Brown sugar is kept soft and fluffy by storing the baking ingredient in an airtight container and by introducing a source of moisture into the container. Hardened brown sugar is rehabilitated using the same method.
Preventing fresh air from combining with the sugar is the best way to prevent brown sugar from sticking together. Using hardened brown sugar can prevent proper mixing and promote uneven baking.
Brown sugar is made by combining white crystallized sugar with molasses. The amount molasses depends on whether one is making light brown sugar or dark brown sugar, with larger amounts creating darker sugar. When the sugar's coating of molasses is exposed to air, the moisture in the molasses evaporates, allowing the sugar to stick together in a hard clump.
Molasses is able to reabsorb moisture, which explains why introducing a moisture source can help soften hardened brown sugar. Small terracotta disks are available for purchase that, when soaked in water, helps retain or reintroduce water to a container of brown sugar. Other sources of moisture include apple slices, white bread or marshmallows.
A speedier option is microwaving the needed measurement of brown sugar with a damp paper towel in 20-second increments, taking care not to burn the ingredient.