Brown sugar can harden when exposed to air for an extended period. To help prevent hardening, place soft brown sugar in an airtight container. Freeze for prolonged storage. If you have brown sugar that has become hard, it's very simple to soften, just follow these quick, easy directions: Microwave Softening Method: Place the hardened brown sugar in a microwave-safe bowl. Cover sugar with two damp paper towels. Tightly cover bowl with plastic wrap. Heat in microwave on high at 30 second intervals until softened. (Microwave ovens vary in power; cooking time may need adjustment) Divide sugar with fork (sugar will be hot); stir. Use immediately. Overnight Softening Method: Place the hardened brown sugar in a bowl. Cover sugar with two damp paper towels. Cover bowl tightly with aluminum foil or plastic wrap. Let stand overnight at room temperature. Divide sugar with fork; stir. Store in refrigerator.