Published September 1, 1994. From Cook's Illustrated.
What's the best way to keep brown sugar soft—or soften it once it's hard.
When brown sugar comes into contact with air, the moisture in the sugar evaporates. This eventually causes the sugar to lump together. You can prevent this by storing brown sugar in a zipper-lock plastic bag or in a tightly covered plastic container--the latter makes scooping and measuring easier. Storing brown sugar in the refrigerator will also help keep it fresh and soft.
If your brown sugar does harden, heat or moisture can be applied to soften it. For sugar not needed right away, place a slice of bread in a container with a tight lid, add the sugar, and close. By the next day, the sugar will have absorbed the moisture in the bread and softened. Some cookbooks recommend using an apple slice in the same fashion, but we found that the apple was so moisture-laden that some of the sugar began to melt, and the apple imparted an unwelcome flavor.
If you realize at the last minute that your brown sugar has hardened, heat must be applied to soften it. The microwave on low power does the job in a minute or two, but can be difficult to control. The sugar can melt in some spots and remain lumpy in others. We found a 250-degree oven to be more reliable. Placing a measuring cup full of water in the oven to add moisture, a step recommended on some brown sugar packages, was found to be unnecessary in our tests.
Spread the sugar in a pie plate or on a square of aluminum foil. After about three minutes in a preheated oven, poke the sugar with a fork to see if it crumbles; if not, leave it in the oven for a few more minutes. In our tests, brown sugar softened in three to seven minutes, depending on how hard it had become. Cool the softened sugar, and store in an airtight container any not being used immediately.