sugar syrup cooked to 121C

The Science Behind Cooking Sugar Syrups For Candy Making

Making candy is often described as a science, requiring thermometers to boil that sugar syrup to the exact right temperature. Alternatively, you can cook the syrup to a prescribed stage, “cook to hard crack”, which can be daunting. If you don’t follow the detailed instructions your caramel, marshmallows, honeycomb, or just about any other sugar candy are doomed to fail (or at least, that’s how you feel). Or you might have done the opposite, and just guessed your way through. Risking that your caramel was just a little too hard and your honeycomb too chewy.

Although there definitely is an art to making candy, there’s also a lot of science to it, hence the detailed instructions. In a previous article, we’ve shown you how sugar syrups change consistency when you cook them. Here, we’ll be digging into the science behind all of those instructions.

Sugar candy variation

It’s pretty incredible how using one core ingredient, sugar, can make so many different types of candy. Fluffy cotton candy, airy honeycomb, crispy peanut brittle, chewy caramels, soft marshmallows, and crunchy toffee are all built upon sugar, but still very different. Some are light and airy, others dense and crunchy.

We have to thank the fascinating way of how sugar interacts with water for this wide spectrum of textures! All these candy styles involve making a solution of sugar in water, just how much sugar in just how much (or little) water, determines to a large extent how the candy behaves!

Even though most sugar candy will contain other ingredients such as fats, leavening agents, or proteins, for now, we’ll assume that these candies are just sugar and water. It will help us explain the science, without too many distractions.

3 core sugar candy science concepts

To explain and understand how sugar solutions behave and make these different types of candy, we have to dig deeper into the science of sugar solutions. Three concepts especially are important here, forming the basis of most candy making science:

  1. Boiling point elevation
  2. Solubility
  3. Glass transition

Boiling point elevation

At sea level, pure water has a boiling point of 100°C (212°F) and a freezing point of 0°C (32°F). However, you can change this boiling point and freezing point by dissolving other components in water such as salt or sugar. The interactions between the different components impact how easily the liquid boils or freezes. Just how much the boiling or freezing point changes depends on how much of that additional component (e.g. sugar) is dissolved in the liquid.

You can dissolve a lot of sugar molecules in water, it has a high solubility. As such, you can greatly impact the boiling and freezing point of water. You use the latter when making ice cream. Adding sugar to your ice cream will decrease the freezing point of that ice cream. It is what makes your ice cream soft, despite being frozen to temperatures well below 0°C.

Salting roads in winter is also based on the concept of freezing point depression. The high concentrations of salt lower the freezing point of water, preventing it from freezing on the road!

When making candy, on the other hand, you’re making use of the boiling point elevation. The more sugar you dissolve, the higher the boiling point of that sugar solution becomes. A sugar solution with 60% sugar (sucrose) already has a boiling point of around 103°C (217°F). Further, increase that concentration to 90% and the boiling point is closer to 120°C (248°F)! As long as you stay at the same altitude and use the same sugars, there is a fixed relationship between the concentration of a sugar solution and its boiling point.

scoops of cream cheese ice cream
Ice cream would be rock solid, where it not for the freezing point depression caused by the addition of sugar!


As we quickly alluded to above, sugar easily dissolves in water. Even at room temperature, you can dissolve twice the weight of sugar in water (approx. 210g per 100ml). You can dissolve even more sugar in water by increasing the temperature of your solution. Not only will more sugar dissolve, it will also dissolve faster. A spoonful of sugar dissolves almost immediately when added to a cup of hot tea, whereas the same amount will take a while to dissolve in a cold glass of water.

By combining the concept of ‘boiling point elevation’ and the increasing solubility of sugar in warmer water, you can create highly concentrated sugar solutions. Simply bring a sugar solution to the boil. By doing so you’ll be evaporating water. As a result, the concentration of sugar increases, which again increases the boiling point, evaporating even more water. You can concentrate sugar solutions until they’re almost 100% sugar!

Saturating a solution

Even though you can dissolve a lot of sugar in hot water, once you cool the solution down again, the solubility will go back to its initial value. This is what scientists call a supersaturated solution, more sugar is dissolved than is energetically stable. Over time, the sugar will crystallize out of the solution, which is simply more favorable. As a result, you will end up with sugar crystals & a sugar solution.

How you induce and control the crystallization of this ‘excess’ sugar is a core technique of candy making. A lot of candies such as fudges or creams need some of the sugar to re-crystallize. The crystallized sugar gives the candies firmness but also really nice melting properties.

You use the concept of supersaturating a solution for a lot of candies, including rock sugar!

quite dense honeycomb
Good honeycomb candy forms a ‘glass’ when it’s cooled down. It’s simple too concentrated for the sugar molecules to move around and crystallize.

Glass transition

Not in all cases can sugar recrystallize again after its been concentrated in water. By concentrating a sugar solution and reducing the water content you create a thick and viscous material (see photos of different sugar syrup cooking stages here). The solution can become so extremely viscous and thick that molecules have a very hard time moving around. At some point, the molecules get ‘stuck’. Even though energetically spoken it’s more efficient for a sugar crystal to crystallize, it simply can’t move enough to organize itself into a crystal!

The conditions under which this happens, that a solution is just too concentrated and viscous to crystallize and flow is called the glass transition. The sugar solution will literally form a glass!

State Diagram: plotting the 3 concepts in a graph

These three core concepts are the foundation of candy-making. By combining them into one visualization you can predict just how your sugar solution will behave and what type of candy you can make. Scientists do this using a state diagram (we used something similar when discussing ice cream science), which we’ll dig into here.

The Axes: Temperature & Concentration

We start the diagram by drawing two axes, the x- and y-axis. On these two axes we plot the two conditions that determine how a sugar solution will behave:

  • x-axis: the concentration of sugar in the sugar solution
  • y-axis: the temperature

Combined, these two factors can predict how a sugar solution will ultimately behave. As you can imagine, a hot sugar solution with concentration A might behave very different than that same solution at room temperature. The same for solutions at the same temperature but with different concentrations.

sucrose state diagram
A simplified version of a state diagram of sucrose (note, these lines are rough estimates, shown for illustrative purposes, their exact location might differ slightly).
Each area between the axes and the orange (boiling point elevation), blue (solubility), and grey (glass transition) lines represents a specific behavior of that specific solution.
Based on: source.

Plotting the concepts

Next, scientists will want to add the three core concepts we just discussed: boiling point elevation, solubility & glass transition into the diagram. It is actually surprisingly hard to get good, accurate data for these three phenomena. It can take scientists tons of experiments to determine the exact boiling point of a solution or the actual solubility.

In the graph above the orange line at the top represents the boiling point elevation. You can see how increasing the concentration of sugar, slowly increases the boiling point of the solution. The lowest point is 100°C (212°F) which is the boiling point of pure water, so with 0% sucrose.

The middle blue line represents the solubility of sugar. Notice here that as the temperature increases, the solubility also increases. This agrees with what we discussed earlier.

Lastly, notice the line indicating the glass transition. Any point underneath this grey line will most likely turn into a glass. Notice how glasses only form for very high concentrations of sugar and at moderate temperatures. It is under these conditions that molecules simply can’t move well enough to form crystals.

Unique for a sugar (mix)

Also, keep in mind that these phenomena all depend on exactly which sugar you’re evaluating. These lines will lie at different concentrations and temperatures for different sugar types or even blends of sugar.

Whereas this is all very nice, keep in mind that making such a state diagram takes a lot of data. What is more, a diagram is only valid for one specific sugar or mix of sugars. In other words, the diagram for sucrose, will be slightly different to that of glucose which again differs from that of fructose.

It is also why using different sugars when making your candy will impact just how that final candy turns out.

Using the state diagram

Now that we know the science behind the state diagram, let’s see how we can use this when making candies. We’ll walk you through a few scenarios.

Sugar syrups

For some applications, all you want to do is dissolve sugar in water. This syrup should then be liquid and flow easily. Think of a glaze that you’d pour over a cake. If you want to store this syrup for longer, without any crystals forming, we need to stay to the left of our ‘solubility’ curve in the state diagram.

As long as the concentration of your sugar in combination with the temperature you’re storing it at (most likely room temperature), stays to the left of that curve, you should be good. This is the area marked as ‘solution’.

Looking at the diagram on the right, a sugar solution with 50 or 60% sugar at room temperature won’t crystallize. At higher temperatures, e.g. 80°C even a 70% solution is stable.

sucrose state diagram

Creating crystals

There are a lot of candies, such as fudge and creams, where you do want some of the sugar to crystallize. Kurma cookies are another good example of this category as is rock sugar. In all cases, you do want the sugar to crystallize to create the correct texture and structure.

In order for this to happen you need to make a solution that is in the area between the solubility and glass transition curves. The area marked as ‘solution + crystals’. A sugar solution of this composition and temperature will crystallize over time. If you make a sugar syrup at room temperature with 80w% sugar you will have made a solution that crystallizes.

Forming a glass

Lastly, you might want to make a candy that’s very crunchy and crispy, which snaps when you try to break it. This can bring you in the glass territory. You will want to make a sugar solution at room temperature containing over 90w% of sugar! Of course, you can’t just make that by mixing water and sugar together, the sugar won’t dissolve. Instead, you will first have to bring a sugar solution to the boil, cook it to about 120°C (248°F) to concentrate the solution, before cooling it down to room temperature.

You can illustrate this process in the state diagram we just discussed, using the red arrows shown. Notice how the 3rd arrow goes through the ‘solution + crystals’ area. If you do not cool down this type of candy quick enough, it might crystallize in this region, ruining your glassy structure!

sucrose state diagram, making a glass

The reality is more complex (unfortunately)

Even though the basic concepts we discussed are all true for sugar candies, in reality it is a little more complicated. As we mentioned, the state diagram that we showed is for sucrose (‘regular’ sugar). However, sucrose isn’t completely stable at higher temperatures. When you cook it you might be breaking down some of the sucrose into invert sugar – we do this on purpose when making a Dutch style of pulled candy. Invert sugar behaves slightly differently that sucrose does. A big difference is that it doesn’t crystallize as easily, so it can actually prevent crystallization from occurring even though you thought you were in the ‘solution + crystals’ zone.

And invert sugar isn’t the only factor at play. Adding other syrups such as corn syrup can also prevent crystallization of sugar from occurring where it normally would have! It’s a feature we use when making a lot of different types of caramel for instance.

That said, we never said candy making was easy ;-). However, use this bit of scientific understanding to help you tweak and problem solve your candy and you will come a long way. We’ve written a few guides that bring this knowledge into the ‘real’ world to help you with exactly that. Read more about rock sugar, caramel, sugar syrups, or crystallized sugar cookies to bring it all to life!

top view of nut caramel tart
Caramel tart, we wouldn’t want the sugar to crystallize in this tart!


Phase/state transitions of confectionery sweeteners: thermodynamic and kinetic aspects, 2010, R.W. Hartel et. al., link

On Food and Cooking, 2004, Harold McGee, p. 682, link

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  1. Thanks for sharing the idea . i would like to know what is added to hard candy so that it doesn’t melt back
    after it has been prepared

    • Hi Samuel,

      So nothing is added to the candy to prevent it from melting back! It’s the unique behavior of sugar that makes it hold its shape. Once there’s so little water in the sugar candy that sugar molecules can’t move around anymore, it will just hold its shape (that’s what happens in that ‘glass’ phase)! Hope that clarifies it.

  2. Hello ! so according to the diagram, beyond approximately 67.5% sugar concentration in the solution, there is a risk of recrystallization of the sugar if the temperature of the solution drops back to room temperature (20 degrees). This means that if we exceed the concentration limit for dissolving sugar at room temperature, we risk recrystallization. could you please confirm?

    • Hi Yacine,

      Yes, that is correct. At room temperature you can dissolve sugar up to approx. 67,5%. This doesn’t mean the sugar will recrystallize immediately, in some cases it may take a little time. But, the higher the concentration, the likelier it is to recrystallize.

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