Take time to thoroughly dry the food processor bowl before blitzing the berries and sugar. Similarly, your airtight container must be totally dry before transferring the finished sugar for storage. Moisture can cause the sugar to turn sticky and clumpy.

If you don’t have a food processor, you can grind the freeze-dried berries in a standard household coffee grinder instead, as I’ve done in my lavender sugar cubes recipe. You may need to work in smaller batches, rather than grinding the berries all at once.