Learn the science behind:
Rock sugar, or rock candy, comes in many shapes and sizes. You might be familiar with the brightly colored version on a stick, you might regularly use it in your (savory) Asian recipes, or, you might use it to sweeten your tea. Your rock candy might consist of large smooth pieces, or be rough and angular.
Despite the wide variety in uses and types, all rock sugars are pretty similar at their core. They’re made up of large chunks of sugar crystals, slowly grown on a stick, or thread. It’s been around for centuries since it can be made with pretty simple tools, though wasn’t fully understood until folks started to properly understand the concept of crystallization. Sugar happens to be great at crystallization as long as you ensure the conditions are optimal for it to happen
- Rock sugar is made up of crystals
- Sugars can crystallize
- It only works with quite pure sugar
- Supersaturation – You need enough sugar
- Nucleation – Starting your rock candy!
- Growing your rock sugar crystals
- How rock sugar is made in factories
Rock sugar is made up of crystals
So what really is rock sugar, or rock candy? That depends a little on where you buy or make it, but all rock sugars are made up of large ‘rocks’ of sugar. These ‘rocks’ are noticeably larger than your ‘regular’ granulated sugar. They are very hard and some can break your teeth if you’d try to bite into them. But, the composition of these large rocks is the same as those of small pieces of granulated sugar! Some rock sugars are made up of just one large rock, whereas others are made up of a lot of smaller ‘rocks’ that have all grown together. Each ‘rock’ is a single compact sugar crystal, made up of almost 100% sucrose (regular sugar).
Sugars can crystallize
Not every ingredient can form these crystals. Sucrose happens to be great at it, but other sugars such as glucose and fructose won’t be able to form these intricate crystals at all! It’s all about the correct molecular structure and interaction between molecules to make it happen.
A crystal is a highly organized, solid structure. That is, molecules, or atoms, arrange themselves very tightly in a repeatable pattern. In our case, this structure is made up of sucrose molecules. Each sucrose molecule organizes itself in this ‘highly ordered microscopic structure’ next to its neighboring molecules. They’re so well aligned, that in the case of rock candy, surfaces are very smooth and angular.
Another very common example of crystals in food are ice crystals made of water. These form when water freezes. Again, the individual molecules line up closely in a strict and orderly pattern.
It only works with quite pure sugar
Since a crystal needs to have a very ordered structure, it needs to be quite pure. If another type of molecule interferes with the compact arrangements, this will mess up the ordered structure. As a result, impurities can only be present in low quantities to not have a negative impact on your rock candy.
Supersaturation – You need enough sugar
In order for sucrose to crystallize you need it to be energetically favorable for the sugar to do so. Sugar wants to crystallize once there’s so much sugar dissolved in a liquid, that it is no longer desirable for the sugar to remain dissolved. Instead, it will want to turn solid and crystallize. This happens when a solution of sugar is supersaturated. A supersaturated sugar solution is one that contains more dissolved sugar than is energetically stable. So, to make rock candy, your main challenge is to create this supersaturated solution and you do so by taking a few steps.
Step 1 for making rock sugar: Dissolve the sugar
To make rock sugar, you first need to dissolve the sugar that you want to use to make the crystals. At room temperature, you can dissolve a reasonable amount. However, at some point, additional sugar won’t dissolve any longer. Instead, it will just lie in the water (most likely sink to the bottom) in its crystalline form. At this point your sugar solution is saturated, you’ve dissolved as much sugar as you can.
Rock candy process step 2: Increase the temperature
By increasing the temperature of the solution, the solubility of the sugar increases. In other words, at higher temperatures, you can dissolve more sugar in water than you can at lower temperatures (you use this when making lots of different types of candy). By adding more sugar you again saturate the solution, adding as much as will dissolve at that temperature. You can simply continue adding sugar to your hot sugar solution until no more dissolves. If you then want to add some more, just warm it up even more and you will be able to dissolve more as well!
The warmer the sugar solution, the more sugar dissolves. A good way to check whether you’ve dissolved enough sugar is by measuring the temperature of your solution using a thermometer. Most recipes will also include a temperature to hit so you’re sure it’s warm enough to dissolve all the sugar you need.
There’s no need to use an actual candy thermometer. Any simple thermometer will do, we love using our basic IKEA thermometer for just about any temperature measuring job!
Step 3: Cooling it down to supersaturate
Once the sugar syrup cools down to room temperature, the solubility of the sugar will go back to its original value. You’ve now dissolved more sugar in the water than is energetically stable at that temperature: the solution is supersaturated. You have created a driving force for crystallization to happen. The excess sugar prefers to be a crystal rather than be dissolved in water.
This does not mean all the sugar will crystallize instantaneously, but it does mean that over time, if given the chance, that excess sugar will crystallize until you’ve again dissolved the maximum amount of sugar and reached a new balanced situation. At that point, you’ll have sugar crystals, hanging in a sugar syrup. Not all the sugar will crystallize since some of it will prefer to remain dissolved. But all the ‘excess’ will have crystallized.
At room temperature (25°C) the solubility of sucrose in water is 210g/100ml. So, to supersaturate the sugar solution, you need to make sure you dissolve more than 210g of sugar per 100ml.
When making a supersaturated solution for making your own rock candy you want to make sure you dissolve quite a bit more sugar than would be unstable at room temperature. If you want to grow big crystals, you need to make sure enough sugar will crystallize to form those crystals.
Nucleation – Starting your rock candy!
In order for the sugar to start crystallizing in the supersaturated solution a first crystal needs to be formed, a nucleus. A nucleus forms the starting point on which other sugar molecules can then grow that crystal. The first crystal forms through a process of nucleation and will need a little nudge to be formed.
There are several ways to start this crystallization. First of all, you can simply add a crystal that you already have available. If you already have a bag of sugar, adding a few of those sugar crystals into your supersaturated solution will initiate the growth of crystals! The added sugar crystals are your nucleus and form a fertile ground to grow on for other sugar molecules.
Impurities & shear
You can also initiate crystallization by the presence of impurities. If the wall of your container (or stick or thread) isn’t completely smooth, it might just be the ideal starting point for the growth of a crystal. Also, stirring (shearing) the supersaturated solution can induce nucleation.
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.
Growing your rock sugar crystals
Once you’ve created your first crystal, the rock candy crystals will continue to grow until the solution is no longer supersaturated. During this time just how you treat your sugar solution determines how the resulting sugar crystals will look like. You can control the size and to some extent even the shape of the crystals!
By continuously breaking up the crystals that are growing (e.g. through shear) you can ensure that your crystals remain quite small. You simply don’t give them a chance to grow any larger.
On the contrary, by only providing a few nuclei (starting crystals) and not disturbing them any further, you can make quite large crystals. This is what you do when making rock sugar. You just leave it be and let the crystals grow.
Ice cream making uses the same science when cooling down and freezing the ice cream mix. By shearing (mixing) the ice cream while it is churning you’re breaking up large crystals into smaller ones, ensuring the ice cream won’t turn gritty!
How rock sugar is made in factories
Rock sugar is made through the exact process that we discussed above. However, if you’re not making it at home, you’d use industrial ingredients. Instead of making it from pure sugar, you’d directly make it from sugarcane and sugar beets directly. This way you can skip some steps in the sugar manufacturing process.
If the rock sugar is made from sugarcane or sugar beets directly these crops are first crushed and milled to free up all the sugar that is naturally present. Sugar manufacturers then bring the syrups to a boil to concentrate the sugar solution. This creates that supersaturated sugar solution.
Next, they’ll leave the sugar solution to sit for several days. They might string threads through the syrup to serve as starting points for the crystals to grow on or they might submerge plates into the solution onto which the sugar crystals can grow. The temperature is tightly controlled, to ensure the solution is saturated just right.
Once the crystallization time is over, the strings, sticks, or plates are removed and the sugar crystals can be ‘harvested’. They’ll have grown over time!
Manufacturers can steer this process in a lot of different ways. Some manufacturers might make pure rock sugar. This rock sugar will be white or even transparent in color and will contain almost only sucrose molecules. In other cases, they might leave in some of the minerals or other ‘impurities’ which can color the sugar crystals brown. These impurities will interfere with the crystallization process, and the yield of a process that makes these sugars tends to be lower.
Chan, S.T., Lee, C.K., Leong, Y.H., Tan, S.Y., Teng, H.S., Teo, K.G. and Wee, E.H. (1986), Studies on chinese rock sugar: Analysis and taste. J. Sci. Food Agric., 37: 194-198. https://doi.org/10.1002/jsfa.2740370214
A. Gholamhosseinpour, M.J. Varidi, M. Elahi and F. Shahidi, Evaluation of Traditional Production Process of Rock Candyand Optimization of Sucrose Crystallization (Part 1), American-Eurasian J. Agric. & Environ. Sci., 4 (1): 72-75, 200, link
R.W. Hartel, Chapter 2: Solid-liquid equilibrium in foods, in Physical Chemistry in Foods, CRC Press, 1992, link
Lim, T.S.E., Chia, K.F., Loo, L.M. et al. Rock Sugar Crystallization: The Effect of Mineral Impurities. Sugar Tech (2021). https://doi.org/10.1007/s12355-021-00991-7, link
PubChem, Sucrose, link ; for solubility data
The Tiense Suikerraffinaderij, Onze producten (Dutch), link ; a sugar factory in Belgium
The woks of life, Rock Sugar & Brown Rock Sugar, 12-Jan, 2021, link
Tsugitaka Sato, Sugar in the Social Life of Medieval Islam, p. 45-49, Brill, 2015, link