Reader Paul C. writes in with this:
I was very interested to read your pieces on tempering and fat crystals. Does the fact that unstable [crystals] form at a lower temperature than stable crystals have anything to do with the softness of untempered chocolate?
An excellent question, and the answer is yes, it does. The forming temperature of a crystal is the same as its melting temperature. However they’ll start to soften at still lower temperatures. Unstable crystals thus contribute to the softness of untempered chocolate in several ways. Their structure is fundamentally weaker, but they’re also softer at room temperatures. On a similar note, reader Natasha observes:
The last time I made a chocolate ganache and covered a cake with it (it had slightly more chocolate than cream), after a day uncovered in the fridge it was basically hard, and had that “brittle snap” when you’d cut into it. So isn’t this the same thing? I guess I’m confused as to why temper it at all. And in which cases you’d need to temper it.
Indeed, refrigerating untempered chocolate does make it brittle, for at around 37 degrees even unstable fat crystals will be quite rigid. The whole point of tempered chocolate is to create and retain that brittle texture at room temperature. Without it, chocolate-coated candies like truffles wouldn’t hold their shape (or have a shine) in the candy case. Certainly, tempered chocolate is an optional extra for a pastry like an Opera cake. It’s not strictly necessary, and indeed many Opera cake recipes leave it out entirely. However it is a nice-to-have in that it provides a textural counterpoint to the cake, which is almost totally soft and velvety otherwise. The crisp edges and subtle luster of a tempered top also make a nice presentation.
Tempering also helps prevent the gray streaks that can appear on a large expanse of re-hardened chocolate, which is a result of what’s called “fat bloom” (basically cocoa butter leeching out of the chocolate and crystallizing). So, in answer to the last question, tempering is usually desirable in any pastry or candy application that features an exposed, pure chocolate surface.