Water activity in food – The theory

For any food scientist or someone who’s involved in determining shelf life and quality of foods, the concept of water activity should be familiar. It plays a huge role in whether or not a product will still be good after a few days of storage on a countertop. In a previous post we’ve discussed the very basics of water activity. However, if you’re one of those people who uses this concept in your professional or daily life, you need to really understand the concept. And that’s what we’ll be doing here. Giving you a level up on water activity in food.

What is water activity (aw)?

The water activity is a measure of the ‘available’ water in food. Why put available between apostrophes? Because available water is quite of a vague concept. When is water available? That is why in this level up article we’ll discuss a more scientific definition of water activity.

Let’s start with the most basic formula:

aw = pvapour / p0

In words, this equation says that the water activity can be calculated by dividing the vapour pressure of water in your food by the vapour pressure of pure water in those same conditions.

Vapour pressure

So what is this saturation pressure? Imagine you have a glass of lemonade. Lemonade is essentially water with some flavour. If you leave this glass of lemonade on the countertop if will be a lot emptier after a few days. This is because water has evaporated. Since the space in which the glass stands is so large, water can spread out so broadly, it all disappears.

However, if you place this glass in a closed container, water will still evaporate, but after a while still will stop. Or, a better description is that the rate of molecules evaporating is the same as that of molecules condensing back in the lemonade. The vapour pressure describes how easily those molecules evaporate. A very volatile liquid, thus one that evaporates quickly, has a high vapour pressure, imagine it pressing down more on the liquid, aiding other molecules to evaporate.

Comparing to pure water

So what does the formula for water activity tell you? It tells you how easy the water in your food evaporates compared to pure water. So if the water in your product evaporates just as easily as that in pure water, the value for the aw-value will be zero. However, if it doesn’t evaporate at all, the water activity will be zero.

Coming back to the ‘available’ water concept we discussed at the start, if water cannot evaporate as easily from your food as it does from pure water, it will somehow be bound, thus less available!

A water activity can never be higher than 1,0. Water with another addition will always have a lower vapour pressure than that of pure water.

sandwich with kroket and mustard, typically Dutch food
A sandwich with a Dutch kroket, a great example of a variety of water activity values. The crust of the bread is pretty dry (low water acitivity) whereas the center tends to have a high water activity. The same goes up for the kroket, dry crunchy outside, soft moist inside.

Determining water activity

There is no one sensor or measurement technique (yet) that can be placed into a food and will immediately tell the water activity. Instead, we have to look at that equilibrium in vapour pressue and for that we use the relative humidity. Relative humidity is probably a more familiar term, describing the humidity of the air, you will see it being used in weather forecasts.

Equilibrium relative humidity (ERH) has a very similar definition to that of water activity. It also relates the current pressure of water in the air to its maximal pressure. A very humid environment will have a high relative humidity value and thus a lot of water in the air, whereas a dry environment will be the reverse.

We can measure the relative humidity of the air. There are techniques for that and to analyse the water activity of a food, we use that technique. By leaving a food in a closed container for long enough the water will evaporate and come into equilibrium in the surrounding air. If that water activity is high a lot of water will evaporate and increase the humidity of the surrounding air. If the water activity on the other hand is very low, the surrounding air will remain dry.

A difference with the water activity is that the relative humidity is a value between 0 and 100 instead of 0 and 1. However, by diving the relative humidity of that chamber by 100, you will know the water activity of your food!

aw = ERH / 100

Truly understanding water activity – sorption isotherms

By now you now what water activity is and how it can be measured. The next steps will be to dive even a little deeper into the topic. How does water activity change with water content for a product? This is quite a tricky question to answer and will be different for each product.

Your first instinct might be that twice the amount of water in a product will result in twice the water activity, a linear relationship. This is not the case however as we will explain with a few examples.

Just knowing the water activity of a certain food is interesting, but knowing the change in water content for varying humidities will tell you even more. In such an analysis a product will be put in a closed chamber again. However, instead of leaving it and waiting for it to equilibrate, moisture is added. By then measuring the increase in weight of food product you know the increase in moisture content. At the same time the humidity in the chamber in measured. That way you know how much water the food will absorb with an increase in humidity.

Langmuir sorption isotherm water
An example of how a sorption isotherm could look like. This one can probably be fitted using a Langmuir isotherm.

The isotherm above is an example of how the result of such an analysis could look like. The higher the water activity (=humidity in this closed chamber), the higher the moisture content. However, the moisture content rises a lot faster than the water activity. At a water activity of about 0,75, just about all moisture has already been absorbed. Within the world of food science this data will then be fitted to mathematical equations, resulting in a model between moisture content and water activity. In this case this will most likely be a Langmuir isotherm.

BET sorption isotherm water
Another possible example of how such a sorption isotherm might look like. This product behaves in quite a different way when placed in a humid environment than the Langmuir example. This isotherm will be fitted using a BET equation.

Using a sorption isotherm

So what can you do with these sorption isotherms? First of all, there is a lot of scientific research going into these kinds of analyses (example 1 and 2). If helps researchers understand the structure and texture of foods. If you know how water will be absorbed it might tell you something about pores in the product, or how easy water absorbs with the molecules on the surface of food.

Example 1 – Icing sugar

Le’s have a look at another isotherm (see below). Let’s assume this is an isotherm of icing sugar. When icing sugar is freshly made it has a water activity of 0.3. Thus the moisture content is still really long, on that first almost horizontal part of the curve, maybe only 1 g/g. We know that icing sugar starts to clump when it absorbs water. Therefore, we want to know at what water activity sugar starts absorbing water. In this case that happens around an aw of 0.6. So when this icing sugar is placed open in a room with a humidity of 60% you’re bound to get clumps! It helps the manufacturer to know what is the maximum humidity at which this sugar should be stored.

Example of a sorption isotherm of a random food
Another example of how a sorption isotherm of a food could look like.

Example 2: Two component food

Another application for which this type of analyses might come in handy is when you have a product made up of different components. As we’ve discussed for the example of a pie with a dry crispy crust and a moist filling, there can be quite a big difference in water activity of the two components. However, thermodynamics tells us that over time the water activity values will even out. The graph below will show you how they will move.

Let’s take the pie as an example. Just assume for now that the orange line represents the filling, whereas the blue line represents the crust. The black dots represent the initial values of the pie components when it’s just been made. You can see that the filling contains a lot more moisture and has a higher water activity than the crust. Using some mathematics you can calculate what the overall water activity will become of these two components, this is the dotted line. You will now see that the crust will take up some moisture, however, it’s not that bad. The filling though will lose a lot of moisture! At the end the crust might still be crispy, whereas the filling will have probably become rock solid.

Example of a sorption isotherm of two different components
Two sorption isotherms of two different products.

Using what you’ve learned in this post you should be able to understand the basics of a sorption isotherm and how these can be used to improve food products. You’ve also learned the physics behind the concept water activity. This should help when developing products, especially when moisture content seems to be a hurdle!

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