Working with hot sugar, either for candy-making or advanced pastry work and cake decorating, is one of the most challenging of hobbies. For one thing, hot sugar can be downright dangerous, causing nasty burns if you still it. It's also extremely finicky, working best at minimal humidity and reacting powerfully to just a few degrees' difference in temperature. Even when everything else works properly, it can sometimes acquire an oddly yellow or brown hue when cooking. It's maddening, but easily avoided.
The Skinny on Sugar
Sugar is a culinary chameleon, giving your finished candies wildly variable textures depending how they're prepared. The key characteristic you'll manipulate in candy-making is temperature. Heating and cooling your sugar carefully to specific temperatures can give results ranging from the crunch of hard candy to the creamy smoothness of fudge and the tooth-threatening stickiness of toffee. Almost every recipe begins by dissolving your sugar in water, then boiling it to a predetermined temperature.
That Mellow Yellow
Sometimes, as you're heating your sugar to the correct temperature, you'll notice that it begins to take on a pale, golden-yellow hue. This isn't a problem if you're setting out to make caramel, since the sugar will brown anyway, but it can detract from the appearance of candies that require clear or tinted sugar. Ordinary table sugar, or sucrose, doesn't caramelize until it reaches a temperature of 340 degrees Fahrenheit, so this premature darkening can be a frustrating mystery to novice candy-makers.
Breaking Down the Science
A number of different factors can cause your sugar to turn color at low temperatures. The filtering process used to whiten the sugar can sometimes leave trace amounts of protein, which brown at low temperatures through a process called a Maillard reaction. More importantly, the heat breaks down part of your sugar into its component molecules, the simple sugars fructose and glucose. Fructose begins to caramelize at 220 F and glucose at 300 F, so the more sucrose molecules break down into glucose and fructose, the greater the risk of your sugar yellowing.
Pick Up the Pace
Fortunately, the cure for this problem is straightforward. The longer and slower you cook the sugar, the greater percentage of its sucrose breaks down. The obvious -- and correct -- response is to heat your sugar more quickly. As soon as it's thoroughly dissolved in the water, turn your burner to a medium-high heat and insert your candy thermometer. As the syrup boils and its percentage of water plummets, the temperature will begin to rise. Pay close attention, and remove the pan from its burner as soon as the sugar reaches the correct temperature. You might need to set the saucepan in cold water briefly, to prevent it cooking further.