The white stuff we know as sugar is sucrose, a molecule composed of 12 atoms of carbon, 22 atoms of hydrogen, and 11 atoms of oxygen (C12H22O11).
Like all compounds made from these three elements, sugar is a carbohydrate. It’s found naturally in most plants, but especially in sugarcane and sugar beets—hence their names.
Sucrose is a disaccharide – two simple sugars “stuck” together: fructose and glucose. At the molecular level, each grain of sugar consists of a small crystal made of an orderly arrangement of molecules.
If you look closely at dry sugar, you’ll notice it comes in little cubelike shapes. These are sugar crystals, orderly arrangements of sucrose molecules.
When you add granulated sugar to water, some of the sucrose molecules start separating because glucose and fructose molecules are attracted to the water molecules. Why? When a molecule of water and a molecule of sugar gets close to each other, the water disrupts the connection between the two simple sugars, the glucose and fructose molecules separate, and each of the simple sugars form temporary bonds with the water molecules around them.
In other words, each molecule of sucrose is like a couple going for a swim. When they get in the water, their relationship takes a break and they float away from each other (at least for a while).
The dissolving process involves two steps: First, the water molecules bind to the sucrose molecules; and second, the water molecules pull the sucrose molecules away from the crystal and into the solution.
In general, only a certain amount of a solid can be dissolved in water at a given volume and temperature. If we add more than that amount, no more of that solid will dissolve. At this stage, we say that the solution is saturated. The additional solid just falls to the bottom of the container.
This is why you can’t just keep adding sugar to water and expect all of it to dissolve. Once the solution hits the saturation point, there isn’t enough free water molecules to dissolve the new sugar you’ve added.
If you were able to see the molecules of sucrose and water, you would notice that, in the beginning, when you add a small amount of granulated sugar to the water, most of the sucrose molecules are leaving the sugar crystals, pulled away by the water molecules.
Here’s the important bit: you would also notice that some of the dissolved sucrose molecules are crystallizing, that is, not only are sucrose molecules LEAVING sugar crystals but sucrose molecules that had dissolved and were floating in the solution are REJOINING the sugar crystals. The reason is this: sucrose molecules are constantly moving in the solution, so nothing prevents some of them from binding again to sucrose molecules in the sugar crystals.
As long as the rate of dissolving is greater than the rate of crystallization, the sugar crystals remain dissolved in the water.
As we add more granulated sugar to the solution, the rate of dissolving decreases and the rate of crystallization increases, so at some point, both rates are equal. In other words, the number of sucrose molecules leaving the crystals is the same as the number of sucrose molecules joining the crystals. This is what happens when the solution is saturated. The crystals and the solution are in dynamic equilibrium, joining and leaving at the same pace. This means that the size of the crystals stays the same, even though the sucrose molecules are constantly trading places between the solution and the crystals
Past that point, if we add more sugar, the process of dissolving will continue, sure…. But it will be outpaced by the process of recrystallization.
It is impossible to dissolve more sugar than the quantity of liquid will support. The saturated ratio for a sugar solution is 1:2. In other words, you can create a saturated solution of one cup of water and two cups of sugar. More sugar than that, recrystallization occurs, and you no longer have a stable solution.
By the way – in cooking terms, a saturated sugar solution is called a “simple syrup.” Just thought you should know.
As we’ve seen, when you add sugar to water, the sugar crystals dissolve and the sugar goes into solution. But you can’t dissolve an infinite amount of sugar into a fixed volume of water. When as much sugar has been dissolved into a solution as possible, the solution is said to be saturated.
However, the saturation point is different at different temperatures. The higher the temperature, the more sugar that can be held in solution.
When you cook up a batch of candy, you cook sugar, water, and various other ingredients to extremely high temperatures. At these high temperatures, the sugar remains in solution, even though much of the water has boiled away. But when the candy is through cooking and begins to cool, there is more sugar in the solution than is normally possible. The solution is said to be supersaturated with sugar.
Supersaturation is an unstable state. The reason the solution is unstable is it contains more solute (in this case, sugar) than can stay in solution—so as the temperature decreases, the sugar comes out of solution, forming crystals. The lower the temperature, the more molecules join the sugar crystals.
METHOD 2: ADD SOME FAT
Fats in candy serve a similar purpose. Fatty ingredients such as butter help interfere with crystallization—again, by getting in the way of the sucrose molecules that are trying to lock together into crystals. Toffee owes its smooth texture and easy breakability to an absence of sugar crystals, thanks to a large amount of butter in the mix.
METHOD 3: ADD SOME ACID
If you don’t want to buy invert sugar, a simple way to prevent crystallization is to “invert” the sucrose by adding an acid to the recipe. Acids such as lemon juice or cream of tartar cause sucrose to break up (or invert) into its two simpler components, fructose and glucose. And if they are already broken up before cooking in water, there is a much smaller chance of crystallization.
METHOD 4: DO STIR THE SOLUTION AS IT COOLS; DON’T STIR AS IT COOKS
Supersaturation is an unstable state. The sugar molecules will begin to crystallize back into a solid at the least provocation. Stirring or jostling of any kind can cause the sugar to begin crystallizing. For this reason, it is important to avoid stirring the sugar as it cooks or while the temperature is raising to whatever candymaking stage you need.
When sugar granules stick to the sides of the pan, use a pastry brush dipped in a little water to wash them back down into the pan. Don’t stir and for god’s sake don’t use a whisk. Agitation at this stage can push the crystals back together and preemptively start the crystallization process. By the same token, you should use spotlessly clean utensils – a stainless steel spoon is recommended. The tiniest grain of dust or speck of fat will serve as a “seed” for sugar crystals to reform around.
If you MUST stir the sugar – swirl the pan gently, don’t stir it.
This is why leaving a popsicle stick in a glass of supersaturated sugar solution will form rock candy: the stick is providing the “seed” for crystals to form. Once the process begins, it will continue until much of the available dissolved sugar will leave the solution and joined the crystal structure.
ON THE OTHER HAND….
Once the mixture is cool, stir with a spoon or a spatula. A lot. Stirring prevents the sugar crystals that start to form from growing too big.
In general, a sugar crystal grows from a “crystal seed,” which is a clump of sucrose molecules, a speck of dust, or a gas bubble. Stirring causes the sucrose molecules to be pushed into one another, forming crystal seeds throughout the syrup. The resulting crystals will be smaller when more of the crystal seeds are present, because the sucrose molecules can join any of a larger number of crystal seeds.
When making fudge, once the solution has gotten cooled sufficiently, you start vigorously stirring or scraping it. It is important to let the fudge cool down FIRST because if you stir during the cooling phase, crystal seeds will probably form too soon and, as a result, may crystallize out of the solution, and the texture of the fudge would be grainy.
As you stir the fudge, many crystals form at once, and the stirring helps the sucrose molecules bind to one another and start forming small crystals. The main goal is to keep stirring continuously, which generates a larger number of small crystals. As the temperature decreases further, the sucrose molecules spread among the many crystal seeds and bind to any one of them, keeping the size of the crystals small. This creates the rich, melt-in-the mouth texture typical of fudge.
Most candies are made from syrup yet their texture can vary substantially. Two factors play a key role: the length of time for crystals to grow, and the way the syrup is handled while it cools down.
In the case of rock candy, the syrup is left for several days, which provides plenty of time for the formation of large crystals. In the case of fudge, because the syrup is stirred continuously, a large number of small crystals is formed. When making glass candy, gummies, or marshmallows, the syrup is cooled down so quickly that no crystals can form at all.
Making candies is actually chemistry in action. You manipulate the size of sugar crystals—even if you cannot see them—to produce an array of textures. This skill has been developed over hundreds of years, before the science of candy-making was understood. But even then, this art form tells us something interesting about chemistry: It is not only the combination of ingredients that defines a product but also the way they are mixed together.
