Light or dark. How do you like it?
Often folks will talk about a coffee roast in simplified terms of light and dark. While this is a helpful clue for how a coffee may taste in the cup, the ultimate color of roasted coffee is only one indicator. In fact, HOW the coffee reaches a particular color is the bulk of the story.
How a coffee develops over time as heat is applied to the beans during a roast cycle is often referred to as its roast profile. This is the recorded observation of temperatures, times and sensory benchmarks throughout an entire roast. If the roasted coffee is a roastmaster's art, the profile is a time-lapse peek of the artist doing their work.
Most commercial roasting equipment has the ability to monitor and control certain variables throughout a roast cycle. For non-commercial roasters using smaller capacity equipment, controlling some of these variables is a little more difficult. Nonetheless, all coffee goes through the same stages until the roast is terminated by the roastmaster. Or there is a horrendous fire.
Below is an outline of the various stages of roasting. If you're using simplified roasting equipment (e.g. air popcorn popper tech), you may be zooming through these stages so quickly they are hard to recognize—but they all exist.
Stages of Roasting
Without digging into the science, it's important to note that bean chemistry, structure and characteristics are evolving throughout the roast cycle. Coffee doesn't simply roast when it begins to turn brown, it begins as soon as you introduce it to heat.
Green coffee that's ready for roasting typically begins with about a 10% - 12% moisture content. This is simply water trapped inside of the bean and just like any organic matter, no "roasting" or browning will take place while water is present. Because it takes a meaningful amount of energy to vaporize this water from dense coffee beans, it will take longer to roast a batch of coffee when your equipment is colder to start. This is not particularly noticeable with small, home roasters, but commercial equipment is typically preheated before the coffee charge is dropped into the roasting chamber. This is called the drop temperature.
Now the water is gone and the coffee is actually starting to roast—meaning, it is beginning to change chemically and physically. There are three aromatic inflection points in this yellowing process worth noting: Grass, Hay and Bread. (These aromatic features were originally noted and published by long time coffee expert, Willem Boot.)
At a bean temperature of somewhere around 200°F - 250°F the water in the coffee beans is converting to gas and the coffee is taking on a yellowing color and the aroma of freshly cut green grass.
At around 300°-ish, activity is picking up as sugars and amino acids interact in what is known as a Maillard reaction (beer makers understand the magic of Maillard reactions). Sugars begin to caramelize and the flavor and color transformation taking place within the beans trigger the look and notable aroma of hay.
As the Maillard reaction continues to transform the coffee color and composition, a rustic, brown color—like bread crust—will emerge. As the roasting gases spill into the air with some mild smoke, the aroma of freshly baking bread is unmistakable. This will typically take place around 330°F -350°F.
Again, during this "yellowing" process, the transformation is not simply visual, but aromatic. A great deal has been taking place deep within the coffee throughout this stage. This all culminates in the next stage: the well-known "first crack."
3. First Crack
For the first time in its evolution, the coffee beans exhibit audible activity as the beans begin to snap and crack; much like the sound of popping corn. This dramatic activity is sometimes thought of as the first major event taking place in coffee roasting, but this is a bit of a misunderstanding.
As noted above, significant transformation has been taking place within the coffee bean. The first crack is the theatrical evidence of that transforming activity as the pressure in the bean finally reaches a point where it can no longer be contained. As the beans expand and break away from their center cut, they swell to about twice their original volume.
For clarity, the first crack does not refer to the very first audible crack of a coffee bean in the roasting chamber. Instead, it refers to the stage of roasting where all the beans begin to crack, distinguished from the second crack which occurs later in the roast and at higher temperatures.
As you monitor your roasting cycle and note the time of the first crack, you're not necessarily looking for the "first crack of the first crack," but that moment when more than just a single bean has popped!
During this first crack stage, the beans step into exothermic activity where the chemical reaction in the coffee actually begins releasing energy (as compared to the earlier endothermic activity where the beans are absorbing energy).
A way to understand this is to imagine holding a lighted match to a piece of wood. It initially starts to blacken, then it smolders, begins to turn red, and finally catches fire. The match heating the wood is endothermic. When it catches fire...that's exothermic. With this imagery, you can see why it's important to recognize this part of roasting as things accelerate during exothermic activity (this happens again, and even more spectacularly, during the second crack!). And, of course, noting the rapidity of the cracking tells you something about the heat being applied to the coffee.
4. Roast Development
This roasting stage—beginning after the first crack through the end of the roast—is really the big kahuna of flavor development. The time duration of this phase is usually referred to as Roast Development ("RD") or Development Time ("DT"). Much of a coffee's distinctiveness in terms of aromatics, sweetness, acidity and balance take place during this stage—and those traits are best teased out gradually.
Fiddling with how RD ultimately influences coffee in the cup is the artistic exploration of the roast master—there is no hard and fast rule on how long or short your RD time should be, but here are a few guidelines to get things started:
The particular roasting equipment (relative size and control features) will influence how you plan your roast development. For large, commercial roasters, aiming for a longer RD is reasonable because bean temperature can be more easily controlled and increased gradually. For smaller home roasters, it can be difficult to roast gently and gradually enough to attain a longer RD without entering second crack (RD is the time from first crack through end of the roast).
Some roasters recommend adding a fixed number of minutes (3-4, or even more) to the roast following first crack while others will urge a percentage of 20%-25% of total roast time as the target. There is no hard and fast "rule," but we prefer the percentage model because it dynamically takes into consideration the rest of the story of how you're roasting a given coffee.
We will delve more deeply into development time in a separate article, but for now, the point is that gentle heat application (and decreasing rate of bean temperature rise throughout the roast cycle) for gradual development is key. If you're applying too much heat, you'll reach second crack stage before you know it and your roast runs the risk of taking off on its own. Slowly increasing bean temperature about 30 degrees or so is probably the max before reaching second crack (and some very energetic exothermic activity).
As noted, depending on your roasting equipment, you may or may not have all the control you'd like over the roast development stage, but it's helpful to understand the importance of this phase and thoughtfully control as much as you can...and in the form that suits you.
And what we mean by that circles back to an important refrain of ours: coffee is a mix of art and science; and how you embrace those two philosophies is deeply personal. To improve your artistic ability, we believe it's helpful to understand some of the science. As a roaster who hovers over his coffee like a doting mother—watching, listening, smelling, reacting—it helps to understand what's going on inside the beans. Not so you can become a button pusher, but a better equipped artist.
5. Second Crack
At the end of development, the beans will begin to crack for a second time. The sound will be softer than first crack, but it can come on more quickly and actively. It's crucial to pay attention at this stage and have a clear idea of what you're aiming for because characteristics of the beans' origins are now changing and even deteriorating at a rapid rate.
Your cue that second crack is imminent will be the increase in smoke. It will start off somewhat sweet, but it will thicken quickly. Oils will start releasing from within the bean and collecting on the surface. In this second, exothermic phase, heat is being generated from the beans themselves and with the increased heat and accumulating oil, fire is a very real threat.
Stopping the roast at the beginning of second crack is your narrow target zone for full city roast, which can yield pleasant and sweet flavor results. But acidity and unique character are dissipating quickly, giving way to the "French" and "Italian" styles which have a characteristic all their own. Some folks like coffee roasted this way, but it's worth noting that it usually doesn't make sense to spend the coin for top shelf green coffee, because those special features you're paying for will likely be all but lost as you wade into the second crack. This dovetails into the next phase, below.
This stage is offered up largely with tongue-in-cheek. This is not a formally described "stage" in coffee roasting, but is indeed a stage in coffee roasting we don't want to overlook. As coffee continues to roast into and even past its second crack, it continues to darken, release its oils, char and eventually reduce to nothing but carbon. Seriously.
The Agtron "gourmet" scale used in coffee color analysis, ranges from 100 to zero as it moves from light to dark. The "100" denotes the color of coffee that is considered undeveloped or undrinkable. A "95" is considered the lightest possible roast color where coffee is drinkable (although, we would not recommend drinking coffee this "light"). As the readings descend from 100, coffee is increasingly darker until it reaches zero on the Agtron scale. At this point, the organic matter in coffee has been burned to pure carbon and is, again, no longer drinkable. If your coffee has reached an Agrtron of zero, you will have already emptied your fire extinguisher.
We point all this out to emphasize the importance of well-reasoned roasting—too light is undrinkable; too dark is undrinkable. Somewhere in between is the magical sweet spot for each coffee and it's up to the artisan to hunt that down in a thoughtful, educated and creative pursuit. It can be tempting to fall prey to conversations which might suggest either "the lighter the better" or, "the darker the better." Over the past 30 years of specialty coffee evolution, both of these mindsets have at times confused roasters and consumers and oversimplified roasting styles.
We believe each lot of coffee has a sort of song to sing—it's not found on sheet music, and it's the goal of the roastmaster to help it find its voice.
Quenching is all about terminating a roast immediately after deciding the coffee is roasted to perfection. This is more than just generally cooling the coffee, because the goal is to reduce the bean temperature very rapidly so the beans stop absorbing heat and continue roasting. Popcorn poppers don't have this feature, but can be "hacked."
Quenching is typically done by applying cool air to the coffee. In drum roasters, this is done by dumping the beans into a cooling tray where cool/ambient air is drawn through the beans. Sometimes, a short blast of water is sprayed over the coffee inside the roasting chamber to accelerate the temperature drop. In small amounts, this does not really have a negative impact on the coffee. An old trick of some commercial roasting companies was to add enough water to the coffee that some was absorbed by the beans to add weight back to the coffee (and since coffee is sold by the pound...).
Again, the goal of quenching is to stop the cease both exothermic and endothermic heat as quickly as possible so the coffee roasting doesn't "coast through the stop" and end up more developed than you intended.
"Coffee needs to rest after it is roasted to attain best flavor."
Myth or science?
We say, art. And anecdotal evidence...
One way to address this topic is to think about grilled steak (we find it easy to think about grilled steak). When a steak (or any grilled meat, for that matter) is taken off the grill, it actually changes character and nature as it "rests." Any experienced chef will tell you this. I think the folks at the famed, French Laundry restaurant drop their grilled steaks into a vat of clarified butter while it rests (yes, I know), because the fat pressure of the butter equals the fat pressure inside the meat, preventing those juices from seeping out.
In any case, when cooking meat, juice is forced toward the center of the cut, away from the sides, by the heat source applied to the surface. Cutting immediately into the steak tragically releases this concentrated juiciness onto the plate. Allowing meat to relax for 5 or so minutes allows the liquid to redistribute throughout the entire cut, resulting in a more succulent and tender experience.
But I digress. This is not what happens inside a coffee bean. We use this more common example to illustrate how foods evolve (and can improve) after they are cooked. Coffee is one of those foods.
Like a grilled steak, there is not a hard-and-fast scientific amount of time to allow coffee to rest for it to reach its optimal flavor. Some will argue a day, others insist 3 days or up to a week or more. We suggest giving coffee at least overnight and have found that in nearly every case, the coffee tastes better a day or two after roasting than it does the day it is roasted.
HOWEVER...none of this is to imply that you can't enjoy a fantastic coffee experience right out of the roaster. By all means you can! To mirror the resting of days, you can grind your coffee to accelerate the "resting" (degassing), but it's a bit harder to judge how much time is ideal because coffee is so much more volatile when ground. The point is simply that coffee improves meaningfully with some time to bleed off some CO2 after it's roasted.