X-Rays Reveal Why Old Chocolate Turns White

Liquid fat turns out to be the culprit

This chocolate may look fine, but it’s just a few steps away from disaster Lee Frost/Robert Harding World Imagery/Corbis

The white bloom marring the face of a chocolate bar hidden away for a bit too long, thankfully, doesn’t mean the candy can’t be consumed. But it does seem to change the flavor. Now researchers have figured out exactly what happens to compromised chocolate, and are offering hints on how to prevent the problem in the first place.

The white splotches and streaks on old chocolates are appetizingly called a fat bloom. Fat blooms happens when liquid fats from the cacao bean move to the candy’s surface, reports Emily DeMarco for Science. To figure out exactly how this process happens, a team of researchers captured this process in real time using high-powered x-rays to take a close look at chocolate’s crystal structure. First, the team mixed up chocolate ingredients — cocoa, milk powder, cocoa butter and sugar. Then, they added oil and watched with the help of an x-ray generating synchrotron.

Immediately, the liquid fat trickled through the pores of the chocolate sample. Then they watched (in horror, one assumes) as the oil melted the precious chocolate crystals over several hours. They reported their findings in the journal Applied Materials & Interfaces. With the crystals out of the way, the fat in the cocoa butter could also move. Fat that made it to the surface recrystallized. The innocent, sweet confection was rendered unsightly with this scrambling of its crystalline structure. Instead of glossy, fresh-looking chocolate, the surface became pitted and discolored and the interior was irrevocably altered.

The research team included experts from Nestlé  based in Germany, the U.K. and Switzerland. And now that they know exactly what those white streaks are, and how they're formed, it's possible chocolatiers​ could work to fight the fat blooms. DeMarco writes:

The study suggests that reducing the porosity of chocolate when it’s being made could help stem the appearance of the off-putting bloom and improve the overall quality of chocolate.

Humidity and temperature changes hasten bloom by helping the oil move. So try to keep secret stashes relatively dry and cool. Demarco adds, "Eighteen degrees Celsius [about 64 degrees Fahrenheit], it turns out, is the sweet spot."

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