All roasts are not created equal.
Coffee is delivered to your local roastery as “green coffee." It's called that, you guessed it, because of its pale, green color. Green coffee doesn’t taste anything like the coffee you know and love. And it’s extremely hard, so if you don’t want to take my word on flavor, you'd better have a grinder that’s both very powerful and OK to break.
The point of roasting coffee is not only to develop those flavors, but also to make the coffee brittle enough to grind and soluble enough to dissolve in water, allowing those flavors to escape into your mug. But roasting for all its complexities, is basically as simple as turning green coffee brown.
What Is Coffee Roasting?
We turn that from green to brown by using some sort of machine to heat it until it’s really, really hot. That machine (along with the person who operates it) is called a roaster. Roasters come in all shapes and sizes, and theoretically be anything from a popcorn popper to your kitchen oven — even a large pan can be a roaster. But professional coffee roasters are generally able to roast them evenly and consistently.
There are two popular kinds of commercial roasters: drum roasters and fluid bed roasters. With a drum roaster, coffee is dropped into a rotating drum and heated underneath by gas flames. In a fluid bed roaster, hot air shoots up from the bottom of the roaster, suspending the beans in mid-air and heating with that hot air. Both of these kinds of roasters have their proponents, but both of them can produce excellent coffee.
Let's Talk Roast Level
The thing most people know about coffee roasting is roast level; you’ll see dark roast or light roast or French roast on a bag, and many of us have our preferences. Roast level is pretty closely correlated with the color of your beans, though occasionally a coffee will taste lighter or darker than it looks (for example, at Trade quality control, we assign a roast level to the coffee based on how it looks, but sometimes we change it after tasting).
It is possible to objectively measure the level of roast using a device called a spectrophotometer (often referred to in the industry by the brand name Agtron), but if you take two different roasters and line their roast levels up against eachother they don’t always add up. I’ve definitely had “dark roasts” from some specialty roasters that tasted lighter than “medium roasts” from others.
Roast level mostly impacts flavor, but there are a few myths about roast and caffeine as well. People sometimes assume that darker roasts have more caffeine or are stronger. We can pretty safely say that roasting coffee darker cannot create any more caffeine in the bean. On the other hand, some people say that dark roasts actually have less caffeine, as the caffeine evaporates or burns off in the roasting process. While this could be true at the extreme dark roast level, most coffees don’t get anywhere near the temperature necessary to reduce levels of caffeine.
As far as flavor goes, while a lot of chemical reactions occur while coffee's roasting from light to medium to dark, a lot of it has to do with caramelization; sugars browning and getting more complex. A lighter roast will be more gentle, with more subtlety and unique flavors developed in the green coffee (based on origin, growing conditions, variety, and processing) likely to stand out.
Darker roasts will likely feature those darker sugar flavors and, if you go further, burnt sugar flavors that we generally refer to as "roasty". It is an oversimplification, though, and one I’ve been guilty of in the past, to say that dark roasts all just taste like roast. There are different ways to develop those flavors that make certain dark roasts more or less bitter and sweet, and different green coffees have flavors that hold up better or worse when roasted dark.
The Phases of Roasting
Whether you’re dealing with light or dark roasts, it’s certainly true that roast level isn’t everything. The roast level of your coffee certainly has an impact, but if I start with two batches of the same green coffee and roast one in 15 minutes, and the other one to the exact same level in 10 minutes (using more heat), they will taste very different. Furthermore, even if two batches of the same coffee are roasted to the same temperature in the same amount of time, they won’t necessarily taste the same if heat has been applied differently at different times of the roast. That’s why the most common representation and way to describe the journey a roast takes from green to brown is a “roast curve,” represented usually by a graph of the time of the roast vs. its temperature.
The roast starts with room temperature coffee being dropped into a pre-heated roaster. In a roast curve, the time it takes for the temperature of the coffee to equalize to the temperature of the roaster is that first big dip. The first few minutes of the roast — where we’re generally trying to give the coffee enough heat energy to get on its way, while not burning the outside of the coffee — is commonly called the drying phase (though coffee is losing moisture throughout the whole roast). By the end of this phase, coffee is starting to turn from green to yellow. This means we’re seeing the Maillard Reaction, the same reaction between amino acids and sugars that makes food like steak turn brown during cooking. A little bit of sweetness and acidity is starting to develop, but coffee at this phase still tastes more like straw than anything you’d enjoy in your cup.
The next and longest roasting phase brings us to right around the time when coffee starts to taste like coffee. Again, lots of Maillard reactions are taking place here, along with the start of caramelization, which makes the sweetness in coffee more complex, and the development of many aromas that make coffee delicious. This phase ends with an important reaction called “first crack”, when the water vapor builds up so much pressure inside the bean that it pops, much like popcorn. After first crack, we enter a period sometimes called “development” (though, much like drying, the flavor of the coffee is developing the whole time). By this point, if you finish your roast, the coffee will have started to taste recognizably like coffee. But roasters keep roasting it for a while to develop those sugars. Eventually the coffee cell walls will break down further and crack again (we call that “second crack”). At this point, you’ll definitely be at a dark roast. If you’ve ever bought beans that look oily, that’s likely a sign that they’ve been taken past second crack.
Generally, for each coffee a roaster buys, or each of the blends they put together, they try to figure out the specific roast curve (or roast profile) for the flavors they want. They can do so by changing the total time and temperature, highlight the particularly acidity by getting to first crack faster, or bring out some sweetness by developing longer at the end. There are an almost infinite amount of ways any given coffee can be roasted, which is why it’s fun to taste a coffee from the same farm or co-op roasted by two quality roasters.
Coffee roasting in principle is pretty easy to understand: you get green coffee, you turn it brown. But all the complex reactions that happen in that 10 or 15 minute color change are hard to get right — never mind to get right every single time. It takes a quite impressive balance of creativity and discipline to roast coffee, which is why we’re so excited to work with so many people who are so damn good at it!