Why exactly does “fat bloom” happen when melted chocolate re-solidifies? So asks reader Wendi. That’s a great question and it all has to do with cocoa butter crystals. Chocolate is full of them, at least when it’s in solid form. That’s partly a factor of cocoa butter’s composition, which is unusually uniform. It’s made up of just three different types of fats (as compared to butter which can have dozens). The reason that uniformity makes cocoa butter so crystal-prone is because similar molecules tend to stack up on one another like LEGOs under the right conditions, forming solid masses.
But here’s the rub: depending on the way in which melted cocoa butter is allowed to cool and harden, there are six different types of crystals that those three different fats can form. Only two are stable and uniform and the rest are comparatively unstable and random. Tricking the fat molecules into forming only (or at least mostly) the stable, uniform kind is what tempering is all about.
But I digress. Depending on how you’re going to use the chocolate, random crystallization can be a problem. For reasons that are probably obvious at this point, random crystals have a softer texture than uniform ones, so there’s less “snap” in un-tempered chocolate. Also it’s duller in appearance because all those random formations don’t bounce light rays back as efficiently. But worst of all are unsightly grey streaks which happen when molecules of cocoa butter get squeezed out of the main mass (another side-effect of random crystallization) and crystallize haphazardly on the surface. This is why, if you’re planning to use melted chocolate as a coating of some sort, it should be tempered, since you never know when those crystals will suddenly appear during cooling and ruin a lovely finish.
Unless you’re using an inexpensive coating chocolate. In which case you have nothing to worry about from a crystallization standpoint. And why is that? Because while cheap coating chocolate has cocoa solids in it, it has little if any cocoa butter. That’s what makes it cheap, because cocoa butter is the most expensive component of the cacao bean. No cocoa butter means no rampant crystallization and a consistent finish. You just melt the stuff, apply it and go on your merry way. As I argued in the post below, that’s great in many cases — like holiday cookies.