I wanted to create some kind of Valentine’s Day treat for the Lab. Steve, Maggie, and Maddie (the MadArtBot) were kind enough to 3D print some hearts for me, from which I could make candy molds:
That’s right, anatomically-correct human hearts. Aren’t they romantic?
Okay–let’s make some candy.
The Chemistry of Mold-Making
The first thing I needed to do was make molds from the plastic heart objects. I acquired a pound of Silicone Plastique, a silicon putty designed by a chef for making food-grade molds. The putty arrives in two parts, a base and a catalyst, one white and the other blue. Separate, they are inert, but mix them together and they’ll cure into flexible, durable silicon.
You take the amount of putty you need (equal parts by weight) and blend them together until you see that the color is consistent–this is how you know that you’ve combined the two parts thoroughly. At this point, you’ve got about 15 minutes to apply the putty to the object you want to make a mold from.
First you apply a skim coat, smearing a small amount of putty onto the object, making sure to cover and push air out of any little details. Next you cover the entire object in at least a 1/4 inch of putty to finish the mold. That’s it! The silicon will cure in about an hour and after popping out the original object, you’re left with a permanent, food-grade mold that can be frozen or subjected to temperatures up to 450 F, and can be used to mold anything from butter and chocolate to soap and plaster. Pretty cool, right? You can see step-by-step directions and a video here.
The Chemistry of Sugar
I decided to make cherry-flavored lollipops with my heart molds, following this recipe (which includes a video with the inventor of Silicone Plastique).
We can make hard candies because sugar molecules tend to form crystals. How hot the sugar gets, how much water is in the solution, and the manner in which and speed at which it cools all help to determine whether you end up with fine crystals (as in fudge), coarse crystals (as in rock candy), or one giant transparent mass, which is what I was going for.
When you dissolve sugar in water, you raise the boiling point of the liquid above that of water. As the sugar syrup cooks, water evaporates, causing the boiling point to rise even more (and continue to rise as more water evaporates). To achieve a certain texture of candy, you want to bring the solution to a boil and watch the temperature–this will tell you the sugar concentration of the liquid. At 235 F, the solution is about 85% sugar, and if you stopped here you could make fudge. For hard candy, you want a sugar concentration of almost 100%, which means bringing the syrup to at least 300 F. To confectioners, this is know as the “hard crack” stage, because of the cracking sound the candy will make if you pull it into threads.
While the syrup heats up, sugar molecules form bonds with water molecules. As the candy cools, however, there is less water to keep the sugar molecules apart and they will once again begin to bond with each other and form the crystals dictated by their structure. You can stimulate crystal growth by introducing a crystal “seed”–a surface the crystals can attach to and grow from (think of rock candy growing on a stick or string). You can also agitate the mixture by stirring or shaking, which will cause the molecules to bump up against each other more frequently than they would otherwise; this method will result in finer crystals.
In order to form transparent sugar “glass,” you need to prevent the growth of organized crystals. One way to do this is to cool the mixture rapidly, so that the molecules do not have time to set into structured arrays and instead settle haphazardly throughout. Another way is to introduce an “interfering agent.” This recipe uses one such agent, corn syrup, whose long glucose chains form a barrier, preventing the sucrose molecules from bonding.
To make the lollipops, I brought sugar, cream of tartar, corn syrup, and water to a boil and then let the temperature come up to 300 F. I then removed it from the heat and let it cool to 275 F before adding flavoring and coloring. This mixture then went into a preheated glass measuring cup inside a 250 F preheated oven for 5-10 minutes.
I set the molds in a bed of uncooked rice, so that I could keep them level, and inserted lollipop sticks before pouring the candy solution into the molds. At room temperature, the lollipops took about 45 minutes to set; after the first couple batches I started finishing them in the freezer. Once the candy was set, I just popped the suckers out of their molds. Ta da!
This Valentine’s Day, I encourage you to start a love affair with kitchen science. A big thanks to my valentine, Harold McGee, and his ever-helpful book On Food and Cooking for providing our sugary chemistry lesson.
An “amuse-bouche” (which literally translates to “mouth-amuser”) is a complimentary morsel to start the meal, a tasty little gift from the chef. We hope you enjoy these edible tidbits.