Learn what causes a hard white crust to form on your sugar when it's left open in humid weather. Discover what's happening to the crystals.
1 Answer
Open sugar develops a white crust in humidity because the sugar on the surface dissolves in the moisture from the air, and then recrystallizes as the water evaporates, forming larger, less transparent crystals.
Detailed Explanation:
Sugar, or sucrose, is highly soluble in water. When exposed to humid air, the sugar crystals on the surface of the sugar pile absorb moisture from the atmosphere. This absorbed moisture dissolves the sucrose, creating a concentrated sugar solution on the surface. As the humidity fluctuates or decreases, the water in this solution evaporates. As the water evaporates, the dissolved sugar molecules come back together and recrystallize. However, the recrystallization process doesn't perfectly recreate the original fine sugar crystals. Instead, it often forms larger, more irregular crystals. These larger crystals scatter light differently than the original fine sugar crystals, making them appear white and crusty. The crust is essentially a layer of recrystallized sugar that has a different texture and appearance due to the moisture absorption and subsequent evaporation process. The higher the humidity, the more pronounced this effect will be.
Pro Tip:
To prevent sugar from forming a crust, store it in an airtight container in a cool, dry place. Adding a few grains of rice to the container can also help absorb excess moisture.