Nothing fits a good wintery day better than a warm chocolate milk, don’t you agree? The heat of the milk warms you up just a little and it gives you enough of a boost of energy to get going again. It’s a favorite on our winter skiing holidays!
That said, it’s also a great vehicle to explain a science concept concept: sedimentation. You may have never thought of it, but it’s actually pretty special that your chocolate milk is so nice and evenly brown. If you’ve made chocolate milk at home, with warm milk and cocoa powder, you will have noticed that the cocoa slowly sinks to the bottom. Of course, if you drink it quick enough and stir it, you won’t see this happening, but if you leave it long enough, it will sink, this is sedimentation.
This opposed to most chocolate milks you can buy in the supermarket. A lot of these do not split, no matter how long you leave them on the counter for. Manufacturers have managed to stabilize these chocolate milks, so the cocoa particles don’t sediment to the bottom. So what’s going on here?!
What is chocolate milk?
Chocolate milk for most people is sugar and cocoa powder mixed with milk. If you make it at home you probably heat up your milk slightly and mix in the sugar and cocoa, stirring vigorously until everything looks homogeneous.
The scientists way of describing chocolate milk is to say it is a suspension of solid particles (= cocoa) in a continuous liquid (= milk + dissolved sugar). In a suspension, as is the case for chocolate milk, the particles literally float in the liquid. Other examples of suspensions are icing sugar in oil or yogurt.
Cocoa powder particles do not dissolve in water, they are more hydrophobic so prefer sitting in a fatty phase. When you mix them through the milk when making the chocolate milk you are using the forces of your spoon to mix them through and have them float everywhere evenly. As you can see in the photo below though, the powder will slowly sink down to the bottom again, this is called sedimentation and is an example of a food physics phenomenon.
A note on sugar
In practice, most chocolate milks contain sugar as well. The sugar dissolves in the milk, making it part of the liquid phase. The individual sugar molecules will leave the sugar crystals and dissolve into the water individually. Energy is actually released when sugar is dissolved, which is why it takes effort to actually get it back pure again. No matter how long you wait, the sugar won’t sink down to the bottom, whereas the coca particles will.
The cause of separation: sedimentation
So, you’re left with these particles floating around in milk in your chocolate milk. If these particles aren’t stabilized in any way, they will slowly sink down to the bottom of your glass of chocolate milk. This process, called sedimentation, is driven by gravity. Gravity pulls down the particles as well as the liquid. The phase with the highest density will eventually sink to the bottom, whereas the phase with a lower density float on top.
In the case of chocolate milk the density of cocoa powder is higher than that of milk, so it sinks down instead of floating upwards. An example might help clarify this, blowing up a flotation aid for kids fills it up with air. The density of air is way lower than that of water. As a consequence, if you place it on the water, it will float. A stone on the other hand has a way higher density than water and will sink immediately. You may have also noticed that very small stones sink more slowly than large ones!
Rate of sedimentation
The speed at which sedimentation occurs in a chocolate milk can be calculated using the formula below. We won’t actually calculate our sedimentation rate, but using a formula like this can help you predict what will happen in your chocolate milk:
Sedimentation rate (v; m/s) = 2/9 * [ (ρc – ρm) / μ ] * g * R2
ρc = density of cocoa particles (or the particle of whichever suspension you’re investigating) – kg/m3
ρm = density of milk (or the liquid in your suspension) – kg/m3
μ = dynamic viscosity of the milk (or liquid in general) – kg/(m*s)
g= gravitational acceleration (this is a constant value) – m/s2
R = radius of the cocoa particles (the calculation assumes the particles are perfectly round, which, in real life, they aren’t, so the calculation doesn’t give the exact correct value)
Understanding the rate of sedimentation
So let’s have a closer look at that formula. You will first see the density coming back in to the formula. We discussed this before by comparing stones and a flotation device and whether they sink or float on water.
Next, you can see that by increasing the size of the particles (R), the speed of the sedimentation goes up. In other words, if you have a powder with a lot of large particles, they will sink more easily. This effect is also the reason why most milk gets homogenized in factories. Homogenization breaks down the fat particles in milk into very small fat droplets, reducing the sedimentation rate so much that it doesn’t occur anymore during the life span of your milk!
Last but not least you can see that an increase in dynamic viscosity of the milk will slow down sedimentation. This is because it is harder for the particle to travel down in a more viscous liquid. An extreme example of this would be honey vs milk, you can probably imagine that particles will stay floating in honey more easily because of its viscosity.
When travelling down however, these particles are slowed down by the liquid. The calculations of this force become a little more complex, I’ll come back to that another time, but both the viscosity and density of the liquid play an important role. Think of a particle sinking down in honey versus one sinking down in water, or air. It will take longer for the particle to sink down in honey than in air!
Preventing sedimentation – Making non-splitting chocolate milk
In order to make a chocolate milk that doesn’t separate you can use the formula again to find solutions. As we discussed there are 4 main strategies:
- Reduce the difference in density between the liquid and particles – if you’re making your own chocolate milk, that is hard, but manufacturers of powders can try to reduce the density of their powders by incorporating air pockets.
- Reduce the particle size – the photo you see at the top of the post shows a chocolate milk made with cocoa powder (left) and a chocolate milk made with a special chocolate milk pack. Their powder is a lot finer than most cocoa powders. That might well be done to slow down sedimentation. The commercial pack surely took longer to split.
- Increase the viscosity of the liquid – we will come back to this on more extensively further down
- Increase the gravitational acceleration – you can’t actually increase this value. However, you can substitute it for centrifugal acceleration. If you put your chocolate milk in a centrifuge it will definitely separate more quickly!
Increasing viscosity – Thickening agents
A lot of manufacturers seem to use this strategy when it comes to their chocolate milks. They add ingredients to the milk to thicken it up, form a gel, so the particles become trapped and cannot sink down anymore or only very slowly. These chocolate milks will stay a nice even brown during shelf life.
The most common strategy to increase the viscosity in chocolate milk world seems to be the addition of carrageenan. Various brands of chocolate milk use one or both of these methods. There is quite a bit of science involved in properly using these solutions though and we’ve written about those in a separate post.
Guar gum can also be used to thicken chocolate milk and is often used in combination with xanthan gum. Guar gum is made from ‘guar’, which is a plant and is also a polysaccharide. It is a long chain of mannose molecules with little side chains of a molecule called galactose. This long chain is the reason it can thicken liquids.
Have a look at the sources at the bottom of this post for more in-depth application examples for chocolate milk!
Shaking to overcome sedimentation
A lot of us consumers don’t like chocolate milk that has separated. They/we want to open a pack and drink it straight away. There’s nothing wrong with this of course, especially if you like the brand. However, the addition of some sort of stabilizer is a consequence of this.
Of course, there’s one other way producers can use to overcome sedimentation. It’s the simple remark ‘shake before use’. By shaking you mix the cocoa and milk particles again and voila, there’s your chocolate milk. And we consumers buy into it, these products are nothing more than milk, cocoa powder and sugar (in the Netherlands the brand Cocio sells this) and we buy them, for the ease of not having to measure out ingredients anymore. It’s downright smart!
Palsgaard Technical Paper, How to make a delicious chocolate milk, October 2010, link
M. OSPINA, MÓNICA & U. SEPULVEDA, JOSÉ & A RESTREPO, DIEGO & R. CABRERA, KENNETH & SUÁREZ, HÉCTOR. (2012). INFLUENCE OF GUM XANTHAN AND GUAR ON THE RHEOLOGICAL PROPERTIES OF COCOA- FLAVORED MILK BEVERAGE. 10. 51-59., link
Saha, D., & Bhattacharya, S. (2010). Hydrocolloids as thickening and gelling agents in food: a critical review. Journal of Food Science and Technology, 47(6), 587–597. http://doi.org/10.1007/s13197-010-0162-6, link