Whenever making caramel, marshmallows, honeycomb or just about any other sugar candy you’ll often find that recipe either tells you to boil a sugar syrup to a certain temperature. Or, they’ll tell you to cook it to a certain stage, for instance, soft ball or a hard crack stage. The first time you made the candy, you might have neglected the precise instructions and just guessed your way. Chances are though, that that didn’t turn out too good. Your caramel may have been too runny or your toffee turned out soft and bendy instead of cracking hard.
There are a lot of recipes where following the instructions to the T isn’t that important (e.g. for gumbo). Unfortunately, sugar candy isn’t one of those. Whenever making sugar candy you’d better follow the instructions with regards to temperature and consistency, it guarantees a good product. What’s even better, there’s a scientific explanation behind all these instructions.
Boiling sugar for candy
Sugar candy is, of course, mostly sugar, with maybe a few additions. Even though it’s all a lot of sugar, it nevertheless exists in a lot of shapes and forms. To name just a few: cotton candy, peanut brittle, a caramel syrup and toffee. The core difference between all these candies is how much water they contain besides the sugar. The water content is mostly responsible for the consistency and is achieved by boiling the sugar & water mixture to that temperature mentioned in your recipe.
For simplicity, in the rest of this post, we’ll assume that your candy is made up of only water and sugar. It will help to explain the power of sugar and water. As mentioned above, the water concentration in a sugar candy has a lot of influence on its consistency as we’ll discuss for three different scenarios:
Low concentrations – solutions
Sugar dissolves in water quite wel. If you take a glass of room temperature water and add a teaspoon of sugar that sugar will dissolve in the water after a little stirring. Adding more sugar will still result in most of the sugar dissolving in the water. You can dissolve more than 100g of sugar in 100g of water at room temperature (20°C). The more sugar you dissolve, the more viscous (thicker) the solution of sugar becomes. When there’s only a teaspoon of sugar in the glass of water it will still flow like water. But if there’s a lot of sugar in there it will become more syrupy.
Whenever you’re making sugar candy in which all the sugar is dissolved you will end up with a liquid syrup. The syrup can be thicker or thinner, but it is a liquid and it will flow.
Intermediate concentrations – crystal + solution
However, if you add too much sugar not all the sugar will dissolve anymore, no matter how hard you stir. Instead, part of the sugar will dissolve whereas the rest will remain a solid sugar crystal. You will see these sitting at the bottom of your glass of water.
The final candy will also be a combination of a thick syrup, with crystals inside. A fudge is a good example of such a candy. Part of the sugar has crystallized into crystals whereas part remains in solution.
High concentrations – glass
If the concentration of sugar is really high, it won’t even form a solution + crystals. Instead, the solution is so concentrated that a glass state can be reached. Hard candy is a typical example of such a candy.
State diagram for sugar
These different phases are shown in a so called “state diagram”, see image below. In this diagram you can see at which weight percentage of sugar the mixture of sucrose and water is a solution, a glass or consists of crystals + a solution.
On the x-axis the sugar concentration is given whereas the y-axis gives the temperature. The oragen, blue and gray line indicate the transitions between the different states of the sucrose-water mixture. The orange line indicates the boiling point. At temperatures above this line the sugar + water mixture is boiling. The blue line indicates the solubility. At concentrations left of the blue line all the sugar dissolves whereas on the right side not all sugar will dissolve anymore and remain crystalline. On the right side of the gray line the sugar + water solution will become a glass. It has become so thick that the sugar is caught inside this glassy matrix.
By following the red line you can trace the compositions we just discussed for room temperature.
Boiling sugar and water
In order to dissolve sugar in water recipes will tell you to heat up the sugar and water mixture. The blue line in the state diagram indeed shows that at higher temperature more sugar will dissolve in the water. Not only wil more sugar dissolves, it will also go a lot faster. Sugar will dissolve almost immediately if you add it to a cup of boiling tea, whereas it will take some more time in a cold glass of water.
If you want to make a sugar candy with a lot of sugar in there, you will often decide to bring the sugar and water mixture to the boil even. The sugar will dissolve quickly, but at these temperatures you can also dissolve quite a lot of sugar. What’s more, the temperatur of the boiling mixture is an indicator of the amount of sugar and water present.
Boiling point elevation
The orange line represents the boiling point of the sugar solution. As you can see, this is not a horizontal line, instead it goes up slightly. Adding sugar to the water will thus increase the boiling point of the overall solution. The more sugar, the higher the boiling point. This is a physical phenomenon, very similar to that of the freezing point depression.
This increase in boiling point is represented by the orange line in the state diagram of sucrose above. Since this line is only an estimate, it isn’t very accurate. In scientific literature the values for high concentrations (well above 80%) aren’t very well known anymore more, it becomes harder to accurately determine the boiling point.
Boiling point curve is a given constant
At sea level this boiling point curve will always be the same for mixtures of sucrose (regular sugar) and water. In other words, if the boiling point of my sugar solution is 120°C, I will always have the same concentration of sugar and water, in this case approx. 90% sugar.
For other sugars, such as glucose, fructose or corn syrup similar charts can be made. However, the curve will look slightly different for each sugar. Some boil at a higher temperature, whereas others do so at a lower temperature.
Controlling sugar stages – Temperature
Since a boiling point is always linked to a certain fixed water:sugar composition the temperature can be used as a guideline when cooking syrup. For a certain sucrose/water mixture the boiling point will be a good indicator of the amount of sucrose and water.
The reason for using your thermometer when making candy should make more sense now as well. It is a very exact and precise way of knowing the concentration in your sugar syrup. Since the concentration of the sugar syrup will determine the consistency of your final product, there is quite a direct relationship between these two. If you stop boiling at a certain temperature no more water will evaporate. As a result the composition of your candy will stay the same.
Candy stages – Ice water test
Before we had regular access to thermometers there was another way to test whether the sugar syrup had been cooked to the right temperature. This uses the composition and structure of a sugar mixture to determine whether it’s done or not.
As we mentioned, each concentration of sugar will result in a different structure. High concentrations will make a glass, whereas lower concentrations will form a soft malleable structure. This can be tested by taking a bit of the boiling sugar mixture and cooling it down very rapidly in ice water. The fast cooling will force the structure into its actual consistency.
Cooks and chefs at the time developed a scale to determine whether the candy was hard enough. They made this scale by using the consistency of the sugar mixture when it was dropped in the ice water.
- Thread: 102 – 113°C
- Soft ball: 113 – 116°C
- Firm ball: 118 – 121°C
- Hard ball: 121 – 130°C
- Soft crack: 132 – 143°C
- Hard crack: 149 – 154°C
In a way, this consistency test thus served as a thermometer!
Forming a glassy sugar structure
It is worthwhile to discuss the formation of a glassy structure in more detail. As we mentioned above, sugar candy often involves boiling the sugar solution. It is a good measure of sugar concentration and also makes it easier to dissolve sugar. When making a glassy sugar structure, it is simply essential to do so. You cannot make a glass of water and sugar by simply mixing the two. Instead, part of the sugar will simply stay crystalline and not even dissolve. It will remain in a separate phase.
This is why you have to boil the sugar. By boiling the sugar you first dissolve all the sugar. You then start boiling the solution to get rid of more and more water. As a result, the concentration of sugar will increase and that of water will decrease. This mixture is very prone to crystallization, which is why caramel can be ruined by crystallization of the sugar.
However, if you prevent this crystallization you will have a highly concentrated solution of sugar, well above 80w-%. If you then cool this down rapidly a glass will be formed. It is important to cool quite rapidly and in a stable area. Remember, at various points in time the solution prefers to be a combination of crystals with a solution. By adding sugar crystals or by providing a place for sugars to grow crystallization can start. If this is avoided, a glassy structured is formed. A peanut brittle is a great example of such a sweet.
In the state diagram below we depicted what happens when making this glass through the use of red arrows.
There are a lot of candies with a sugar concentration over 70% but below 90%. According to the state diagram these should contain sugar crystals. However, not in all cases will they actually contain them. This can be achieved by preventing crystallization of the crystals to occur. One way to do this is by adding an inhibitor (this is commonly done for caramels for instance).
In order to understand this we should add an additional detail. A state diagram is a representation of the thermodynamics of in this case a sugar and water solution. The thermodynamics shows what is energetically the most desirable state. However, sometimes it is not possible to get there because of kinetic influences. The addition of inhibitors is an example of this. The inhibitors prevents the crystals from forming, even though from an energy perspective it would be the thing to happen. The inhibitors are simply in the way, preventing it from happening. In our post on caramel science we discuss this in more detail.
For making a nice brown sugar candy, you shouldn’t only look into consistency. You also want a certain flavour. This is why you would caramelize the sugar first.
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