Coffee beans sing distinct tune

Crackling sounds could be key to automated acoustic roasters

CUP OF JOE  As they roast, coffee beans emit a chorus of crackles that begins with low, loud noises and ends with quick snaps that sound like Rice Krispies in milk.  

Preston S. Wilson/UT-Austin

Listen to the audio

The snap-crackle-pop of coffee beans could tell automatic roasters when to turn down the heat.

Hot beans sing a distinct ditty that reveals their stage in the roasting process, mechanical engineer Preston Wilson of the University of Texas at Austin reports in the June Journal of the Acoustical Society of America.

Java roasters know to listen for some basic sounds, commonly called “first crack” and “second crack,” but until now no one had measured and analyzed these noises, Wilson says. He roasted a small batch of green coffee beans — an espresso blend — in an electrically heated drum roaster and recorded the crackling sounds as the beans got toasty.

The first crack noises, which sound like popping corn, ring out between 400 and 600 seconds after roasting begins. Those crackles are louder, deeper and less frequent than the second chorus of cracks at 620 to 730 seconds, which snap rapidly like Rice Krispies in milk.

Measuring the beans’ sound signatures is a first step towards making an automatic acoustic roaster, Wilson suggests. If engineers can design a device that listens to and evaluates roasting beans’ chatter, people might one day be able to wake up and hear their coffee. 

After about 400 seconds of roasting, coffee beans emit a chorus of crackles that sounds something like popping corn and lasts around 200 seconds.

Coffee beans’ second song plays from about 620 to 730 seconds after roasting begins and sounds like the quick, soft crackling of Rice Krispies in milk.

Meghan Rosen is a staff writer who reports on the life sciences for Science News. She earned a Ph.D. in biochemistry and molecular biology with an emphasis in biotechnology from the University of California, Davis, and later graduated from the science communication program at UC Santa Cruz.

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