The six crystal forms of cocoa butter

Cocoa butter's triglycerides can pack into six distinct crystal lattices, conventionally numbered I to VI. They differ in melting point, density, stability and appearance.

Form I melts around 17°C and forms fastest. Form V melts around 34°C, just below body temperature. Form VI is the most stable of all, melting around 36°C, and it is what bloomed chocolate has become.

Form V melts at almost exactly body temperature, which produces the clean melt that defines good chocolate. It is dense, so it contracts on setting — which is what releases a moulded piece from its mould. It gives gloss and a hard snap.

The difficulty is that Form V does not form readily on its own. Cooled uncontrolled, cocoa butter forms the unstable lower numbers first because they nucleate faster. Tempering is the procedure for seeding Form V specifically and letting it dominate.

Chocolate cooled without tempering sets soft, dull and streaky, with no snap, and it will not release from a mould. Over days to weeks the unstable forms convert toward the more stable ones, and that conversion pushes fat to the surface — which is fat bloom, the grey film on old chocolate.

So bloom is not spoilage and it is not mould. It is a polymorph transition finishing what an incomplete temper started.

Every tempering technique — seeding, tabling, machine tempering — does the same three things: melt everything to erase all existing crystals, cool to form crystals including Form V, then rewarm just enough to melt the unstable lower forms while leaving Form V intact.

That is why the temperature windows are so narrow and why an accurate thermometer is not optional. You are working in the two-degree gap between melting Form IV and melting Form V.