By Harold McGee
Unstable cocoa butter crystals are crystals that melt relatively easily, which means at relatively cool temperatures, between about 59 and 82°F/15–28°C. The desirable stable crystals (sometimes referred to as “beta” or “beta prime” or “Form V” crystals) melt only at warmer temperatures, between 89 and 93°F/32–34°C. The temperature range in which a particular kind of crystal melts is also the range in which it forms as the chocolate cools. Unstable crystals therefore form when molten chocolate is cooled rapidly, so that the stable crystal types—the ones that begin to form at warmer temperatures—don’t have time to gather most of the fat molecules to themselves before the unstable crystals begin to form. Stable crystals predominate in melted chocolate when the cook carefully holds it at temperatures above the melting point of the unstable crystals, but below the melting point of the stable crystals. This tempering range is 88–90°F/31–32°C for dark chocolate, somewhat lower for milk and white chocolates thanks to their mixture of cocoa and milk fats.