Why Old Chocolate Turns White, and How Chocolatiers Stop It

Snap a fresh square off a good bar of chocolate and it breaks with a clean, crisp click. The surface is glossy enough to show a reflection of the kitchen light, and the piece melts smoothly on the tongue. Then one day you find a bar that has spent a few months at the back of the cupboard, and it has gone dull, with a pale, dusty, grey-white coat over the top.

That coat has a name: chocolatiers call it bloom. The interesting part is that nothing new has been added to the bar and nothing has been taken away. The chocolate is made of the same ingredients it always was. What has changed is the way its fat is arranged.

What people often get wrong

A white film on food looks like mould, so many people assume the bar has gone off. In this case it is the chocolate's own fat. Researchers at the University of Guelph, writing in Nature Communications in 2021, describe fat bloom as a greyish white film on the surface that comes from an uncontrolled change in the way cocoa butter crystallises. The bar has not rotted. Its fat has slowly rearranged itself into a different crystal.

Why the same fat can set in six ways

Cocoa butter, the fat that gives chocolate its melt, is unusual. When it hardens, its molecules can stack into at least six different crystal structures, known to chocolate scientists as Forms I to VI. Each one has its own melting point and its own feel. The low forms are soft and crumbly. Form V is the one everyone wants: it is shiny, it snaps when broken and it melts in the mouth. Form VI is harder and melts too slowly to be pleasant, and given several months, Form V slowly turns into it. That slow change is what leaves the dull, dusty coat.

Warmth helps the change along. On a hot day the fat can partly melt and rise to the surface before it sets again, which is why a bar that has sat in a sunny window often blooms. Cool storage slows the change down, because the molecules have less energy to shuffle into a new arrangement.

How tempering makes the right crystal

Tempering is the chocolatier's answer. As the Exploratorium in San Francisco explains it, tempering is simply a way of melting and cooling chocolate so that the cocoa butter crystals end up more uniform, which is what gives the shine and the snap. The chocolate is melted, cooled so that crystals start to form, then warmed again to just below the melting point of Form V. That melts away the softer forms but leaves the Form V crystals behind, and when the chocolate cools for the last time, the rest of the fat follows their pattern.

• 6 crystal forms of cocoa butter, at least
• 0.1% phospholipid added in the 2021 study
• 20 °C the temperature the chocolate was cooled to

How we know it can be made easier

Because tempering is fussy, scientists keep looking for shortcuts. In the 2021 study, Jay Chen, Alejandro Marangoni and their colleagues added a tiny amount, 0.1% by weight, of certain fat-like molecules called phospholipids to melted chocolate and cooled it quickly to 20 °C. The additive pushed the cocoa butter towards Form V, and the finished chocolate had a gloss and strength comparable to a commercial tempered bar. So the next time a bar comes out of the cupboard looking grey, remember that it is still the same chocolate, made of the same things, only with its fat sitting in a different crystal.