This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Taffies or pulled candies have vinegar, lemon juice, or cream of tartar added during cooking. These substances cause inversion of the sucrose, which prevents crystallization. Dextrose, corn sirup, or molasses may be added to the sucrose to prevent crystallization. Pulled taffies become white, or if made with molasses become much lighter in color, on account of the air bubbles incorporated during pulling.
The cooking temperatures of pulled candies vary according to the ingredients and their proportion used, but follow the same general rules as for caramels. Those containing large quantities of corn sirup are cooked to lower temperatures. Taffies are firmer than caramels and are therefore cooked to higher temperatures.