Recipes for fudge tell you to heat a sugar syrup to two hundred and thirty degrees Fahrenheit.
Taffy recipes give similar instructions, but with a temperature of two hundred and sixty-five degrees. And to make hard candy, you heat the mixture to three hundred degrees.
Why Is There A Difference In Temperature?
The difference between fudge, taffy, and hard candy is in the amount of water each one contains: the less water, the harder the candy.
The temperature of the syrup is what tells you how much of the syrup is water and how much is sugar.
At sea level, pure water boils at two hundred and twelve degrees and never gets any hotter. But the boiling point for sugar is much higher than it is for water. And the temperature of the syrup, is a rough average of the temperatures of the two ingredients.
Up to two hundred and twelve degrees, the temperatures of the two ingredients may be the same. But above that, only the sugar can get hotter.
The Sugar-Water Radio
When you first heat sugar solution, it boils at a temperature specific to that particular sugar-water ratio. As the water boils out, the solution gets more concentrated and the boiling point goes up.
Eventually, all the water boils out and the temperature stays at the boiling point for sugar alone.
The Perfect Piece Of Candy
A syrup with more sugar and less water makes harder candy and also boils at a higher temperature. As you boil the syrup you know water is disappearing because the boiling point goes up.
When the boiling point reaches the right temperature, you know you have just the right ratio of sugar to water. That way, when the mixture cools you'll get the kind of candy you want.