Ever heard of “green” coffee?!
That’s what we in the coffee industry call unroasted coffee. Coffee “beans” are actually seeds of a fruit widely known as coffee cherries.
After being picked, the fruit of the cherry is removed by either washing it away, or by drying in the hot sun. The result is a small seed (there are two per cherry), which is green in colour, very hard in structure, and without much taste, other than subtle “grassy” flavours.
To bring out the wonderfully complex flavours and aromas inherent in these seeds, or “beans”, we roast them. There is a plethora of science around specialty coffee roasting, but for our intents and purposes, let me walk you through the basics.
The Coffee Roasting Machine
A coffee roasting machine is essentially a big cast-iron drum which turns at a constant rate over a gas flame.
It’s a lot like cooking with a cast-iron pan on a gas range - small, incremental changes to how much gas you’re adding will affect the temperature of the pan. Cast-iron has efficient heat retention, which makes the process of warming up and cooling down the roasting machine relatively slow.
However, the benefit is that changes to the gas settings while roasting allows for a gradual and more “even” heat application. This, along with the constant rotation of the roasting drum, makes it possible to roast several kilograms of coffee beans evenly.
At the top of the roasting machine, is a large bin which feeds into the roasting drum.
This “hopper” is where green coffee is loaded into the machine.
Once the machine has been warmed up to the necessary temperature (typically around or above 400°F), a pre-measured amount (or “batch”) of beans are dropped into the machine, with the drum already turning.
Each batch is started at a relatively high temperature - this allows for the beans to quickly rise in temperature to match that of the machine, so as not to completely dry them out early.
Green coffee has moisture content which affects the way it roasts. With too much temperature, beans with lower moisture content can dry out and burn during roasting, and beans with higher moisture content can take too long to roast at lower temperatures, which again will dry out the bean, and negatively affect flavour in the end result.
As the batch progresses through its roast cycle, and the beans come up to the correct temperature, gas settings on the machine are lowered incrementally, at various stages and temperatures, so as to keep the beans roasting, while reducing the risk of burning.
To help manage temperature inside the drum, there is a fan inside the machine, with a damper to control how much airflow is directed into the drum. Changes in airflow at critical moments help keep rapidly rising temperatures under control.
Now, as the gas is lowered, the coffee continues to roast and rise in temperature.
As Scott Rao, a US-based, world-renowned coffee expert has found, the key to great-tasting coffee, is the balance between knowing when and by how much to drop the gas settings during any given roast cycle, while maintaining a “steadily declining rate of rise” in the beans themselves.
The “rate of rise” is just that - how quickly (or slowly) the temperature of the bean is rising. This is monitored on modern machines (or machines retrofitted with modern technology), by a series of temperature probes inside the roasting drum, which are connected to a computer running specialized roasting software.
I won’t get too much into the software side of things, but the overall benefits of using roasting software are the ability to more accurately monitor temperatures during roasting, and to be able to save and replicate particularly good roast cycles.
These are saved as “roast profiles”, which provide a record of the time and temperature in which every gas or airflow change was made during the cycle. As a roaster gets to know a specific coffee from a specific region, through roasting and tasting, they will make alterations to its profile until they are happy with the result. That result is then repeatable, using their desired profile.
Perhaps the most crucial point during a roast cycle is a stage called “first crack”.
When the moisture and oils inside the bean reach a specific temperature, pressure inside the bean will expand to the point where it literally cracks open. This results in an audible sign of when your batch of coffee has reached the start of its’ “development phase”.
The beans are now at the ideal internal temperature for flavours and aromas to develop. Caramelization of sugars and reactions to amino acids happen during this phase.
First crack sounds almost like popcorn popping inside the machine. After a short amount of time, the cracking will stop.
If a roast cycle is long enough, second or even third cracks will also occur. Coffee roasted to “second crack” or longer will result in a dark roast.
There is a small scoop attached to the roaster, with a metal “scoop” end inside the roasting drum, and its wooden or otherwise heat-resistant handle protruding from the outside of the machine.
This is called a “tryer”, and it allows you to pull a very small amount of beans (think 10-15 beans total) from the drum while its’ roasting so you can take a look and gauge the beans by colour to see when they’re done. There is also a sight glass at the front of the machine, which allows you to see the beans and monitor colour as they roast.
When the beans have reached the desired colour, hopefully after following the desired profile correctly, a door on the front of the machine is opened, and the batch is dropped into a cooling bin attached just below the door to the front of the machine.
Airflow is directed through the cooling tray, and “stirring arms” are engaged within the bin to keep the beans moving around, ensuring even and constant airflow through the bean pile.
Coffee beans need to be cooled quickly after roasting, as otherwise they would continue to “cook”. The cooling bin/stirring arm combination provides a consistent environment for which the beans to cool, which also helps with repeatability of any given coffee’s roast profile.
As the freshly roasted batch of coffee cools in the cooling bin, temperature can be raised on the machine to begin roasting the next batch.
The time it takes to roast a batch of coffee depends on the size of the batch, and the roast profile (most small-batch, specialty coffee roasters will roast in batches between 12 and 20 lbs, the roast time ranging between 8-10 minutes).
Once the coffee in the cooling bin has cooled to around room temperature, it can be dropped through a trap door in the cooling bin, into storage bins, where it is labeled and is now ready to be packaged up and shipped out.
There is a lot more to learn about the science of coffee roasting, such as thermodynamics, plant science and more, and if you’re curious or just want to nerd out a little, look into Scott Rao’s book, “The Coffee Roaster’s Companion”.
It’s a must-have for those looking to take their roasting knowledge to the next level - but at least for now, you know the basics!
Author John Ohrn