Ask A Scientist: Rock Hard Sugar
Question: Why does brown sugar harden when stored improperly while white sugar doesn’t?
Answer: Ah, brown sugar. How delightful it is when those wet sandy, crumbly piles fold effortlessly into your oatmeal. And yet, can’t heat very same brown sugar can drive a normally sane baker from zero to batcrap in no time flat when they discover that they are the owner of an unyielding sucrose brick. What’s up with that?
The reason why brown sugar locks down tighter than a supermax when stored improperly stems from the exact ingredient that makes brown sugar brown. Molasses!
Most brown sugar labeled as brown sugar at the grocery store is simply refined white cane sugar that has molasses added to it. Light brown sugar has a molasses content of 3.5% by volume, while dark brown sugar has a molasses content of 6.5% by the same measure. The reason behind fortifying ordinary refined cane sugar rather than using genuine brown sugar (think Moscovado or Demerara) is to ensure consistency as well as to reduce manufacturing costs.
The humectant molasses that makes that brown sugar soft and brown stays that way only if there’s limited exposure to air. If brown sugar is left out in the open, that molasses will eventually evaporate, leaving the rest of the sugar to harden.
This being known, the old trick of storing brown sugar with a price of bread or a citrus peel to keep the sugar soft has a simple explanation as well. Both the bread and peel have just enough moisture to keep the sugar soft, like a miniature humidifier. Science, it’s amazing!