So, I spent a delightful afternoon celebrating the end of the semester helping my friend Nora make her signature salted caramels, which she gives her lucky family every year for Christmas. While we watched the sticky sweet solution bubble away, she asked me what happens to the sugar to make caramel sticky and fondant, her other favorite holiday candy, so smooth?
I knew that we could find the answer in the science of sugar molecule structure, different methods of cooking, stirring, and cooling sugar syrups result in different sugar structures when the candy cools. Sugar, purchased from the store as a sack of white grains is crystallized molecules of sucrose. Molecules form crystals by stacking together in tightly packed, efficient structures, built of many molecules in a highly stable shape. Sucrose is composed of two smaller sugar molecules, fructose and glucose, each in a ring. Below, glucose is the hexagon on the left and fructose is the pentagon on the right:
To make most candy, you dissolve the sugar in water, which breaks down the rigid structure of the molecules, and separates the glucose from the fructose. Then you have sugar in solution. But, the water can only handle a certain amount of sugar before it is saturated. Increasing the temperature of the solution increased the amount of sugar that the water can handle, however. This is why the proto-caramel syrup stays in solution, even as the water evaporates and the amount of sugar increases relative to the amount of water.
Now that you have a bubbling, hot sugar syrup, you have to decide what kind of candy you want to make: crystalline or non-crystalline. Crystalline candies like fudge and fondant purposefully let the sugar crystallize as it cools. Caramels, and taffy, and smooth hard candies form when the sugar solidifies without crystallizing.
To form the mint fondant that Nora loves, like the inside of a peppermint patty, but better, the minty sugar syrup is rapidly stirred as it cools, usually at a very specific temperature. The disturbance encourages the sugar to crystallize and create a specific texture.
Candy thermometers are essential for heating the syrup to the right temperature so that you have the correct ratio of sugar to water, and for stirring the solution or adding an ingredient at just the right temperature. How hot the syrup gets is directly related to how hard the candy is when it cools, hotter syrup makes harder candy. Once my mom and I tried to make peanut brittle without a candy thermometer. The brittle was so solid that trying to eat it put your teeth at risk, and we had to throw it all out. Very sad. This chart, and a thermometer, would have saved us.
We had to let the caramels reach 248 F, a firm, but not hard, ball temperature. Then, when you remove them from the heat and cool the solution. For caramels like we made today, you don’t want the sugar molecules to build crystals as they cool. One way to do this is to keep the syrup very still, no stirring at all. Another way is to add ingredients that will get in between the sugars and prevent them from building big crystals. Fats, like butter and cream, are great for this, and Nora’s caramel recipes used both. Yum. Because the fats interfered with the sugars, we could stir the caramel solution until it bubbled and thickened to the perfect temperature. When we spread it out on a cookie sheet to cool, the caramels solidified, and Nora cut them into sticky squares, with no sugar structures to break up the smooth candy. Delicious. Trust me, I licked the pan.