In the Netherlands we don’t have any mountains, we have a hill and are pretty proud of the fact that our highest point is only a little more than a 100m above sea level. What’s more, most of our country is below sea level! Nevertheless, I adore skiing, as a kid it was always my favorite holiday of the year. We used to drive down south, to the Alps in France where we would zoof down the skills, enjoying the outside and being active all day. For lunch we ate French baguettes that I bought at the ‘boulangerie’ in the morning, eating them in the snow. And we always had (and still have) a hot chocolate milk break. Skiing and hot chocolate milk with whipped cream (‘chocolat chaud avec Chantilly’) just don’t go without one another.
When skiing, chocolate milk is always made fresh to the order (often it’ll come from a machine though). When you leave your chocolate milk for too long (that doesn’t happen often when on skiing holiday though) it will start separating, just like homemade chocolate milk down. But the other day I realized that a lot of store bought chocolate milks do not separate, and that, of course, triggered a blog post!
There are quite some chocolate milks on the market you don’t have to shake before use. Simply poor and drink. But if you make your own chocolate milk and leave it in the fridge it will always separate out in a milk and a chocolate layer. Apparently, there’s something about the pre-made chocolate milk that’s preventing it from separating!
This is a typical food physics phenomenon. Typical topics of food physics involve the mixture of ingredients that don’t react, so no chemical reaction occurs. Chocolate milk is a typical example of such a material. Other examples of products interesting for food physicists are mayonnaise and salad dressing.
What is chocolate milk
- Fill a cup halfway with milk
- Add in as much cocoa powder and sugar as you like (I prefer lots of cocoa powder, only little sugar, a ‘dark chocolate’ version) and mix well until all clumps are gone.
- If you like, heat the mixture in a pan or microwave for warm chocolate milk. Heating the milk will also make it easier to dissolve the sugar.
- Add the rest of the milk (not adding all the milk at once will make it easier to get rid of clumps, just as is the case for pancakes)
When making chocolate making you are actually performing two different processes:
1 Dissolving sugar
On the one hand you are dissolving sugar in milk. Milk consists of mainly water and sugar readily dissolves in water. By heating up the milk the dissolution of sugar in milk will go even faster (imagine dissolving sugar in ice tea vs. hot tea, the hot tea is a lot easier). The reason it goes faster is because at a higher temperature all particles move a lot faster (molecules never stand still at temperatures used in your kitchen, they’re always moving). Dissolving sugar in water breaks up the crystals of the sugar. Energy is released because of the breaking down of crystals and such a solution of sugar in water is stable. You will have to boil out the water for instance to get pure sugar again.
2 Creating a dispersion
The other thing you’re doing is making a dispersion of the milk and the cocoa particles. A dispersion is a mixture of two materials, one liquid (the milk) and one solid (the cocoa powder), which don’t react nor dissolve in one another. Instead, the solid particles simply float around in the liquid.
Cocoa particles do not dissolve in milk. Cocoa powder actually contains quite some fat, which doesn’t mix with water easily, they will always try to separate again.
The cause of separation: sedimentation
So, you’re left with these particles floating around in 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 solid particles in the liquid. The overall force of gravity depends on both the size and density of the particle. The heavier the particle, the more gravity pulls at the particle, thus the faster it will settle down.
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!
Making chocolate milk that doesn’t separate
In order to make a chocolate milk that doesn’t separate you can use the formula to tackle this issue. The first thing you can do if prevent the particle from moving. You can increase the viscosity of the liquid. However, we generally don’t appreciate chocolate milk which is as thick as honey. So you cannot do this too much (furthermore, it will make it harder to mix in your cocoa powder as well). What else can you do? Influence the particle size of your powders, but for completely preventing sedimentation over a long period of time, you’d have to go down quite bit.
Chocolate milk and carrageenan
There’s another option, it’s creating some sort of network in your chocolate milk that keeps the cocoa particles floating. This is the method a large chocolate milk producer in the Netherlands uses. Their chocolate milk contains the following ingredients:
Semi-skimmed milk, sugar, cocoa (1,6%), dextrose (0,7%), stabilizers (carrageenan, guar gum)
Of course, you recognize the first three, we used those to make our own chocolate milk. The only reason I can think of for dextrose to be there is to add additional sweetness. Then there’s the carrageenan and guar gum. These two make sure that the cocoa particles don’t ‘fall down’ in the chocolate milk.
Working principle of carrageenan & guar gum
Carrageenan are long polysaccharide chain with sulphate groups attached to them. There are various classes of carrageenans and all work slightly differently, the three main types are: kappa, lambda and ioto carrageenan. They all have a different number of sulphate groups, which gives them different properties.
The sulphate groups that sit on the long polysaccharide chains are what give the carrageenans their stabilizing properties. These groups are negatively charged and repel each other as well as bind with positively charged ions in a solution. They will form complex three dimensional structures and since they are hydrophobic (don’t like water) they will prevent water from moving too much. I won’t go into too much detail, it’s pretty sciency, but if you’re interested here are some interesting articles: from a supplier of carrageenan and a book.
Guar gum was the other stabilizer present in chocolate milk. Guar gum is made from ‘guar’, I didn’t even know this was a real plant. Guar gum is also a polysaccharide. It is a long chain of mannose molecules with little side chains of a molecule called galactose. This long chain again is the reason it can thicken liquids, also guar gum is very good at thickening mixtures. Again, I won’t go into the details of the chemical properties here since it’ll be too much for this post, but there’s a lot of information to be found on the internet.
The last way 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!