I tried several times to cook the glaze for a strawberry glaze pie and the sugar, water, and cornstarch would thicken but then the sugar would turn grainy. I have also tried to boil water, sugar, and butter to make a carmel type sauce to pour over corn curls and it thickens and seems to be fine but then turns grainy right after I take it off the stove.
It sounds like your syrups are crystallizing on you.
For a smooth glaze or caramel, you want tiny little sugar crystals. When you heat your sugar and water mixture, after a certain temperature the water becomes super-saturated with the sugar.
After this super-saturation point, things get dicey. If the solution is disturbed--say by stirring, or an undissolved sugar crystal or other foreign particle getting into the solution--the sugar can rapidly come out of solution and form big crystals. This could be why your confections are grainy.
- Always use a clean pot or pan.
- Make sure the sugar is completely dissolved before it comes to a boil
- Dip a pastry brush in water to wash away any sugar that sticks to the side of the pot or pan as the sugar heats.
- Combine the sugar with a little water (it should have the consistency of wet sand) before cooking.
- Avoid stirring the sugar when it comes to a simmer.
- Cover the pan loosely with a lid or baking sheet.
- Add a little acid (such as a touch of lemon juice) or corn syrup to the sugar-water mixture before cooking