Learn the science behind:
Ever made your own chocolate milk? You carefully mixed the milk, cocoa powder, and maybe some sugar. And let it sit for a little while. If you waited too long though, chances are you had to stir a few more times. A dark brown layer may have formed on the bottom. Your chocolate milk split!
You’ve been witness to a very common phenomenon: sedimentation. So why does it happen and why does some store-bought chocolate milk never split?
- What is chocolate milk?
- The cause of separation: sedimentation
- How to prevent chocolate milk from separating
- If you can’t fix it – Shake it!
What is chocolate milk?
If you’d ask a non-scientist, you’d probably get an answer along the lines of: milk mixed with chocolate or cocoa powder. Or, a chocolate-flavored drink.
It’s a suspension
If you’d ask a scientist they might say it’s a suspension of solid particles (= cocoa) in a continuous liquid (= milk). That’s because cocoa doesn’t dissolve in milk. Instead, cocoa particles float within the milk.
This is very different from a milk + sugar solution. When you mix milk and sugar, the sugar dissolves. In doing so, the sugar crystals break down into individual molecules. These individual molecules are so small, you can no longer see them with the bare eye, nor a light microscope.
Cocoa particles, on the other hand, don’t dissolve. They don’t completely break down into individual molecules. Instead, you might just still be able to see them!
How to make chocolate milk
So what happens when you make chocolate milk? To make this suspension, all you need to do is to mix three ingredients together:
- Cocoa powder (or chocolate)
- Sugar (for some sweetness, optional)
You add sugar for a hint of sweetness. The sugar will dissolve in the milk, you won’t be able to find back any sugar crystals.
The cocoa gives the chocolate milk its chocolate flavor. However, it won’t dissolve. By stirring you disperse the particles throughout the milk, but they’ll remain particles.
To make good chocolate milk, you should stir it well, just before you’re planning to drink it. Otherwise, the cocoa will separate from the milk.
On a side note, to make a delicious chocolate milk, you’d want to use a cocoa powder that tastes good as well! And, tastes of powders differ more than you might expect! We’ve written about some of the differences between cocoa powers you can look out for.
Hotter speeds things up
You can make chocolate milk with both hot and cold milk. But, hotter milk speeds things up, for a few reasons:
- At higher temperatures, molecules move faster. As a result, sugar dissolves a lot more quickly in hot milk than it does in cold.
- Cocoa powder contains a lot of fat. This fat is solid at room temperature, but, it melts at about body temperature. When the fat is melted, it’s easier for the cocoa particles to disperse and mix.
But, even though hot milk speeds up the making of chocolate making. It also speeds up the separation!
Using cocoa vs chocolate
For the rest of this article, we’ll focus on a chocolate milk that’s made with cocoa powder and milk. However, the same theory applies when you’d make a chocolate milk with milk and chocolate!
Keep in mind that chocolate is made up of cocoa butter (a fat), cocoa powder and sugar. The sugar will dissolve, the fat will melt, so you still have cocoa particles floating in the milk.
The cause of separation: sedimentation
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In any suspension, the particles and liquid will split over time. This is because of a process called sedimentation.
Sedimentation is driven by gravity. Gravity pulls down the particles as well as the liquid. The ‘heavier’ phase will sink down, whereas the ‘lighter’ phase will float on top.
Have you tried to mix oil and water by shaing them together? No matter how well you shake though, over time they’ll separate again. Here, the oil will always lie on top and the water on the bottom. The science behind this is very similar to that of chocolate milk.
Why the milk doesn’t sink – Comparing densities
‘Heavier’ isn’t the correct description though, instead, we have to compare densities. Density is a measure for the weight of a specific volume of something. It’s a measure for how much 1 liter, or 1 gallon for that matter, weighs.
If something has a low density, that means that one liter of something weighs very little. Bird feathers for instance. 1 liter of bird feathers would weigh almost nothing.
The opposite, if something has a high density, means that 1 liter of something weighs a lot more. An example could be steel, or concrete. One liter of concrete is a lot heavier than 1 liter of bird feathers!
In the case of chocolate milk the density of cocoa particles is higher than the density of milk.
Density is a very useful concept to understand in food. Beer brewers and hard cide rmakers use a related concept: specific gravity. With it, they can determine whether their liquids contain enough sugar!
Calculating the rate of sedimentation
You can calculate how fast a chocolate milk splits, using the following formula:
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)
Theoretically, you can calculate how fast your particles will sink down. In reality though, it’s not that easy. Your cocoa particles may have varying densities or sizes. However, even without actually calculating it, you can still use this formula to understand how to slow down sedimentation!
The formula states exactly which factors influence the sedimentation rate, and thus how fast chocolate milk splits. Let’s see what we can learn from this formula.
Larger particles sink faster
We’ll start at the end of the formula, where it says:
R stands for the size of our cocoa particles, their radius, which is half of the diameter. Let’s have a look at what the impact of increasing the radius from 2 to 4 is on the sedimentation rate. We’ll assuming that all other variables remain the same and equal 1:
SMALL particle: sedimentation rate = 1* 22 = 4
LARGE particle: sedimentation rate = 1* 42 = 16
As you can see, increase the particle size with a factor 2 has a huge impact on the sedimentation rate. It goes up by a factor 4.
So, we can conclude that larger particles sink faster.
Thicker liquids slow down sedimentation
Next, let’s have a look at another factor we can influence: μ.
μ stands for the viscosity of our milk. Viscosity is a measure for how easy our liquid flows. A very viscous liquid is thick and syrupy, it won’t flow easily.
So let’s see how viscosity impacts the sedimentation of particles. Let’s assume that we have a thin liquid with a viscosity of 2 and a thick liquid with a viscosity of 4. Again, all other factors remain the same, and we’ll assume that they add up as 1.
THIN liquid: sedimentation rate = 1 / 2 = 0,5
THICK liquid: sedimentation rate = 1 / 4 = 0,25
Note how a thicker liquid slows down sedimentation!
If you’d try to visualize this, it should make sense as well. Imagine mixing cocoa powder in water and in a thick pudding. Intuitively, you’d say it’s easier for the cocoa to sink down in the water than it is in the pudding. Which is exactly what the formula tells us as well!
You can’t control it all
The formula contains a few more variables that are trickier to control. You can’t easily adjust the density of the milk (ρm) and the cocoa powder (ρc). Also, you can’t adjust the gravitational acceleration in your glass of milk, this is a fixed variable.
How to prevent chocolate milk from separating
Using those newly gained insights. We can now turn our attention to trying to slow down or even prevent the separation of chocolate milk. We’ll zoom in on the two factors that we found in our formula:
- Particle size of cocoa
- Vicosity of milk
Strategy 1: Increase the thickness of the milk!
Remember how by increasing the viscosity of the milk, you can slow down sedimentation?
So, you can slow down this separation by thickening your milk. One way to do so would be by using a little bit of corn starch. Simply add 1 tsp of corn starch to cold milk. Gently heat the milk until it just comes to a boil. The milk will still be liquid, but be a little thicker. This increased viscosity slows down sedimentation!
You’re not bound to corn starch specifically, most other starches will work as well, e.g. potato starch. Tapioca starch will have a bigger impact on texture, but it might actually make it a little creamier, if you don’t mind it! Starches are all good thickeners, thanks to the large molecules that can bind water.
Use thickening agents
A downside to using starches is that it noticeably changes the thickness of the milk. Also, starches can bind some flavors of the chocolate milk, muting the overall flavor.
Commercial chocolate milk manufacturers therefore use different thickening agents. They still work the same way, but aren’t as noticeable.
A common strategy is the addition of carrageenan, guar gum and/or xanthan gum. All three are also very large molecules, just like starch. However, they do not need to be heated to thicken a liquid. Even without thickening the chocolate milk that much they make it harder or even impossible for the cocoa particles to sink. The large complex molecules entangle cocoa particles, making it harder for them to sink.
Strategy 2: Use smaller, finer particles
By using a cocoa powder with a very fine, small particle size, you can slow down sedimentation. It may be hard to find these types of cocoa powder though. However, manufacturers use this all the time.
When we made a chocolate milk using a package of chocolate milk powder we immediately noticed how fine the powder was. As a result, chocolate milk made using this commercial powder was stable for a lot longer!
Strategy 3: Use a different type of milk?
So far, we’ve assumed that you’re using cow’s milk to make your chocolate milk. And we had initially assumed that it would work the same, regardless for the type of milk you’d use.
But then we decided to use oat milk to do our experiments with. And, we were amazed! Not only did it take a lot longer for cocoa particles to sink down in oat milk, they also sedimented in a very different way (see photo above)!
Oat milk contains fibers
So why is this happening?
To find an explanation, we need to look at the composition of the two milks. Cow’s milk contains proteins, fats and sugars. All of these are very small and won’t have a big impact on sedimentation. Factory processed cow’s milk is very stable and won’t separate.
Oat milk on the other hand also contains some fibers. Fibers are very long molecules. These could actually be working in a way similar to those thickening agents we discussed earlier. They entangle the cocoa powder. Oat milk as a whole though isn’t stable. It’s why you need to shake it before use.
Oat milk separates as a whole
So, what we see happen with our chocolate milks is that in cow’s milk the milk itself is stable, but the cocoa sinks down. In oat milk on the other hand, the cocoa is trapped within the oat particles. However, the oat milk itself isn’t stable and over time a layer of water forms on top!
Since the separation of oat milk chocolate milk takes a lot longer than that of cow’s milk chocolate milk, it could be a great way to slow down sedimentation!
If you can’t fix it – Shake it!
If you don’t want to change your chocolate milk though, just to prevent sedimentation, not all is lost. Sedimentation is not a harmful process. The chocolate milk is still safe and delicious to drink. All that’s happened is that it separated.
Just stir or shake your chocolate milk to mix all the cocoa back in and enjoy! No need for fancy tricks :-).
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