What Actually Happens During Coffee Roasting?

updated May 1, 2019
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(Image credit: Kris Krug)

While the coffee beans you buy and make your morning cup with are brown, they don’t start out that way.

I’ve had many a person in the coffee industry tell me how shocked they are to realize how many people think coffee beans are just picked off the coffee tree as-is. So we’re here today to break down that myth, in case there are any of you who think someone just picks the beans and makes a cup.

(Image credit: Matt Biddulph)

How Coffee Gets from Bean to Cup: The Short Story

Coffee starts out as coffee cherries — the bean is the seed of this fruit that grows on coffee trees. To get it into bean form, coffee cherries are picked, processed (which can happen in a variety of ways), and dried.

Once dried, coffee beans are then hulled, graded, and sorted. At this point it’s called “green coffee,” and it’s ready to be shipped from origin countries to the places where it will eventually be roasted. Approximately seven-million tons of green coffee is produced worldwide each year.

Roasting is what transforms that green coffee into the aromatic beans that we are used to. Simple as that. If you want the easiest explanation of coffee roasting, it’s that you take the green beans and turn them into brown ones.

(Image credit: Kristian Mollenborg)

The Science (and Art) of Coffee Roasting

Of course, the process is a little more complex than that. Coffee roasting involves a whole lot of science, and if you’re up for some reading on decarboxylation, fractionization, and isomerization, I’d recommend this article: Basic Chemical Reactions Occurring in the Roasting Process.

Roasting coffee is both technical and an art. As Jim Kelso, head of quality control at Stumptown Coffee once told me, “The best roasters I know operate according to their senses and use data and tech only as a backup to their instincts.”

(Image credit: Matt Biddulph)

What Actually Happens When Coffee Beans Are Roasted

When a bean is roasted, the chemical and physical properties of the green coffee beans are transformed, giving us the aromatic and flavorful bean that we then grind and make a drink out of. Try to make a drink out of green coffee and you’ll probably be disappointed.

Roasters have to take a lot into consideration when roasting beans — everything from density to the altitude that the coffee was grown at to the ambient air temperature of the roasting facility.

And while roasting is an essential part of the process, it really all starts with the bean itself. “In the roasting process there are ways to increase/decrease acidity. perceived sweetness, and mouthfeel, but really the most important thing is sourcing great coffee,” says Ron Heathman of MadCap Coffee, a roaster in Grand Rapids, Michigan. You can ruin a good-quality bean, but you can’t magically make a bad-quality bean great. “We are not alchemists. I can’t put garbage in the roaster and get gold out,” says Heathman.

That’s why coffee buyers, as well as roasters, spend a lot of time cupping and selecting their beans. “The roasters whose coffee I admire all cup a lot of coffee,” says Heathman.

During the roasting process, the beans are kept moving, so they don’t burn. As the beans roast, the internal temperature of the beans changes, and as it does, so does the color. Sweet Maria’s has an excellent visual breakdown of the changes in color of beans during the roasting process.

While you may be used to four general descriptors for coffee roasts — light, medium, medium-dark, or dark — which are defined by the beans’ roasting temperatures, specialty coffee roasters talk about roasting profiles.

Roasting profiles, essentially a collection of all kinds of data, are what affect the ultimate flavor profiles of a coffee. In other words, you need to roast well to get a good-tasting coffee. When a roaster gets a new coffee in, it may take a few rounds of roasting before getting the roast right. “We log and cup not only each type of coffee we roast but each batch. It is a labor-intensive process but we love it and it really is the only way to get feedback and know if a particular coffee profile needs adjusting,” says Heathman.

In the grocery store you may find a “light” or “dark” roast, but you’ll be hard-pressed to find this language to describe coffees at specialty coffee roasters. They work with what they know about the bean to begin with and go from there to figure out the exact roasting profile that will work the best for that particular bean. “Most specialty roasters have a jumping-off place for each coffee they approach,” says Heathman. “I have heard some roasters try to make it into rocket science. It’s not. It just a little regular science and some heavy lifting.”

So what’s the “perfect roast”? That’s up to personal preference. Really, it’s less about the roast and more about the coffee itself. As Stumptown writes on their website, “We don’t like to impart a fingerprint or a roast signature on the coffee. Instead we prefer to highlight what makes that particular varietal or farm exciting … We’re here to bring out the best aspects of the coffee, and if we do our job correctly no one should even be thinking about the roast when they drink it.”

In other words, be sure to thank your roaster for a job well done.