Moisture plays a crucial role in a lot of foods. Making sure that there is not too much and not too little moisture makes up a really big part of cooking and storing food. Baking your pie crust for long enough, cooking a jam until the right consistency, or caramelizing sugar, all revolve around controlling moisture. It becomes even more crucial when you want to store your food for a longer period of time. Moisture has a tendency to move around and controlling that movement is very important (we discussed that in more detail here!).
How much moisture a food absorbs and under which conditions it does so, is thus very helpful information to have for product developers. This is where moisture sorption isotherms come in!
It starts with the water activity
In order to read and understand moisture sorption isotherms, you need to know what water activity is. Water activity is a measure for the amount of available water in a product. This is water that is free to move, can be used by micro organisms, etc.
A drier food will generally have a lower water activity. However, it is important to keep in mind that the water activity of a food is not the same as the amount of water in a food. How much water is actually available is impacted by various factors. For instance, other ingredients can impact how much of the water is available. Sugars for instance can bind water, making it unavailable.
We discuss water activity in more detail in a dedicated post.
Water activity & Equilibrium relative humidity
Water moves between products and phases in a such a way that it aims to even out water activity. It will move from a food with a higher water activity to one with a lower water activity (it’s why your pie crust can get soggy over time). It doesn’t just do so between foods, but also with the surrounding air. If the air around your freshly baked bread is very dry the bread will release moisture to the air. However, if the air is very moist, chances are that some of that water in the air will sit in your bread crust, making it soggy. Scientists use this principle to measure the water activity of a food.
If you place a food in an otherwise empty chamber, moisture from the product will sit in the air. Within quite a short time span the water activity of the air and that of the food will be in equilibrium. Remember, it’s an enclosed chamber so no fresh air can come in! Since the air will contain a lot less moisture than your food, the final water activity will be that of the food.
It so happens, that we’re actually pretty good at measuring the water activity of air, or better said, the humidity of the air. By measuring this so-called equilibrium relative humidity (ERH), you can derive the water activity of the food by dividing it be 100.
Relating water activity and water content: Moisture sorption isotherm
As mentioned previously, the water activity (aw) does not describe how much water is in a food. Luckily, there is a way to relate the two and this is where moisture sorption isotherms come in. A moisture sorption isotherm is a graph that related the moisture content (on the y-axis) to the water activity (on the x-axis) of a product.
Making a moisture sorption isotherm
Nowadays, making a moisture sorption isotherm only requires a relatively simple piece of measuring equipment and some patience. The equipment will submit your sample to a range of relative humidity values. At each measuring point the equipment will carefully weigh the sample to determine whether the sample is taking up water, or releasing it. It is then able to plot the moisture content in w-% versus the water activity of the sample.
Reading and interpreting moisture sorption isotherms
Moisture sorption isotherms can be quite tricky to read and really understand. So let’s have a look at what is going in in a moisture sorption isotherm, using the graphs below. In this graph the x-axis contains the water activity of a sample, the y-axis describes how much moisture your product contains at that point.
All the graphs shown here are adsorption isotherms. This means that they have been made by starting at a low water activity, increasing the humidity over time. For some products the opposite curve (desorption) looks identical to the adsorption curve. However, for a lot of products these two aren’t identical, we’ll come back to that later.
Langmuir – Type I
Let’s imagine our product from the orange graph below is currently stored at a very low humidity, e.g. 20%. This would translate into a water activity of 0.2. At this point you can see that the moisture content of your product is also quite low, probably <0.2 g/g. However, you see that this product is very sensitive to increases in humidity. If the water activity goes up, the moisture content goes up very rapidly as well. It easily grabs on to that moisture and absorbs it. As such, this is probably a very hygroscopic material. Slight increases in humidity in the air will cause the product to absorb extra moisture.
These types of materials, that very easily grab onto moisture from the air, are often described as being Type I. A so called Langmuir model can be used to model the behavior of these materials.
BET Type II
Not all foods grab onto water so easily, most aren’t as hygroscopic. Some foods may give a moisture sorption isotherm more like the one below, in the shape of an S. (Keep in mind that the illustrations are extreme examples of products. Most foods will start with a horizontal line at lower water activities and will show this S-like behavior at higher values.)
So what happens to this product? If you store it at a very low humidity (that is, low water activity), it will contain a limited amount of water. By increasing the moisture in the air, the product will start absorbing moisture, which causes its weight to increase. However, at some point, it stops taking up more water, despite the increase in humidity. Only when the humidity increases quite a big further will it start to take up more water again.
Corn starch and milk powder both show this type of behavior. Having two distinct ranges within which they absorb extra water. A reason for this type of behavior may be changes in the structure of the product. In the case of milk powder for instance, the adsorption of moisture will change the structure of the lactose in the milk, it crystallizes. One this has completed the product might not take up water as easily anymore, until a next threshold is passed.
This type of behavior is often referred to as being a BET-isotherm, or type II.
BET – Type III
The last big category we’ll discuss here are products that don’t take up moisture easily, up to a specific threshold (see below). These products can be referred to as BET or type III.
In the graph you can see that the line is initially quite flat. This means that despite the increase in humidity, the product won’t take up additional moisture. However, at some point a threshold is crossed and the product is able to take up moisture. Certain icings and cheeses may behave this way.
When to use a moisture sorption isotherm
Product developers can use a moisture sorption isotherm in a wide variety of cases. We will focus on a two specific use cases:
- To determine storage conditions for your product.
- To determine how different components from your product might interact
Storing apple chips
Researchers investigated how best to storage apple chips, dried crunchy slices of apples. As you can imagine, these apple chips contain only very little moisture. This is what makes the chips nice and crunchy. Storing the chips in a humid environment would make them soft and soggy by taking up moisture from the surrounding air.
So scientists ran a moisture sorption isotherm on the chips, so understand at which values exactly the chips would start to absorb moisture. They found that the chips would already contain too much moisture at a water activity as low as 0.18! This is very low, the air around us is almost always wetter. As such, they now knew that they needed to use special packaging, that would not let in any moisture to keep the chips crunchy.
You can do very similar analyses for a lot of other foods. Powders such as icing sugar, are also very prone to taking up too much moisture, causing them to clump. Knowing just exactly at what point this happens helps product developers design appropriate packaging.
Multi component products
If you’re in the business of making pies, with a crunchy crust and moist, juicy center, you will want to make sure that your crust stays crunchy for as long as possible. Same for any other food that contains multiple components that touch one another. You want them all to stay of as high a quality as possible and that generally means: not too much movement of moisture.
In such a case, manufacturers may revert back to moisture sorption isotherms. When you know the moisture content of your crust and filling for instance, you can then determine the water activity of each. You then know how different they and how moisture will move between the two.
By seeing the full isotherm, as opposed to just one water activity value you can determine how much of a challenge those two components will bring. For instance, if you see that one isn’t very much affected by a surrounding change in humidity, you will know that it likely isn’t going to be impacted as much by an adjoining component with a different water activity.
Limitations of a moisture sorption isotherm
Whereas a moisture sorption isotherm can give you a lot of information, it does have its limitations.
One to definitely keep in mind is the impact of temperature. An isotherm is only applicable to the temperature at which it was measured. If you increase or decrease the temperature your product might absorb moisture at different water activity levels.
If you change your recipe, you will have to redo a moisture sorption isotherm. A change in ingredients will impact how they interact with water and thus the isotherm.
All the graphs in this post are adsorption isotherms. In other words, the curve has been made by exposing the sample to increasingly high values of water activity. The adsorption of moisture can permanently impact the structure of a food though. Therefore, reversing, this process through desorption, will often (in the case of food) give a different shape of the graph. In a lot of cases, the product won’t become as dry as it was anymore, the moisture is held up within the structure. It might still be of the same water activity, but will contain more moisture. If this happens, you’ve encountered hysteresis.
Once a product has gone through these irreversible changes it will not get back its original quality.
That said, despite these limitations and additional consideration in mind, moisture sorption isotherms remain very powerful tools. If your icing sugar has started to clump again, just think back of these curves, something must have gone wrong along the way…