An Introduction to Food Rheology – the Flow of Material

Within the food industry it is very important to understand how your food will behave. Will it be deformed (too) easily (a muffin), will it flow through a pipe (a pasta sauce), will it stay on your product (caramel topping of your cookie), will it thicken up, or not (bechamel sauce)? If you want your product to be made efficiently and stay good during storage and transport it’s useful to know more about these physical properties of your food. Luckily, there’s a whole food within food physics looking into this: rheology.

This post will explore the basics of food rheology, starting with figuring out what rheology is. Then we’ll zoom in on some very common terms in the world of food rheology: viscosity, shear rate and shear stress. Last but not least, we’ll zoom in some more practical examples of flow behaviour of some foods. As a home cook or chef you might not be able to measure something like a viscosity value. Nevertheless, it is will worth understanding the basics of these concepts. It will help you understand what happens to your food during cooking, cooling and transport.

What is rheology of food?

Rheology describes the flow of materials. Rheology is not only applicable to food, it can be the flow and behaviour of any (most often liquid) material. Let’s use a few examples to bring the concept to life:

• Ketchup doesn’t flow out of the bottle by itself, instead, it will only flow once you’ve shaken or pressed the bottle or tube. By measuring the rheology of materials that show this behaviour you will see some similarities.
• Water flows out of a cup really easily and pretty fast. Honey on the other hand flows very differently, it flows more slowly and in a thicker syrupy consistency.
• Even tried to mix corn starch with a little bit of water? If so, you might have noticed that it will flow out of your bowl when you tilt it. However, once you start stirring vigorously with a spoon it will become pretty tough! It thickens up, but once you stop stirring and gently move the spoon it becomes a lot easier again!

Rheology looks into all of those phenomena and will help explain what happens, why it happens and how it can be changed (or can’t be).

Food rheology – Viscosity

In order to study rheology you have to be familiar with a few concepts that are used extensively within the world of food rheology. We will start with viscosity. All liquids and soft solids will have a viscosity value. This viscosity describes the resistance of the material when a certain stress is applied. Another more visual way to explain viscosity is through ‘thickness’. A material with a higher viscosity is thicker than one with a lower viscosity. For instance, honey is thicker than water and has a higher value for viscosity.

Viscosity of a material always depends on the temperature. Imagine a bechamel sauce, it might be very fluid (low viscosity) when it’s warm. However, when it cools down it will thicken. For some materials the viscosity at one temperature will be constant, no matter whether it’s being poured, mixed, etc. These are so called Newtonian fluids. Water is an example of a Newtonian fluid.

However, for a lot of materials their viscosity actually depends on external forces or stress. Let’s return to that ketchup. It doesn’t flow when the bottle is hold upside down. Instead, you have to shake the bottle to initiate flow. Clearly the resistance towards flow, or thickness, has changed because of this shaking. A lot of foods behave like this, these are non-Newtonion fluids.

Food rheology – Shear rate & shear stress

Two other very common terms within food rheology are the shear rate and shear stress. Within a lot of analysis techniques these two terms will show up.

The easiest way to explain these concepts is by imagining two parallel plates on top of each other with a little space in between them. That space can be filled with the material you’d like to analyze. During an analysis the top plate will move over the bottom plate. The shear rate describes at which rates these plates move alongside each other:

Shear rate  = speed of top plate (meters per second, m/s) / distance between the two plates (meters, m)

The reason the distance between the plates is included can be illustrated with a simple example. If the distance between two plates is very large, the material that sits in between won’t notice as much, the material has enough space to distribute that movement throughout the sample. In other words, the shear rate is low. If the distance is very small and the plates are very close to each other, the material in between will surely notice a movement.

In more practical terms: the shear rate is influenced by the speed at which you stir your food or the speed at which you mixing arm moves.

Now that we’ve found a way to describe how fast we try to move a material, we should define how much trouble it takes to move this. As we discussed, viscosity is the resistance against flow. So if we know how much pressure it takes to apply a certain shear rate we can define the viscosity. That’s where shear stress comes in.

The shear stress describes how much force (per surface area) is required to apply that defined shear rate. In other words, it describes how much force you need to apply to move that peanut butter.

Viscosity = shear stress / shear rate

For a Newtonian fluid, knowing the shear stress and the shear rate applied to a food will give you a value for viscosity by dividing the shear stress by the shear rate. For non-Newtonian fluids this calculation can also be done, however, the viscosity value you will find will only be true for that specific shear rate. Once the shear rate changes, the outcome of this formula will be different.

Let’s think back of that corn starch in water. When you stir slowly (=low shear rate) the viscosity is low, it is easy to mix. Let’s take some random numbers to see what happens here. If the shear rate would have been 5 (1/s) and the shear stress 10 (Pa) the viscosity would have been: 10/5 = 2 Pas.

However, we know that when we start mixing faster, it becomes a lot harder to mix. Let’s assume that we kicked up the shear rate to 50 (1/s) and that that would have resulted in a shear stress of 500 (Pa). That would give a viscosity of 500/50 = 10 Pas! In other words, the viscosity changes simply by stirring the corn starch water mixture!

Types of non-Newtonian behaviour

Newtonian fluids (like water) are the easy part of rheology. In food though, by far most materials aren’t Newtonian. A product like peanut butter, which is a fat with a lot of small peanut particles inside, or a foamy marshmallow, or processed cheese are complex system. There might be solids, liquids and gases in just one products. There may be a lot of different molecules, large and small, in this product as well. All of these components interact with one another. This causes them to behave differently from plain water!

And even though all these products might be non-Newtonian, they again can all behave very differently! Some materials thicken up by stirring (corn starch + water) others start to flow thanks to shaking (ketchup). Luckily there are a few main trends within the world of (food) rheology, so we will discuss a few of these common patterns in flow.

We’ll be using the image above to explain this concept. First of all, you will recognize some familiar terms. On the x-axis you can find the shear rate and on the y-axis the shear stress we just discussed. The black lines represent several measurements on 4 different foods.

Newtonian

Let’s start with the straight line starting at the bottom left. This describes a Newtonian fluid. If you divide the value of the shear stress by the shear rate at any point on this line you will always get the same value.

Shear thinning

This line isn’t straight, instead, it is very steep at the start and levels off afterwards. This means that when you increase the shear rate at the start they shear stress will increase rapidly as well. However, since the line if curved, the increase of the shear stress will change with a changing shear rate. In this case the line becomes less and less steep. In other words, it become easier and easier to shear this material. When this happens we call a material shear thinning. Mixing the food will make it flow more easily.

Shear thickening

The reverse is also possible. Here the line gets steeper when the shear rate goes up. This means it become harder and harder to stir the mixture when going faster! This is called shear thickening.

Yield value

These three basic phenomena can be seen in a lot of different foods. They can also be combined with the fourth line, called the Bingham plastic. Whereas the other 3 lines start at 0 shear stress, this material requires quite a lot of stress just to start a shear rate. This is called a yield value. Ketchup is an example of such a material. You will need a minimum stress just to start the movement. Materials with a yield stress can again be shear thinning, thickening or Newtonian.

Re-cap

This has been quite an intense post with a lot of science concepts. Here’s a quick summary:

• We discussed the importance of flow of foods (can you pump it? get it out of a bottle?)
• We discussed what rheology is: studies the flows of materials.
• We also discussed that the field of rheology uses it owns terms to describe this flow. The most important ones we discussed: viscosity, shear rate and shear stress.
• We then ended with a description of how materials can flow. Sounds general, but understanding how the flow of your food depends on the process conditions can be very convenient!