rock sugar kandijsuiker

The Science of Sugar Crystallization – What Is Rock Sugar?

In the Netherlands we have this one cookie type that’s used for a lot of different dishes. They’re popular for use in cookie pie crusts, ice cream and a bunch of other recipes. Their name: Bastognekoeken, which apparently refers to a town in Belgium that their maker thought of when he first tasted them. Since their taste is quite unique, they can’t really be substituted for by anything else.

When looking at the ingredient declaration though, there aren’t any apparent ingredients that would contribute to their unique flavour. No unique spice mix (cinnamon only). However, the cookies do contain a special type of sugar: rock sugar (kandijsuiker in Dutch). You may have heard of rock sugar as the sugar you can ‘grow’ yourself, on a stick. But it’s not a very common sugar in cooking and baking and seems to be pretty regional. That said, it is completely different from other sugar candies such as caramel or honeycomb. It is also a great way to explain the science of crystallization.

What is rock sugar?

It’s all in the name, rock sugar is a sugar that is sold in the form of small ‘rocks’. These rocks are large sugar crystals and are ‘rock solid’, they don’t break easily, very different to a sugar cube for instance. Rock sugar can be made of pure table sugar (sucrose), thus from a chemical perspective it’s identical to white sugar. To give the rock sugar some extra colour though, colourants are often added, for example some caramel.

Since the crystals of rock sugar are quite hard you won’t often find it in recipes unless it’s added as a whole, for example as a decoration on the outside, or some pieces inside a sweet bread.

How is rock sugar made?

The production of rock sugar starts with dissolving sugar in water. The sugar and water are heated to a specific temperature and then left to cool down. During that time sugar crystals will start to grow. By adding strings, sticks or plates (in industrial processes) you can assure these sugar crystals grow onto these surfaces.

After several days the sugar rocks will have grown considerably onto whichever surface you were using. These sugar crystals will form your rock candy.

How do sugar crystals form?

Rock candy and a smooth chewy caramel are opposite’s of one another. Whereas we want crystals to form with rock sugar, we don’t want it to happen for the caramel. So how can this crystallization be controlled? Surprisingly, there’s a lot of similarities here with ice cream, where you’re also trying to grow crystals, ice crystals as a matter of fact. The driving force is a little different though, in order to understand this we have to go back to state diagrams, as we also discussed when exploring cooking sugar candy.

Saturating sugar

In the so called state diagram below you can see how sucrose and water behave thermodynamically. When the temperature is below the orange line and the sugar concentration left of the blue line all sugar dissolves in water and you end up with a solution. Sugar will not crystallize at this point. However, if you’re in between the blue and grey line this will not be the case anymore. Instead, not all sugar can dissolve anymore and some will crystallize. This is the area we have to be in to make rock sugar. We need to have too much sugar in order for the crystallization to initiate.

This state can be achieved by making a so called supersaturated sugar solution. This means that more sugar is dissolved than is energetically stable. As a result, part of the sugar will ‘leave’ the solution and crystallize.

Such a supersaturated sugar solution can be made by heating water and sugar and subsequently cooling it. As you can see, at a higher temperature, more sugar will dissolve in the water. By cooling it down though, not all the sugar will remain dissolved and some will crystallize out.

sucrose state diagram
A state diagram for sucrose and water. The x-axis indicates the weight % of sugar, the rest is water. See the article on sugar cooking for sources.

Initiating crystallization of rock sugar

Once you’ve made a supersatured sugar solution you need to initiate the crystallization of the sugar somehow. A supersaturated solution can remain completely dissolved for quite a while, it needs something to help the crystals start crystallizing, to form the first ‘nuclei’ on which the sugars will be growing.

In industry this is done by hanging plates inside the solution, at home you can do this by hanging a thread or stick in the solution, this will provide an area for the sugar crystals to grow on.

Controlling size of rock sugar crystals

Once the crystals start growing you’d probably want to control the number and size of these crystals. You can do this by controlling two different processes:

  1. The formation of new crystals, the nuclei
  2. The continued growth of already formed crystals.

Remember ice cream? There we want to have a lot of small crystals. We do this by cooling very rapidly, this helps to form new crystals faster than that they grow.

For rock sugar on the other hand we’d like to do it the other way around. You don’t want to have a lot of small crystals, instead, you are looking for a few very big crystals. To achieve this you want existing crystals to grow faster than that new ones form. This is done by cooling a lot slower. As a result, the overall process will take a long time, because the growth of existing crystals won’t be very fast, but we will end up with those few big crystals.

Rock sugar & Bastognekoeken

Based on the ingredient list of the bastognekoeken, you should be able to make them at home using rock sugar. However, even though we tried a couple of batches, we didn’t come close to the taste of the true Bastognekoek. That special flavour seems to be hard to replicate.

Making cookies with rock sugar does give a nice texture if you decide not to grind them, it gives a little crunch.

Bastognekoeken ice cream with just 3 ingredients
Bastognekoeken work great in ice cream, this is how they look like!

History of (Belgium) rock sugar

Making rock sugar can be very low-tech. As a result, the product is already centuries old, as is shown in this video below.


The origin story of bastognekoeken and their name comes from Wikipedia.

The Tiense Suikerraffinaderij, Onze production (Dutch), link ; a sugar factory in Belgium

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  1. Alright, I’m pretty familiar with the science of chemistry and lately I felt the need to tackle replicating the ice cream sandwich. For starters, I have 1 question, am I in over my head? I have no industrial equipment but I am equally familiar with the methods associated with stoichiometry

    • Hi Paul,

      You can definitely give making an ice cream sandwich a try, I don’t see a reason why you can’t. It’s essentially two cookies with ice cream in the middle. We don’t have a recipe on the website but there are plenty of them online.Good luck!

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