Cocoa butter molecules stack themselves into many different forms of crystals, and some are more stable than others. (Think of the difference of strength between a single Lego tower and an interlocked Lego wall.) When you melt the chocolate, the cocoa butter cools into crystalline forms that are less stable. Cocoa butter gets pushed out of the chocolate structure and reforms into white crystals.
Chocolate bar at the end of The Great Melting Race

Chocolate Bloom

Broken chocolate: notice the fissures where the chocolate has separated

Cocoa Butter Crystals

hmm
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