"Revive Your Brown Sugar: Tips for Softening and Preventing Drying Out"

TL;DR
Brown sugar hardens due to moisture evaporation when exposed to air. To prevent this, store it airtight and provide an additional source of moisture. To soften hardened brown sugar, use a microwave with a damp paper towel or a zip-top bag with water. Alternatively, use a Brown Sugar Bear, a soaked terra cotta piece that keeps brown sugar soft. To keep brown sugar soft from the start, transfer it to an airtight container or a zip-top bag and use a Brown Sugar Bear for extra moisture.
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