You might be familiar with the brightly colored version on a stick. Or you might regularly use it as a core ingredient in your (savory) Asian recipes. Or, you might use it to sweeten your tea.
Rock sugar, or rock candy, is used by a wide variety of cultures around the world in a variety of applications. Some are just for fun, others play a core role in flavoring or sweetening dishes. Despite the wide variety in uses, all rock sugars are pretty similar in their core. They are made up of large, chunky sugar crystals. Some rock sugars might be large smooth crystals of sugar, whereas others are more rough and angular.
It’s an old form of sugar, that has been around for centuries. It’s definitely been around longer than it’s been understood scientifically! It also just so happens to be a great tool to explain the concept of supersaturation and crystallization!
What is rock sugar?
So what really is rock sugar? 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.
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 (of which you can find an example in the photo below). Each ‘rock’ is a single compact sugar crystal, made up of almost 100% sucrose (regular sugar).
Crystals are actually pretty special not every material can form a crystal and crystals tend to be quite pure, being made up of just one type of molecule or atom.
Science of Crystallization
So what really is a crystal? A crystal is a very highly organized, solid structure. In our case this structure is made up of mostly sucrose crystals. Each sucrose molecule organizes itself in this ‘highly ordered microscopic structure’ next to its neighboring molecules. As you can see in the photo of a piece of rock sugar above, the sugar molecules have aligned themselves along straight lines to create almost cubic shapes.
Since a crystal needs to have a very ordered structure, it needs to be quite pure. Impurities will mess up the ordered structure, inhibiting the individual molecules to sit very tightly next to each other.
Not every component has the ability to crystallize. In order for a component to be able to crystallize the individual building blocks (e.g. molecules) need to fit tightly into each other to make that ordered structure. Sucrose (which is ‘regular’ sugar) can crystallize.
Another very common example of crystals in food are ice crystals made of water. When water freezes into ice it forms ice crystals. In these crystals, the individual molecules line up very close to one another in a strict and orderly pattern. In the case of water you create crystals by freezing it, but how do you do the same for sucrose?
In order for sucrose to crystallize you need it to be energetically favorable for the sugar to do so. You do so by creating a supersaturated solution of sucrose. A supersaturated solution of sucrose is a solution of sugar in water that contains more sugar than is energetically stable. So what does all of that mean?
Let’s start out with regular sugar. This regular sugar is made up of small sugar crystals. In dried form, sucrose will always want to exist as a crystal. However, once it’s a crystal, it’s pretty stable. It won’t grow or merge together. To change the size of your crystals, you will have to dissolve them and let them form again from scratch.
Dissolve the sugar
You can dissolve sugar in water. At room temperature, you can already 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.
Increase the temperature
However, 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 candy as well!). By adding more sugar you can again saturate the solution, adding as much as will dissolve at that temperature.
Cooling it down to supersaturate
When you now bring the temperature of the warm sugar solution back to room temperature, the solubility of the sugar will go back to its original value. So, you’ve now dissolved more sugar in the water than is energetically stable at that temperature!
You have now created a driving force for crystallization to happen. The excess sugar now 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. The crystalline sugar will have ‘left’ the sugar solution, leaving you with both pure sugar crystals as well as a sugar solution.
This solution that contains ‘too much’ sugar is a supersaturated solution, it’s the start of almost any crystallization process including making rock sugar!
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 sugar 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. Crystallization will stop once the solution isn’t supersaturated anymore, so best to add some more!
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.
Once you’ve created your first crystal, the 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
Rock sugar is made through the exact process that we discussed above. It can be made from both pure sucrose (as we show in the ‘recipe’ below) but it can also be made from sugarcane and sugar beets directly.
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 the boil to concentrate the sugar solution. This creates a 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 interfer with the crystallization process, the yield of a process that makes these sugars tends to be lower.
- 300g sugar
- 120g water
- Little extra suga to coat your toothpicks
- Take two small (approx. 0,.25l) or one large glass jar(s) (0.5 liter / 0.5 quart). Cut a piece of parchment paper a little larger than the diameter of your jars, such that you can easily wrap the paper over the top and secure it with a elastic band.
- Take two small or one large wooden toothpick(s) or skewer(s) (you can also use chopsticks, or even a piece of thread). They need to be a little higher than your glass jar so they stick out at the top.
- Wet the toothpicks/skewers and roll through a little bit of sugar. These are the 'seed crystals' that will help initiate the crystallization!
- Take a pan and add the sugar and water. Bring the mixture to the boil so all the sugar is dissolved. No need to continue cooking once all the sugar has dissolved.
- Pour the sugar solution in your glass jar(s). Leave to cool to room temperature. Be patient, if you add the toothpicks too soo the sugar will simply dissolve in your sugar solution.
- Once the sugar solution has cooled down, take your toothpick and carefully press through the middle of the paper with the clean side, just enough for the top to come through. Now place the toothpick in the solution, cover with the parchment paper, the parchment paper should hold the toothpick in its place. Tighten the parchment paper with an elastic band.
- Now, be patient!
- Within a day or two you should see some crystals growing on your toothpick! It will have finished growing in about a week's time, though this depends on the temperature in your house.
- Take the toothpick from the sugar solution, you'll have some beautiful sugar crystals! You'll probably also have a lot of sugar crystals growing on the sides of your jar. You can re-use the sugar solution by redissolving all the sugar, maybe adding a little extra and placing in another toothpick.
You can experiment a lot with rock sugar. How about:
- Changing the concentration of sugar in your sugar solution. You can go both up and down. You can probably decrease it somewhat while still getting rock sugar. However, at some point no more crystals will be formed.
In the post we mention that the solubility of sucrose at room temperature is 210g / 100ml. We are using 250g/100ml for this experiment, so we're clearly making a super saturated solution, there's 40g too much sugar in the water. (Calculate as follows: 300 / 120 * 100 = 250.)
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