Cake with fondant
Dessert, Vegetarian

How to make candy – Part 7: Crystalline candy

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Fondant and rock sugar are both crystalline candies. And they are a great way for anyone to understand and observe the principles of crystallization. Seriously, this is a great teaching lesson for school children and any interested adult. If you’ve read my post about the theory of candy crystallization, then consider this the practical course.

Fondant gets crystallized quickly at a temperature when the rate of crystal nucleation is maximal. Rock sugar, on the other hand, gets crystallized slowly at room temperature. And in both cases, we end up with a delicious product to eat.


You might know fondant as that fancy cake decoration. It is a kind of sugar dough that can be shaped like playdough. But fondant can not only be used as a cake frosting, it can also be used in candy bars or pralines as a filling or coating.

The important thing is that you start the crystallization at the right temperature. At home, that is 50 °C (120 °C). That is when you start to knead the candy mass until it resembles a smooth dough. I’ve drawn the production process for fondant in the phase diagram below.

Phase diagram of sucrose and water
The production process for Fondant. Modified. Picture Source: Jain Himanshu, Research Gate.

Before I get to the recipe, I have a few questions that you should be able to answer if you’ve read my post about crystallization.

1. Why do we induce the crystallization of fondant at 50 °C (120 °F) and not immediately after the syrup is taken off the stove?

2. What does the structure of fondant look like? What are the two co-existing phases?

3. Why is fondant opaque and not clear like rock candy?

Could you answer all the questions? Here are the answers:

1. The nucleation rate and overall crystallization rate is highest in the “center” of the rubbery zone. If we induce crystallization the second after we take the syrup off the stove, the crystal growth will be favored over the nucleation in the first phase of crystallization. This is because of the low level of supersaturation at higher temperatures. This gives us a fondant with a gritty texture and large crystals in it.

In the red area, nucleation is favored over crystal growth. If we crystallize the syrup in the green area, we will end up with fewer large crystals. Modified. Picture Source: Jain Himanshu, Research Gate.

2. Fondant is a saturated sugar solution with small dispersed sucrose crystals. The two co-existing phases are saturated syrup (a viscous liquid, the continuous phase) and tiny sucrose crystals (the dispersed phase).

3. Fondant is opaque because it gets aerated while kneading.

Did you get all the answers right? Congratulations! If not, you can always go back and check out posts #1-3 of my candy-making series. Let’s get to the recipe.

Rock candy

In Rock candy, we want the opposite of what we want in fondant. We want the sugar to grow into a beautiful large crystal. I’m not a big fan of licking on rock candy but in tea it is magic. I love to observe how rock candy slowly dissolves in my cup of tea.

As for the fondant, I have a few questions for you that can help you to check your level of comprehension:

1. Why does the crystallization process take so much longer for rock candy (up to two weeks at room temperature) than for fondant (up to 1 day at 50 °C (120 °F))?

2. When producing rock candy, you dip wooden skewers that are coated with granulated sugar in a supersaturated syrup. Why do we coat the wooden skewers with granulated sugar?

3. When will the growth of a rock sugar crystal out of a supersaturated sugar solution come to a stop? Assume that the sugar solution doesn’t get changed or renewed during crystallization.

And here are the answers:

1. The lower the temperature, the slower the crystallization speed.

2. The granulated sugar crystals are used as seed crystals to induce crystallization.

3. The sugar crystal can only grow as long as the syrup used for dipping is supersaturated. All the sugar molecules that can be dissolved in the water will stay dissolved. This limits the maximum size of your rock candy. To make the crystal grow further, you would need to replace the saturated dipping solution with an oversaturated syrup.

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