This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
The factors that control the size of the crystals in fondant making also determine the size of the crystals formed in fudge. Fudge is often made of brown sugar. Brown sugar contains a higher percentage of invert sugar than granulated sugar. Thus it crystallizes less readily than granulated sugar.
Fudge also ripens with storage and if placed in a container with a tight-fitting lid becomes much softer and more velvety after 24 hours' storage.
In substituting cocoa for the chocolate in fudge, Reese found that the quantity of cocoa was a matter of individual preference, although 3 to 31/2 tablespoons to each cup of sugar was preferred by the majority of judges. But, if other conditions were standardized, the final temperature to which the sirup was cooked depended upon the proportion of cocoa used. Cocoa contains considerable dextrin. It is probably the presence of this ingredient that affects the consistency of the sirup so that with increasing amounts of cocoa the temperature to which the sirup is cooked should be lower. For a sirup cooled to 40°C. and requiring about 12 to 15 minutes to beat to bring about crystallization, the fudge containing 1 tablespoon of cocoa is cooked to 112.5° or 113°C. For each additional tablespoon of cocoa the temperature to which the sirup is cooked can be lowered one-half degree. These conditions hold for the recipe given in the laboratory outline. With larger proportions of butter, the temperature to which the sirup is cooked should be elevated.