Photograph by Brian Finke, Getty Images/Nat Geo Image Collection
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Final touches are put on chocolate pastries. Bayonne, France.
Photograph by Brian Finke, Getty Images/Nat Geo Image Collection
The Plate

Ultrasound Detects Chocolate at its Peak

To pinpoint the confection's perfect moment of shine, melt and snap, scientists are turning to ultrasound technology.

Medically, an ultrasound scan uses high-frequency sound waves to generate state-of-the-art images of the body’s internal organs—or, if you happen to be pregnant, to track the developing fetus and tell you whether to expect a boy or a girl. It turns out it’s also a potentially invaluable tool for monitoring the quality of your chocolate bar.

Chocolate is intimidatingly complex stuff, containing up to 600 different chemical compounds—three times more, chocolatiers like to point out, than red wine. Many of these contribute to chocolate’s richly varied flavor. Other compounds like serotonin, the “happiness hormone,” and phenylethylamine, the “love drug,” are mood-enhancers. That’s why indulging in a bag of chocolate truffles gives us a lift.

However, it’s a long and difficult path from cacao bean to chocolate truffle.

Cacao beans, fresh out of the pod, are both bitter and blah; to transmogrify these unpromising tidbits into chocolate treats, they must first be fermented, dried, roasted, ground, and refined­. And even then, you’re not out of the woods.

Cocoa butter, the delicious-sounding (hey, butter) major fat in chocolate, suffers from polymorphism, the chemical equivalent of multiple-personality disorder. Depending on the temperature, cocoa butter crystallizes into any one of six different forms, only one of which (Form V) is desirable, generating just the right degree of shininess, melt-in-your-mouth yumminess, and satisfying snap when you break it that high-quality chocolate makers require.

The temperature-controlled process by which cocoa butter is nudged to convert itself into Form V crystals is known as tempering. This involves melting the chocolate such as to liquefy the cocoa butter, then slowly cooling and re-heating until the proper crystallization state is attained. Improperly tempered chocolate is too soft, too crumbly, or too readily meltable to be suitable for use in commercial chocolates.

To date, the only way of monitoring the crystallization state of a developing batch of chocolate has been to snatch a sample from the production line and send it to a laboratory for analysis. During the prolonged analytical process, however, the simmering chocolate can slide into forbidden crystal territory, deteriorating to the point where it either has to be tossed out or, at the very least, re-melted and processed all over again.

With ultrasound scans, however, such catastrophes may soon be a thing of the past. Belgian researchers Imogen Foubert and Koen Van Den Abeele of Belgium’s University of Leuven (KU Leuven) have discovered that rapid, on-the-spot analysis using ultrasound can detect the precise crystallization state of a given batch of chocolate. When the cocoa butter is liquid, the researchers explain, high-frequency sound waves directed at it all bounce back at the observer; as crystals begin to form in the mix, however, increasing proportions of the waves are absorbed, the amount varying with the type and packing state of the crystals. The researchers hope this discovery will lead to the production of factory-grade, ultrasound-based, chocolate-testing devices, capable of pinpointing cocoa butter’s transition to delectable Form V. With ultrasound analysis, there’s a good chance that no future batch of chocolate will be left behind.

That said, even desirable Form V crystals don’t last forever, which is why the shelf life of chocolate is limited. Form V inexorably moves on to Form VI, a transition which takes place slowly at room temperature but more rapidly when chocolate is exposed to heat. The result is a dull, soft, mottled-looking chocolate that no longer melts readily in the mouth. You can test this for yourself by leaving a chocolate bar in a hot car, then allowing it to re-solidify. You won’t be happy.

Ideally, chocolate experts recommend, chocolate, like wine, be stored at temperatures between 59 and 64 degrees Fahrenheit (15-18 C).

Or you can just eat it all immediately, to be on the safe side.