What does the perfect cup of coffee sound like?

    Listen
    These freshly roasted beans were pea green just moments prior

    These freshly roasted beans were pea green just moments prior

    Tiny popping sounds are a roaster’s secret road map.

    Do you have a Pavlovian response to the sound of coffee beans being ground? If you answered “yes,” you aren’t alone. According to an online survey conducted last year by the National Coffee Association, 83 percent of Americans drink coffee to the tune of 587 million cups a day. That’s a lot of joe, and a lot of grinding and steaming milk and percolating.

    But there’s another sound that occurs in creating your next cup of coffee that’s crucial to its taste and virtually unknown to the millions who depend on it to start their days. It’s called the Maillard Reaction and sounds like a bowl of Rice Crispies, but this brief chorus of popping midway through a coffee roast is a crucial link in giving your morning ritual its distict flavor.

    Out on the Brooklyn waterfront in New York City, there’s a picturesque brick building called the Liberty Warehouse. It dates back to the 1850s, and has huge iron shutters and doors. Inside door number 324, you’ll find Ed Kaufmann. He’s the Roasting Director for Joe Coffee, a small chain in New York and Philadelphia. And this is where he roasts his beans. It’s been a busy morning.

    “I roasted a few thousand pounds of coffee for our stores,” he half yells over the sound of whirling fans and industrial machines.

    Rows and rows of burlap sacks sit in the warehouse, stamped with names like “Finca Cielito” and “Finca El Suspiro.” They’re loaded with raw green coffee beans, from Nicaragua, Honduras, and Brazil, and they’re waiting for Ed and other roasters to turn them into the stuff we like to drink.

    “When they arrive to me here, they’re green, and they don’t really taste very much like coffee,” Ed explains. “So this allows me to brown the sugars and roast the coffee to where it’s the color we all know, and so I can brew it.”

    Ed roasts coffee on big machines he describes as “the child of a pizza oven and a clothes dryer.” It takes 9 to 13 minutes at 400 degrees Fahrenheit. And now that he’s got the day’s heavy-duty roasting done, he can play around, experimenting with different roasting techniques—more heat, less heat, faster, slower—and try new coffees before buying thousands of pounds.

    “This coffee’s from Mexico,” he says pointing to a bowl of green coffee beans. “We’ll taste it tomorrow, and it’s basically waiting for us on the border of Texas. And as soon as we taste it, and we say we love it, then they send it to New Jersey. If we don’t love it… they dump it back in the ocean.”

    He pauses and smiles. “No I’m just kidding. They don’t do that.”

    Ed works on a sample roaster to test out small batches of new coffee. Picture a tiny clothes dryer, spinning beans over a gas flame. It’s open in front, so he can pull out a spoonful of coffee to inspect it.

    “At the very beginning of the roast I use mostly my eyes to see how the water in the beans is evaporating,” he says, sniffing small samples of the beans a few times every minute. “I can smell if I’m going too fast or too slow, but I also start to smell things I’m going to smell and taste in the final cup of coffee. You know it smells like dry grass, wet grass, baking bread, and then eventually, starts smelling like coffee.”

    But surprisingly, roasters like Ed don’t just use their eyes, noses, and watches to determine how close to ready their batch of coffee is—they use their ears, too. They actually listen to their beans.

    Ed’s listening for a chorus of tiny pops to iminate from the roaster. The pops are a release of pressure that have built up in the cell walls of the heated beans. It’s referred to as “first crack,” which starts seven or eight minutes into the roast. Up to this point, the beans have been absorbing heat from the fire, but at first crack they begin to roast themselves with heat from internal chemical reactions.

    “There’s the Maillard reaction,” says Ed as first crack begins. “There are breaking and making of complex polysaccharide sugar chains, and caramelizing of those polysaccharides. That’s just a few of hundreds.”

    Then, just after first crack and that cascade of chemical reactions, Ed stops the roast. Listening to what the beans are telling him—the rate of cracking, the intensity of each pop—Ed says that helps him gauge how the roast is going.

    “First crack is such a monumental part of the roast, and it can either make or break the coffee,” he explains. “If you go into first crack with too much momentum, you’re gonna really cook out nuance and acidity and some of the aromatic compounds that you’re gonna wanna smell and taste in your mug of coffee. On the other side if you go in with not enough momentum, you’re gonna leave a lot of plant matter in there, you’re gonna end up with green, tart, cereal, grassy flavors that aren’t absolutely delicious in the cup, when you know they’re there.”

    On the small sample roasters, Ed can listen to the momentum of first crack. “You know, those are my headlights when I’m driving in the night. I use that information a lot,” he explains. But on the big machines, where Ed roasts thousands of pounds a day, first crack is not audible. Ed says the new machines he uses today are especially insulated—more so than ones he’s used in the past—so that faint popping sound is masked by blowing fans and tumbling beans. He says that even on industrial machines where you can hear the beans, the sound coming out is inconsistent. And there’s the problem. Ed can’t hear what’s going on with his beans. He just relies on read outs of time and temperature instead.

    That’s where Preston Wilson comes in. He’s a soft-spoken acoustician at the University of Texas at Austin, where he studies how sound travels in the ocean—stuff the Navy’s interested in, and the oil and gas industry. But Preston also happens to be an avid home coffee roaster and fancies himself an aficionado of sorts.

    “I got interested in the sounds that are made when coffee beans are roasting,” he says, “and just a few months ago, I actually decided to record them and analyze those sounds and have a look at them quantitatively.”

    Preston published his research in the Journal of the Acoustical Society of America. His paper analyzes the sound frequency of each little “pop.” And how the cracking rate changes throughout the roast. Think of it like popcorn—it starts out slow with just a few beans, that escalates to a chorus of popping, and then it finishes with just a few stragglers at the end. Preston charted that progression.

    He also analyzed what happens later in the roast, like if you bring the beans to French roast darkness. It’s called “second crack,” and happens four to five minutes after first crack. Second crack is a higher-frequency sound. People compare it to rice crispies. “Physically what’s happening is the brittle cell walls are cracking and fracturing, they’re just busting apart, Ed says, adding that he usually doesn’t take his beans all the way to second crack.

    Preston envisions using the measurements he’s made to automate the roast process, or to give roasters like Ed a window into what’s going on inside the roaster, by plotting the crack rate on a computer screen.

    “You can imagine a little light coming on on the roaster,” he muses, “it’s like, okay, first crack has begun, yellow light turns on, first crack is getting to the end, maybe a little bit more red colored light comes on, then second crack happens and lights and alarms start going off.”

    Nice idea, but would professional roasters actually find this useful? Ed thinks so.

    “This is like, huge,” he says when asked what he thinks about Preston’s vision, “this is monumental, this is the next thing.”

    Here’s an an exchange between Ed and Preston:

    Ed: “I have to tell you Preston, that, as a roaster operator, somebody who does this on a daily basis, basically, I feel like that information could be more valuable than any temperature readout. Um…when can you be here?”

    Preston: [Laughing] “That’s the missing link, you know. I hadn’t talked to anybody in the industry other than kind of the home-roasting friends that I know. And we all thought it was an interesting thing. And, you know, it’s another knob to turn and to observe while you’re roasting.”

    Ed: “Yeah, we use a lot of other information the same way, but it’s mostly temperature and time. But we’ve never used actual sound. And now that this concept is rattling around in my brain, it’s kind of amazing that we haven’t been using this the whole time. Because it’s kind of what we’re missing in the middle. It’s the everything, almost. I feel like you should really talk to a couple people I know. I think your research is gonna be useful in my field.”

    Preston: “Wow, that’s great I’m glad to hear that.”

    While Ed’s people are talking to Preston’s people, there’s still one line of new research Preston can pursue. The perfect latte…

    “If you steam milk, you know that there are certain sounds associated with that, too,” says Preston. So I could imagine we could do some analysis there. The sound of the steam, and then the way the milk sounds when you tap the cup. Maybe that’s a whole other radio show for another time, once that research is done.”

    As they always say, more research is needed…even for coffee.

    Want a digest of WHYY’s programs, events & stories? Sign up for our weekly newsletter.

    It will take 126,000 members this year for great news and programs to thrive. Help us get to 100% of the goal.