Chocolate and Bloom

Why does chocolate sometimes form a white film on its surface?

This harmless white substance is called bloom and is caused by extreme temperature changes, most commonly on chocolate that has been refrigerated or frozen. Although bloomed chocolate is safe to use, its appearance is off-putting (although the bloom is reabsorbed and disappears when the chocolate is melted).

When chocolate goes from a warm, humid environment to a cooler one, moisture forms on its surface, dissolving the sugar it comes in contact with. After the moisture evaporates, the sugar crystallizes and leaves behind the white film. Similarly, if kept in an environment that’s too warm, the fat in the chocolate changes to a different crystal structure, which looks gray. For bloom-free storage, milk and white chocolate will last six months, and darker chocolate will last as long as a year when wrapped tightly in plastic wrap and stored at room temperature in a cool pantry.