The Sharp & the Broad

Reader Frank writes:

You mentioned that more cocoa butter causes a chocolate to melt faster. Can you explain why?

I would love to do that, Frank! You’ve of course heard the term “melting point”, a temperature at which a substance changes from solid to liquid. A melting point can be “sharp” or “broad.” A substance with a sharp melting point changes from solid to liquid very suddenly. A substance with a broad melting point changes “phases” only very slowly. In general it’s purer, more homogenous substances that have sharper melting points.

Why? Think of a stick of butter. From a normal human vantage point it looks like a homogenous substance: an oblong block of fat. But if you were able to suddenly zoom in on it and see it microscopically, butter isn’t made of just one kind of fat. It’s actually an amalgamation of dozens of different types, each with its own melting point. Some are relatively low, some high. Heat the butter and the different fats begin to melt in turn. The effect on the butter block is that it gets pliable, then soft, then runny before the last few fats finally melt and the whole thing goes liquid. Dairy butter has a broad melting point.

Cocoa butter has a comparatively sharp melting point. It too is composed of many different types of fat (21 of them to be precise), three fats dominate the mix, and they have melting points that are very close to one another. Heat them much above 85 (higher if the chocolate is tempered) and a rapid cascade of dissolving crystals begins: the cocoa butter liquifies. This is what makes cocoa butter such a great cosmetic component. Touch it with your fingers and it softens instantly, ready to be rubbed into skin.

Of course chocolate isn’t pure cocoa butter. It contains plenty of other things: cocoa solids, sugar, milk solids, emulsifiers and flavorings. All of those things serve to blunt the sharpness of the cocoa butter’s melt point. However it’s easy to see now why a higher percentage of cocoa butter yields a faster-melting chocolate, yes?