance, and as a result, isn’t free. Sugar, for instance, is known to hold onto water and so does salt. Water that is bound is not ‘available’. Microorganisms can’t use it to facilitate their growth, nor can it diffuse or migrate throughout your food.
This is where water activity, often abbreviated as aw, comes in. Water activity is a measure for the amount of available water. The water activity is a value between 0 and 1. A value of 0 means there is absolutely no available water, this is very rare in food. A value of 1 means all water in the product is available, which is pure water. Most foods hover within a range of 0.2 and 0.99 for water activity.
Examples of aw-values in food
Generally speaking, the ‘wetter’ a product, the higher the water activity. As you can see in the list below, most of these values are still quite intuitive.
|Food Product||Typical water activity|
|Fresh fruits & vegetables||0.99|
|Roasted coffee beans||0.1-0.3|
The formula for water activity
Water activity is a physical measure and can be described using a basic formula:
aw = pvapor / p0
- pvapor = vapor pressure of your food
- p0 = vapor pressure of pure water (under those same conditions)
The vapor pressure of a liquid is a measure for how easily that liquid evaporates. More formally said, it’s the pressure that results from the evaporation of a liquid above a liquid in a closed container.
Liquids always evaporate to some extent. If you leave a glass of water on your countertop, the amount of water in that glass will decrease over time. Water activity compares just how easily water from your food product evaporates compared to that of pure water. If the water in your food is bound, thus not available, it won’t evaporate as easily, thus lowering the water activity. The water activity of pure water is always the highest possible value.
How do you lower (or increase) the water activity?
Even though water content and water activity are not the same thing, they are related. Generally speaking, keeping everything else the same, adding more water, will increase the water activity for instance. Less water does decrease water activity. For instance, a dry cracker has a lower water activity than a moist bread. So you can decrease the water activity of your food product by drying it.
The other major factor that impacts water activity is the composition of your food. Some ingredients ‘bind’ water, making it less available. Sugar for instance is very good at binding water and thus lowering the water content. Salt can do so as well, but to a lesser extent (without it becoming inedible because of the saltiness!).
Why is water activity important in foods?
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Knowing the water activity of your food can have several benefits, both for the safety of the food, as well as its quality.
Growth of microorganisms
First of all, the water activity tells you whether and what type of microorganisms can grow in and on your food. The higher the water activity, the more (and the more dangerous) microorganisms can grow on your food. However, once you’re below a set limit, only non-harmful microorganisms can grow. Lower the water activity even further and barely any microorganism will be able to grow and spoil your food.
We wrote about this in more detail here.
Quality & crispiness
When two components with a different water activity touch one another, moisture will generally start to move from one to the other. The water from the component with the highest water activity (e.g. a fresh fruity filling) will migrate into the component with the lower water activity (e.g. a crunchy pie crust). Knowing and understanding these differences is essential for developing a stable product. For instance, you might need to add some more sugar to the fruity filling to lower the water activity and get it more similar to the crust.
We wrote about the theory and ways to prevent this type of migration in way more detail here.
Humidity and water activity
The air that surrounds us also contains moisture. How much moisture is in the air depends on the weather and climate. In humid areas, air contains a lot more water than the air in dry desert-like areas does.
Just how much water is in the air, is expressed using the relative humidity. You might have seen it being used in weather forecasts. It is expressed in a value ranging from 0 to 100%, with 0% being completely dry air.
The relative humidity and water activity of a product are closely related. If you leave a product in air of a specific relative humidity the moisture content will balance out over time. In other words, if the air is very humid and the product has a low water activity, the product will absorb moisture from the air. Its water activity will increase. However, if the relative humidity of the air is very low the opposite will happen, the product will dry out over time.
Measuring water activity
Scientists use this phenomenon of moisture balancing out between the air and a food product to determine the water activity of a food. If you enclose the food product in a small chamber, the humidity of the surrounding air will be in equilibrium with that of the sample. By measuring that equilibrium relative humidity value (ERH), you know the water activity of the food. This is expressed in a very simple formula:
aw = ERH / 100
It is important to keep in mind that this measurement measures the water activity of the overall sample. If however your food is made out of different components (e.g. that pie crust with a filling) you will want to measure the water activity of these individual components. Those results will give you insights as to how moisture will move in your final product when all of these components are put together.
Once you know the water activity of your food you can understand how it will behave under various conditions. For example, you might need to package the product in a specific way, to avoid it from either losing or attracting moisture. You will also know whether it will be easy for microorganisms to grow on your food!
Water activity is a core parameter in a lot of applications, and knowing the value is just the beginning. If you want to understand how your food will behave under a range of humidity values, you will want to use moisture sorption isotherms for even more understanding.
FDA, Water Activity (aw) in Foods, April-16, 1984, link
Meter Food, The food manufacturer’s complete guide to water activity, link
Purdue University, Vapor pressure, link
Shelly J. Schmidt and Anthony J. Fontana, Jr., Appendix E: Water Activity Values of Select Food Ingredients and Product, Water activity in foods, link
UC Davis, Water activity in food, link