My mailbox is often a little too eager to delete messages, so I don’t have the text of an excellent question that was sent in on Wednesday. It was basically this: why, once chocolate chips are baked (as in chocolate chip cookies) do they not firm up again completely? Logic seems to dictate, after all, that once a chocolate chip cookie has come completely back to room temperature, the chips inside them — even though they may have lost some of their shape — should return to their original firm texture. But not so. They stay soft-ish. Why?
The answer has to do with cocoa fat molecules, and the crystal structures they form at certain temperatures. Wait, Joe, you mean to say that fat molecules form crystals??? In fact they do. All sorts of edible molecules form crystals: sugars, salts, starches, fats, even proteins. It all depends on whether conditions are such that similarly-shaped molecules can begin stacking up on one another into uniform, 3-dimensional shapes (which are essentially what crystals are).
Cocoa butter molecules do this in a very narrow temperature range, between about 88 to 90 degrees Fahrenheit. Melt a mass of chocolate, allow it to cool to 88 degrees and hold it there, and a very dense and uniform crystalline structure forms. This process, if you haven’t guessed by now, is called “tempering”, and it’s the same process metallurgists use to strengthen materials like steel, for when all the molecules in a given medium are essentially locked together, that medium takes on a decidedly hard character. Tempered chocolate (and all chocolate makers temper their chocolate when they make it) is not only harder than untempered chocolate, it’s shinier, since the uniform cocoa butter crystals are better at reflecting light rays.
So what happens when you heat up a tempered chocolate chip in the oven? As it heats past about 120 degrees Fahrenheit, any crystal structures it once contained are destroyed. Then, when the chip is removed from the oven it begins to cool. Once it gets under 120, crystals once again start to form, but they’re random and disordered. Sure, eventually the temperature passes through the “good” crystal range, but it doesn’t stay there long enough for uniform crystals to form in any great numbers. So the chip eventually reaches room temperature, its cocoa butter a riot of random molecules, with no shine and no “snap”.
Pretty cool, eh?