In addition to being an expert fudge maker, I’m also a nerd. So this week I’m going to explain the chemistry behind fudge. I know you probably slept through high school science class, and some of you have already stopped reading, but I promise it’s not as boring as you think. Pay attention, there might be a pop quiz at the end.
Making a good batch of fudge is hugely dependent on getting all the variables just right. Even small mistakes can result in a disaster. (Ironically, fudge itself was originally a result of botching some other candy recipe). If the ingredients, technique, and temperature are all perfect, you’ll have smooth, creamy fudge. If not–well, let’s just say that instead of fudge you get something fudged up.
Fudge depends on supersaturation and crystallization. Remember learning about those in school? No? Well, keep reading. Controlling these two processes is the key to making traditional fudge. Supersaturation means dissolving more of a substance (in this case, sugar) in a liquid than would be possible under normal conditions. This is achieved by heating cream, which allows us to add more sugar, which in turn allows the cream to reach temperatures well above its normal boiling point. For fudge, the temperature needs to reach 234-240°. This is called the soft-ball stage. If the temperature is too low the fudge will be runny; too hot and it will be too hard. Humidity and altitude both affect the exact temperature required, so it takes some skill to know when the fudge is really done cooking.
Sugar doesn’t like being in a supersaturated solution. It really prefers being a solid and will try to become one at any opportunity. Controlling sugar’s natural desire to crystallize is very important for getting smooth fudge instead of a grainy texture. Even a single sugar crystal–called a “seed”–can cause the rest to rapidly crystallize. Fortunately, there are a few ways to avoid that. Many fudge recipes call for a little corn syrup, which is mostly glucose. These extra glucose molecules get between the sucrose molecules and prevent them from forming crystals. Think of glucose as the chaperones at a school dance, keeping the kids from getting too close. Letting the fudge cool completely undisturbed until it reaches 110° is another important step. Then when it’s cool it needs to be stirred constantly. At that point we want it to crystallize, since that’s what makes it firm up, but the goal is to keep the crystals as small as possible. As the crystals form the fudge will go from being shiny to a bit duller, a signal that it’s ready to pour into a pan to set.
I’ll bet you didn’t know there was so much science behind a seemingly simple treat like fudge! And I didn’t think you’d read this far, so we’re both surprised. The next time you want to try a chemistry experiment, make a batch of fudge. And then eat it all, in the name of science.