Welcome back to What even IS Coffee? This time we’re tackling one of the most dramatic steps in the process of getting coffee from the plant to your cup – roasting. (Strap in y’all, this one is a doozy!)

Roasting coffee is both hilariously simple and endlessly complex. Turn green seeds brown! Make good flavor!

But we all know that nothing that appears that simple is actually that simple. Roasting coffee involves transformation to a fascinating degree.

We start with the green coffee– picked, processed, cleaned and dried, then add heat and energy. This evaporates the last of the water out, and the seed becomes soft enough to grind. The cellular structure becomes more brittle, making flavor compounds accessible to the brewing water.

That’s the short version, but let’s start at the beginning.

As we discussed in our last few posts, once the green coffee seed is freed from the fruit, it’s dried to a moisture content of about 10% before it’s bagged for shipment across oceans.

Once the coffee is exported and imported, it’s brokered to coffee roasters. There are a handful of ways this works, but the bare bones is that roasters either pre-order green coffee before it’s even off the boats, or they buy direct from the warehouses.

When the roasters have the green coffee in their hands, they determine the roast profile and make blending and labeling choices. Roast profile is basically a map of how a roast is supposed to go – all on a graph with time and temperature axes. (Bet you didn’t know that coffee roasters need algebra!)

Different roast curves will bring out different flavors in the coffee – it’s much more complicated than just x temperature for y time! It’s x temperature plotted over y time for the entire duration of the roast, and variations at any point can and will affect the final flavor profile. (It is possible to chart your curve on an old-fashioned graph paper, but most roasters these days opt for software to plot the curves during the roast since it allows collection of more types of data.)

There are a few different styles of coffee roasting machines – the most common are drum roasting and fluid bed or air roasting. Think of the difference between cooking in a cast iron skillet and in an air fryer – One cooks with hot air, and the other cooks with a hot surface. Here at Three Keys, we use a drum roaster for several reasons.

Some things are definitely better cooked one way over the other. If you have a steak that you want a good sear on the outside, you’re not going to try to achieve that in an air fryer. It won’t work. But if you have, say, brussels sprouts, you could cook those well in either appliance. 

The benefit of the cast iron is that you can use it to cook just about anything from eggs to ribeye steak to broccolini, and that’s why we like the drum roaster here at Three Keys Coffee. We find that we have a lot of flexibility with profiling in the drum roaster, which allows us to offer a wide range of roast styles and degrees.

Regardless of the style of roasting machine, after the green coffee is received and the orders for the day are in, the coffee is weighed out into batch sizes. The roaster is turned on and preheated to about 400 degrees Fahrenheit before roasting begins.

Remember the aforementioned moisture content? That is super important because coffee seeds are mostly cellulose– glorified wood chips.

Imagine dumping a bucket full of wood chips on a fire. Your goal is to get the wood chips hot, and to start them changing color, but you don’t want them to also start on fire. The wetter the wood chip, the easier it is to not catch them on fire, right? But you also don’t want your wood chips to start molding while they’re in wood chip storage. So you need the wood chips to be dry enough not to mold, but not too dry.

So phase one of roasting is drying. When roasters plot the temperature curve of their roasts, it starts off with a precipitous drop. The temperature of the roasting machine is brought way down by the addition of the room temperature green coffee.

First, the coffee and the roaster must reach a temperature equilibrium. This is called the point of change– where the temperature stops dropping, steadies, and starts to rise again. Then the coffee finishes drying out the last 10% or so of moisture. Then the cellulose and other components start to undergo their chemical reactions. 

The coffee smoothly progresses from green to yellow, from yellow to tan, and then something cool starts to happen. The remaining moisture inside of the seed reaches a critical point of vaporization and can no longer be contained by the seed. That’s called first crack. The coffee, light brown at this point, starts to crackle inside the roaster. It’s quieter than popcorn, but louder than, say, breakfast cereal. 

This phase– after first crack, before the end of the roast– is the start of development phase. This is the phase where lots of good stuff starts happening. Organic acids and sugars start to transform with caramelization and maillard reactions. If everything that happens before first crack is a warm-up, development is when the roast comes into its own.

After first crack and into development, the coffee can go from light to medium to medium dark. Somewhere around medium dark, depending on the coffee, the seeds will start to crack once again. Second crack is quieter and less dramatic than first crack.

First crack is the release of water vapors, whereas second crack is the release of CO2 gas that has built up in the heat. Many roasters won’t push all the way to second crack, and definitely not beyond it. Beyond second crack is when the coffee starts to actively turn to carbon. (Then you have coffee that you can write on the sidewalk with.)

From there, the coffee is poured out of the hot roaster drum into a tray where it’s cooled before being bagged and sent to your front door.

Once you start to understand a bit about roasting coffee, you’ll see why we find a similar love for coffee and jazz music. Profiling the coffee is like learning a variation on a new theme, and performing the roast day after day and adapting to changes to keep it tasting the best is exactly what every musician with a daily practice does.

Roasting coffee and jazz music are both all about exploration and experimentation informed by practice and theory. And that's the music in your cup.