Roasting delicious coffees has been a passion of mine for more than 10 years. I joined Chimney Fire Coffee as Head Roaster in 2018 when founder Dan moved his operation to Ranmore Common in the Surrey Hills.
Taking green coffee seeds from different coffee growing regions on their journey to becoming brown beans (as most people know coffee to be) takes a bit more than first meets the eye.
Firstly, you need two elements to make this happen. There is the roast master (that’s me), and there is a coffee roasting machine. In our case we have opted for a CoffeeTool cast iron drum roaster, hand made by Stelios Sakantaris in Athens.
The drum rotates above a powerful burner plate which heats up the iron drum, as well as warming the ambient air before it enters the revolving roasting chamber. This hot air is driven through the bed of beans and out the other end of the drum.
However, air alone is not the only heat source for roasting. Our machine allows us to transfer heat to the beans by three means:
- Conduction - heat transfer to the beans from the surface of the drum
- Convection - hot air being driven through the bed of beans while in the drum
- Radiation - Beans passing on heat to each other
To measure the temperature inside the drum, the CoffeeTool has two appropriately placed temperature probes. One, the bean temperature probe (BT), is situated inside the front of the drum, sitting within the bed of coffee to give me an idea of how hot the beans are at any given time. The other probe, exhaust temperature (ET) is placed at the exit of the drum. This measures the temperature of the air as it leaves the roasting chamber.
On the left side of the roaster there is a control panel which allows me to give the necessary commands before, during, and after roasting. Here I can make subtle changes to the input of the gas flame, the amount of air I wish to pass through the drum, and the speed at which the drum rotates. It also gives a live read out of both BT and ET, overall roast time, plus a rate of rise (RoR) which indicates by how many degrees the beans gain heat over a given period.
Before starting the roasting session, I have to slowly heat up the CoffeeTool for an hour before charging the first batch of coffee into the drum. The main objective here is for the cast iron to absorb as much energy as possible, creating the most ideal environment in the drum for consistent roasting.
Every bean, depending on its origin, size, moisture content, and density, demands a slightly different drum environment. It is the roast masters job to create this before charging the green beans into the drum.
Another interesting phenomenon I have to take into account during the roasting process is that the beans go through two chemical reactions:
- Endothermic - the beans absorb heat from the environment they are in
- Exothermic - the beans release energy to its surroundings (i.e. to its neighbouring beans within the drum)
The roasting journey the beans undergo can be broken down in three equally important phases, albeit with very different objectives. During these stages a loss of moisture, myriad chemical reactions, and beautiful aromas are released. All these attributes culminate with the beans changing colour. An audible sound referred to as first crack happens as the bean structure pops, just like roasting popcorn!
Working with this knowledge allows me to be precise and fully in control when applying roasting principles. However, the craft of roasting would be incomplete without using all one's senses - as well as a certain amount of intuition, thrown in for good measure.
My most important objective during roasting is to bring to the fore and lock in the flavours inherent to the raw material with which I’m working.
In case you didn’t know, it’s worth mentioning that coffee is a fruit - a cherry which grows on a tree, just like our British cherries. Like with any fruit we consume, we would expect to eat/drink it when it is at its best. In our case, we want the seeds in the cherry (the green beans) to be picked when ripe and efficiently processed at origin, roasted with care and attention, and of course perfectly brewed at the end.
For the beans to be roasted for optimum taste, I have to consider the roasting degree I’d like the beans to reach. Generally speaking this is a medium roast, which allows for a balance between acidity (sweetness) and body (earthiness). Once the desired roasting degree has been achieved, it’s time to release the beans from the roasting chamber into a revolving cooling tray with a perforated surface. Underneath the tray the CoffeeTool has a powerful motor which, by means of suction, cools the beans from over 200°c quickly and efficiently.
Et voila, job done! I hope that reading this short overview has given you a bit more insight into the craft of coffee roasting I love so much.