This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Caramels, taffy, and brittles are types of candy that are firm but not crystallized. To prevent crystallization larger quantities of dextrose, levu-lose, corn sirup, or other substances are added than when making fondant or fudge. If these substances are not added directly, then larger quantities of substances that produce inversion of sucrose are used, thus giving a higher percentage of invert sugar.
Corn sirup and molasses are the materials most commonly used in caramels to prevent crystallization. The temperature to which caramels are cooked depends upon the ingredients used to prevent crystallization and their proportion. With increasing amounts of corn sirup, because of its dextrin content, the temperature to which the caramels are cooked is lowered. But with molasses or honey the temperature to which the caramels are cooked is much higher than when corn sirup is used. In the recipe given in the experimental outline with 1 cup of corn sirup to 2 cups of sucrose, a temperature of 119° to 120°C. produces the degree of firmness liked by most people. If the corn sirup is increased to 2 cups and the sucrose reduced to 1 cup the same firmness is produced by cooking to a lower temperature or 116° to 117°C. If dextrose, honey, or molasses is substituted for the corn sirup, the temperature to which the sirup must be cooked to give the same firmness is above 120°, sometimes as much as 126°C. or higher, the temperature depending on the type of molasses or the proportion used.
Sugar reactions with proteins. Ramsay, Tracy, and Ruehe found that when milk, albumin, or casein is heated with lactose or dextrose a brown discoloration occurs. As the temperature is raised dextrose and casein become so firmly attached to each other in a protein-sugar complex that no amount of washing will remove the sugar.
The amount of milk in caramels. A large quantity of milk in caramels develops a flavor that can be obtained in no other way. Slow cooking develops more of the brown color and flavor than rapid cooking. The color comes from lactose-protein and dextrose-protein compounds and the caramelization of the lactose with high temperatures and long cooking. If all the milk is added when cooking is first started the milk may curdle. If most of the milk is added slowly after the sirup is thick it very seldom curdles. In using large quantities of milk in caramels the protein of the milk will tend to prevent crystallization.
Large quantities of fat increase the richness of caramels.
Caramels are stirred or handled as little as possible after the sirup stops boiling until they are cool. After the boiling is stopped they soon reach the saturation point, which for caramels is above 100°C, and stirring the sirup or scraping the pan tends to produce crystals.