Crystallization, where syrup develops gritty particles, is a natural process which sometimes occurs after long storage at cool temperatures or even because of air bubbles in the syrup, but it is quite harmless. If you discover that your syrup has developed crystals that you think would spoil a dish, simply warm the can gently in a pan of hot water to dissolve them and then use as specified in the recipe.
While it is not necessary to store opened cans of golden syrup in a cool place, as long as the container is sealed, it is best to keep black treacle where it will stay cool.
