Sudden Coffee

The Science Of Coffee Roasting

Every mug of coffee has a rich history. There are dozens of stages in the coffee production journey, and often hundreds of people play roles in getting the coffee beans (or instant powder) to you.

We sat down with our VP of Operations, Leslie Mah, to talk about one of these stages that’s often misunderstood: coffee roasting.

Before joining Sudden, Leslie was the head roaster of Ritual Coffee for three years, placed 2nd in the 2016 US Tasters Cup, and won 1st in the 2017 Roastmaster Challenge.

Her insight into the complex and fascinating world of coffee roasting isn’t just interesting for science nerds—it can help anyone understand how to pick coffees that are right for them.

Why Coffee Even Needs To Be Roasted

As you likely already know, coffee beans are the seeds of a small fruit that looks somewhat like a cranberry. These small fruits (often called ‘cherries’ in the coffee world) are harvested when they’re ripe, the fruity layers are stripped away, and the seeds go through a couple processing stages that slowly dry them.

But those seeds aren’t yet ready to be brewed.

“There's just a ton of different amino acids and organics and there's nothing that’s very tasty. It tastes like what you may imagine the color green to taste like. Like, if you just ripped the grass from the ground and you just started chewing it.

Coffee beans are the product of a plant, so it shouldn’t be so surprising that they taste like plants by default. But it’s not just about the flavor—those seeds can be hard like small pebbles.

“So the reason we roast coffee is because it doesn't taste good yet when it's green. And the second reason why we roast coffee is because we want to break down that structure and make it more brittle so that we can actually grind it and brew it up.”

So how does roasting actually accomplish these two goals?

The Three Stages Of Roasting

Once these unroasted coffee beans (often called ‘green coffee’) are shipped to the United States, they are received by a coffee roaster. Now, a roaster is actually two things:

  • The person who roasts the coffee
  • The machinery that roasts the coffee

“I like to tell people that most coffee roasters—the machines—look like if you took your clothes dryer and just made it out of straight metal. Essentially, what you're doing is just adding heat and then making the beans tumble around in some form or fashion.

This doesn’t just heat the beans up. It actually results in hundreds of chemical and physical reactions in under fifteen minutes. And these reactions are what produce the flavors we love in coffee.

The Drying Phase

When the green coffee is first added to the hot roaster, it enters an initial ‘drying phase’ where much of the moisture is sucked out of the beans from the heat. As a result, the green hue of the beans fades away into an off-white.

Leslie compares this phase to the early stages of sauteing meat. You cannot get a good sear when the meat has a lot of moisture still in it. The water will keep the pan cool, which will cook the meat without that flavorful sear. In the same way, the beans need to be dried for them to be able to brown.

The Yellowing/Browning Phase

As the beans start to heat up rapidly, they start to experience rapid chemical reactions in form of caramelization and Maillard reactions.

Maillard Reaction — A chemical reaction between amino acids and sugars that gives browned food its distinct flavor.

“At this point, sugars and acids are mixing and changing and figuring out, well ‘what are we going to do with each other to create new exciting compounds’ that will be the eventual flavors you taste in the coffee.”

The beans are also beginning to expand quickly.

“It's kind of like if you had a deflated balloon inside of inside a box, and the box fit the deflated balloon perfectly. And then you started inflating the balloon. That box would expand slowly and then eventually burst.”

First Crack

And then comes the point where the beans have had enough. One of the walls of each bean actually bursts open from all the water vapor and creates an audible “crack” sound.

Like when you're at a party and the music is getting to that point. The beat is rising and then you're just waiting for the drop. The drop is the crack.

Leslie pauses and smiles.

“There will probably be some people in the world very upset about me comparing the roasting process to an EDM party. But you know. Essentially.”

At this point, immediately after the first crack, the coffee is considered a light roast. The roaster can then decide how dark they want to take the coffee, depending on their flavor goals for that bean.

Light, Medium, Or Dark?

Here’s a quick look at how these roast levels are different. You’ll be able to use this information to inform your coffee buying decisions and find coffees that are well-suited for you.

Light: Bright And Fruity

Light roast coffees tend to have vibrant fruity flavors and a crisp acidity. Those vegetal flavors have been transformed into notes of lemon, orange, or cherry—and the bright acids reinforce those fruity flavors.

I would say that a light roast will often be like a gin and tonic. It's like very light. It might be effervescent and bubbly and focus on those citrus elements.”

These coffees can be exotic, fascinating, great all on their own.

Medium: Caramelly And Rounded

Roasted a little longer, medium roasts generally have a more rounded flavor. Those bright acids are calmed down by the presence of sweeter notes like caramel and milk chocolate. That classic coffee aroma becomes strong and the mouthfeel of the brewed coffee gets a little heavier.

As you go into the medium roast, you’re kind of going towards a Manhattan, or maybe a Sidecar. You know, the flavor gets sweeter and has more of a caramel flavor.”

Umeko, our Head of Coffee, says that the fresh apple flavor of a light roast may seem more like a baked apple pie in a medium roast. Sometimes light spice notes like nutmeg or cinnamon form in medium roasts as well.

These coffees really go with anything. Dessert, breakfast, a gentle afternoon cup. The well-roundedness makes them versatile.

Dark: Chocolate, Nuts, And Spice

When beans are dark roasted, they take on a flavor profile that tends to bring out notes of chocolate, nuts, and spices. A gentle bitterness appears as well, creating a fascinating juxtaposition with the bean’s sweetness.

Dark roast beans also usually have a thin layer of oils attached to their surfaces.

Those oils on the beans will actually end up playing a factor in how it tastes in the cup because it will cause the coffee to be more velvety. It will kind of taste more oily and creamy.”

Because of the gentle bitterness, dark roast coffees are often perfect for sweet desserts. This is actually a classic combo that you can find around the world. We suggest trying a dark roast with a slice of German chocolate cake or a truffle.


Roasting is complex, sometimes confusing, and adds a lot of flavor diversity to coffee. Soon, we’ll lean into this fact by offering more than one coffee at a time. This will give you the chance to try multiple roast levels in Sudden form.

We hope you enjoy this upcoming flavor diversity and use it to find the roast that best fits your taste preferences.

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