Candy Making - Rock Candy
Cooking Knowledge, Dessert, Non Recipes

How to make candy – Part 2: Crystallization


In the first part of my candy-making series, I have talked about the importance of the water content in candy-making. There I have discussed that most candy is an amorphous material instead of an ordered crystalline solid. We can distinguish two amorphous states: the rubbery state and the glassy state. Rubber is chewy whereas glass is hard and brittle.

Whether you have a glass or a rubber depends on the water content of your candy. Candies with extremely low water content are glasses (think cough drops, lollipops) whereas candies with a medium to high water content are rubbers (think fudge, marshmallows). The way to control the water content in your candy is by boiling syrup to a specific temperature that corresponds to the desired water content.

An ideal glassy material is free from any crystals. It is a liquid in solid form. It has an unordered structure. In hard-boiled candy preparations, this is the desired structure. You don’t want any sugar crystals in things like lollipops. But you also don’t want a crystalline structure in some rubbery candies like marshmallows or gummy bears.

On the other hand, there are also crystalline and partly crystalline candies. Examples of crystalline candies are rock sugar and dextrose tablets. Now, I know most people wouldn’t associate these with candy. Rock sugar is just a huge sugar crystal we put in our tea to sweeten it and dextrose tablets are usually gifted to children at the pharmacy. Much more important are the partly-crystalline candies like fondants, grained nougat in candy bars, and fudge.

Rock sugar is a fully crystalline candy.

Crystals in the product matrix give the candy a short bite. We don’t have to chew on it forever. In some candy bars, you have a short and clean bite whereas others are very chewy and can be stretched like molten cheese because they contain only very little crystals. Which texture you prefer is up to your taste and one of the reasons why there is such a great variety of candy available for purchase. Everyone loves a different texture.

What happens when we boil and cool down the syrup for candy production?

When we prepare candy we usually start with a 60-80 % syrup where all the sugar in our recipe can be dissolved in water or milk when heated. Then we boil off the excess water. If we would try to prepare a solution consisting of 90 % sugar and only 10 % water at room temperature that would not work because we are below the solubility curve of sugar in water. We would get clumpy sugar crystals. Even if we heat that mixture at atmospheric pressure we have no chance of crossing the solution line in the phase diagram of sugar and water. We will never be able to fully dissolve the sugar unless we work with a pressure cooker. However, if the sugar has previously been fully dissolved in the syrup at a lower concentration we can boil off as much water as we want and the sugar won’t recrystallize immediately.

A boiling candy syrup containing only 5 % water is still liquid. The reason for that is that systems resist structural changes. It takes energy to turn an oversaturated solution into granular sugar crystals. We have a metastable area below the solubility line in which the syrup resists a phase change. You can observe that at home. Try to dissolve 80 g of sugar in 20 g of water at room temperature. It won’t work. However, if you heat the mixture to 100 °C then you can suddenly dissolve all the sugar. Once you cool down the mixture the sugar won’t recrystallize immediately. Instead, it will be in a meta-stable state. If you wait long enough, however, the sugar will eventually form crystals again and won’t be fully dissolved anymore.

This is basic thermodynamics. At room temperature, the crystalline state of sugar is a lower state of energy than the amorphous state. All systems strive to be in the state of the lowest possible energy. However, it takes energy to change the state of a system. It’s quite intuitive. To explain it in simple words: People strive for their homes to be in an ordered (low-energy, crystalline) state. But if your home is currently messy (high-energy, amorphous), then you need to invest energy to clean and tidy up the house. There’s an investment of energy needed from your side. And if you are lazy like the sugar your home will remain in the high-energy amorphous state because you don’t get your ass up.

By boiling the syrup we supply the system with energy so that it turns into an amorphous material. Heat is energy. It’s as if someone is giving you money or feeding you candy to mess up your house. But then to clean it up there is no reward. This is the situation that sugar experiences during candy-making.

In the chart below you can once again observe what I just told you. Once the syrup gets hotter than about 118 °C, the boiling and solution line cross. The sugar is now so concentrated that it shouldn’t remain dissolved. But because we have a metastable area in which the system resists change below the solution line the syrup just turns into a supersaturated sugar solution.

The phase diagram of a sucrose-water system. Modified. Picture Source: Jain Himanshu, Research Gate.

How crystallization works

We know that crystallization of sugar in the metastable area is thermodynamically favored because it is a low-energy state. But crystallization also requires an initial energy investment. We call this energy investment that prevents crystallization from happening kinetic inhibition. The profit in the form of a low-energy state that the sugar receives from crystallization needs to be the same or higher than the initial energy investment needed. It’s intuitive. You would only invest your money in a business where you hope to gain more profit than your initial investment. If you anticipate that you will spend more money than you get back, you won’t buy the stock of a certain company.

The farther you cool down a supersaturated sugar solution, the more supersaturated it will get. The colder the water, the less sugar can be dissolved in it. This decreases the energy burden the sugar has to overcome to crystallize. Suddenly crystallization becomes a profitable business.

The rate and type of crystallization are dependent on where we are in the phase diagram of sugar and water. If we have only a little water in our system, the viscosity is high and the molecules are restricted in their movement so that they can’t bump into each other to form crystals. If we have a lot of water in our system, the crystallization rate is also low because the syrup is less supersaturated. All the sugar that can be dissolved in water will stay dissolved. Only excess sugar crystallizes.

As you can see in the chart below, the crystallization rate is maximal in between the solubility line and the glass transition line of a sugar-water system. This is the rubbery zone. Hard-boiled candies are hard and brittle glasses. The rate of crystallization is practically zero for glasses. Neither nucleation nor crystal growth is happening in hard-boiled candies. But for rubbery candies like fudge or marshmallows, crystallization is happening. As you can also see in the diagram below, the crystal growth is favored close to the solubility and glass transition line whereas the formation of many new small crystals is favored in the center of the rubbery zone.
Crystallization rate depending on the water content of the system. Crystallization only occurs in the rubbery region between solution (syrup) and glass. Source: Cambridge.

Just to make it clear once again, in the phase diagram below you can see where the crystallization rate is maximal in the phase diagram. The red area has the highest crystallization rate whereas the green one has the lowest. The yellow area is in between the red and green areas with a moderate crystallization rate. In all non-colored areas on the chart, no crystallization can be observed.

Crystallization is highest in the red area and lowest in the green area. Modified. Picture Source: Jain Himanshu, Research Gate,

How to control crystallization

If you want to produce candy with a long shelf life then you need to be able to control the rate of crystallization and even inhibit it for many candies. In the chart below I will show you what will happen if you fail to inhibit the crystallization of your candy. All the excess sugar in your candy that can’t be dissolved in water at room temperature will crystallize during storage.

The candy production process and what happens during storage. Modified. Picture Source: Jain Himanshu, Research Gate,

How fast this process is happening is, of course, temperature-dependent. The lower the temperature, the lower the crystallization rate. So at room temperature, the candy will crystallize slower than at 50 °C. You will have eaten the candy far before all the excess sugar has crystallized if you store it at room temperature.

There is a candy that is produced in the way drawn in the diagram above: fondant. Fondant is left to crystallize at about 50-80 °C. Many tiny sugar crystals are formed. But the crystals are so tiny that you can’t observe them in your mouth. Fondant is a saturated sugar solution that contains all these tiny dispersed sugar crystals. However, if you wouldn’t encourage crystallization at a high temperature for Fondant production, this process would take much longer and you would end up with fewer and larger crystals. You would have to store fondant possibly for years at room temperature until it has fully crystallized because the crystallization happens much slower at room temperature than at 50 or even 80 °C. And you would have a grainy structure because of the larger crystal size.

How do you induce crystallization? It’s simple, you add seed crystals to the syrup. And you stir the syrup. Crystals need a surface to adhere to grow. That’s why all the recipes tell you to not stir caramel. For one, you are providing a surface for crystals to cling to (the spoon). And second, you might introduce seed crystals into your syrup with the spoon. Maybe you use the same spoon that you used to transfer the granulated sugar to the pot. Then there might be some crystals left on the spoon that you then stir into the syrup. Also, you are encouraging the formation of new crystals by stirring around seed crystals that you have introduced to the system. Suddenly, there are seed crystals everywhere in the caramel. Not good. However, once the caramel has cooled down to the glassy state, crystallization will not be a problem anymore. The caramel will stay a glass. But all the crystals that have grown before the caramel has turned glassy will, of course, remain in the caramel. The quicker you cool a caramel down, the fewer crystals you will have in it because crystals from and grow only in the rubbery region.

The effect of doctoring agents on the candy texture

Candy manufacturers typically add doctoring agents like glucose syrup, honey, or inverted sugar to their candies. These doctoring agents get in between the sugar molecules and thus prevent crystallization. If there’s a glucose molecule between two sucrose (table sugar) molecules, they can’t join together to form a crystal. It’s as simple as that. Why are glucose syrup, honey, and inverted sugar the most popular doctoring agents? Because they are also sugars and taste sweet. And they don’t crystallize.

The more chewy and rubbery you want your candy to be, the more glucose syrup should be added to it. If you leave out the doctoring agent or only add very little, the candy will have less elasticity. You can bite through it without intense chewing. On the other hand, if you add more doctoring agent than table sugar in your candy, you might even be able to pull strings like you can with molten cheese.

There are no hard rules when it comes to the question of how much of a doctoring agent should be added to a certain candy. Most candies use a ratio of 70-90 % table sugar to 10-30 % doctoring agents. This will give you candy with no or only little elasticity like a cough drop or toffee. The candy has a short bite.

If you want something squishy and elastic like marshmallows, you usually add more glucose syrup to the candy than table sugar. A defining feature of marshmallows is that they don’t have a crystalline texture. Or think about gummy bears. Gummy bears also contain more doctoring agent than table sugar. The first ingredient on the ingredient list is glucose syrup followed by table sugar.

Gummy bears typically contain more glucose syrup than table sugar to make them extra chewy and squishy.

You can either rely on recipes to determine the optimum amount of doctoring agent for your candy or you can simply experiment. Do one batch with half glucose syrup and one with 10 % glucose syrup. Examine the final texture and pick your winner. There is no right or wrong. Candy-making is about personal preference.

That’s it for today! In my next post of this series, I will talk about foams and the incorporation of air into candies.

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