Simple syrup crystallizes when enough of the sugar molecules stick to one another that they become insoluble in the water. In a syrup prepared with a high 2:1 ratio of sugar to water (often referred to as a rich syrup), the chance of sugar molecules clustering and crystallizing is high.
We made three batches of rich syrup by bringing 2 cups of sugar and 1 cup of water to a boil, and then we added ingredients that allegedly prevent crystallization—1/4 teaspoon of lemon juice and 1/4 teaspoon of cream of tartar—to two batches, respectively, and left the third alone. Within 24 hours we saw crystals in the control. The additives bought us more time, but after 48 hours crystals still began to appear. Adding these ingredients in larger amounts helped, but they changed the flavor profile too much.
To find a more effective solution, we had to understand why these additives helped. The cream of tartar and lemon juice are both acids that are able to break down sugar molecules into glucose and fructose in a process called inversion. So not only were fewer sugar molecules available to cluster together in our doctored syrups but the newly present glucose and fructose were physically blocking the remaining sugar molecules from one another.
So we just needed a way to invert enough of the sugar without changing the flavor. After some research, we landed on prolonged exposure to heat. Simmering the syrup for 10 minutes, instead of merely bringing it to a boil, inverted enough of the sugar without affecting flavor.
Here's our method: Bring 2 cups of granulated sugar and 1 cup of water to a simmer in a medium saucepan. Continue to simmer the syrup, covered, for 10 minutes, and then let it cool completely. The syrup can be refrigerated for at least two weeks without crystallization.
