For candy makers, one of the most important characteristics of sugar is that it tends to crystallize easily. If you are making pralines or fudge, crystallization at the proper time is the desired result. When you are cooking caramels or brittle, crystallization is the enemy. Controlling the crystallization of sugar is a large part of what candy making is about.
Any ingredient added to a batch of cooking sugar that helps to prevent it from crystallizing is called a doctor. There are two main types of doctors that home candy makers use: acids and syrups. The acids commonly added to sugar to prevent crystallization include cream of tartar, lemon juice, and vinegar. Adding a small amount of these ingredients to boiling sugar will not only help to prevent or control crystallization, but will also make the finished product softer, so they must be measured carefully and added when the recipe instructs. The syrups used as doctors include corn syrup (both light and dark), molasses, and honey. Maple syrup is not an effective doctor.
Each of these ingredients contributes its own flavor and characteristics to a recipe, so they are not entirely interchangeable. Proper use of doctors will help you to be successful any time you cook sugar.
