Q. Whenever the urge strikes, I make divinity or fudge. On humid days, the candy seems slow to crystallize. Is this really the problem? Or, am I doing something else wrong?
A. There are many variables involved in the crystallization of a molten sugar syrup, whatever its flavor. Humidity is least important. At most, humidity might produce a glossier appearance. A hot syrup cannot absorb moisture because its heat excites water molecules to the point of vaporization. There is therefore a net loss of moisture from a candy syrup until it cools to room temperature.
During storage, however, humidity does have more noticeable effects. For example, fudge and divinity left uncovered overnight in a Tucson home might acquire a rough, powdery appearance. But the same candy under the same conditions in a Houston home might appear shiny at first, and eventually acquire a syrupy glaze.
Far more important to crystallization are the speed of boiling, accuracy of measurement and final boiling point of the syrup.
Speed of boiling:
The slower one cooks a syrup, the more sucrose (table sugar) is inverted or split into fructose and glucose. These two sugars do not crystallize as readily. Furthermore, they interfere with sucrose's orderly crystallization while the cooled or cooling syrup is stirred or beaten. To limit inversion, one should boil and concentrate the syrup as quickly as is possible. To prevent the contents from foaming up, over and onto the burner and to prevent scorching, use a thick-bottom pan with a capacity four times the initial volume of syrup.
Corn syrup, honey, molasses, cream and butter are all "interfering agents." That is, they interfere with crystallization of sucrose. It is therefore very important to measure them carefully. An overly rich fudge crystallizes only after eons of biceps flexing. And you'll burn out the beater trying to make divinity crystallize if the volume of corn syrup was in error.
Final boiling point:
At first, a candy syrup boils at approximately 215 degrees (at this point, it should be thoroughly stirred to dissolve all sugar crystals and the spoon removed). Further boiling brings the syrup to the soft-ball stage, which is 228 to 234 degrees. This viscosity measurement is also given as a range of temperatures; some candy syrups reach the soft-ball viscosity before others. Fudge, for example, which contains starch cocoa particles, fat globules and milk proteins, is boiled to the lower range. One degree high or low can make a very significant difference in texture and speed of crystallization. Fudge boiled to 227 degrees might take 10 minutes of stirring, whereas if boiled to 228 degrees, it might only require 5 minutes of stirring. And if boiled to 232 degrees (the candy is crumbly and dry), the syrup might take 30 seconds to crystallize -- before it can be transferred to a greased plate.