What Are Cocoa Butter Polymorphs?
The short answer
Cocoa butter can crystallize in six different forms, numbered I to VI. They differ in how tightly the fat molecules pack together and in the temperature at which each one melts. Form V is the one tempering aims for, because it gives chocolate its gloss, its snap and a clean melt in the mouth.
How it works
Cocoa butter is not one fat but a mix of them, and as it cools those fats can line up in several crystal structures. The lower-numbered forms are less stable. They melt at lower temperatures and, given time, shift into more stable forms. Chocolate that sets in them looks dull and feels soft.
Form V packs densely and evenly. That tight packing is why well-tempered chocolate shines and breaks cleanly.
Form VI is more stable still, but it does not normally form directly from melted chocolate. It appears slowly in stored chocolate and is linked to fat bloom, though chocolate can shift to form VI without any visible bloom. So the most stable form is not the one you want; the target is the one just below it.
You will sometimes see the forms named with Greek letters instead of numbers. Some sources use one system, some the other.
Where you’ll see it
In explanations of tempering, in food-science papers on chocolate, and in the technical notes for tempering machines and tempering aids such as cocoa butter silk.
Related terms
Tempering is the practical side of this science. Bloom is what happens when crystals change in storage. Seeding works by adding form V crystals from already tempered chocolate.