3. Crystallization: One of the greatest frustrations in toffee making comes when a smooth syrup turns quickly into a grainy mass. This is caused by sugar crystals that have formed on the sides of the pan in the process of being stirred down into the syrup.
Here are several ways to prevent crystallization:
Dissolve sugar completely before mixture boils.
If you notice any crystals on the side of the pan, brush them down into the syrup with a pastry brush dipped into hot water or tightly cover the saucepan and let the mixture cook for about 3 minutes. This causes steam, thereby melting the sugar crystals that may have adhered to the sides of the pan.
Avoid stirring syrup once it begins to boil unless the recipe instructs otherwise.