Ever wondered how to roast coffee? Does coffee get put in a tray, basted with goose fat and left in an oven for 2 hours wrapped in tin foil? Does it need to be rested afterwards? How do you know when it is "done"?
So, with these questions and plenty more in mind, Lewis our Roastery Manager has shared some of the dark arts and let us into some secrets. Nerd Alert Klaxon! If detailed, technical data analysis is your bag - read on! If not, save this blog for a sleepless night…
So first - get your coffee roaster. A coffee roaster is basically a drum that rotates over a heating element. Ours is a gas powered coffee roaster but all roasters work on the same principle. Raw, green, coffee beans are introduced into the drum which rotates over a heating element. As the temperature rises within the drum (drum temp) the temperature of the beans rise. As the temperature of the beans starts to increase this is called the turning point. This is the point at which the coffee beans are heating up. As the roaster increases the temperature within the drum, the bean temperature also rises.
At some point - and that point depends on the type of bean, how it has been processed and the level of moisture within the bean - the bean "cracks". That is to say that the outer shell of the coffee bean splits away and becomes chaff. This chaff is then sucked away and out of the drum and away. The bean is now roasting.
The length of the roast is then determined from this point - the cracking point. A light roast may be a minute or two from the first crack. A dark roast might be 3 to 4 minutes. There is also something known as a "second crack" - at this point the bean has been almost incinerated. Some commercial roasters go beyond this second crack. The reason for this is varied but they sell "dark, roasted, Italian" style coffee, and the truth is that incinerated coffee beans will keep for ever and so there is no danger of them going "off". The downside however is that incinerated beans have lost all their flavour profiles and basically taste burnt. Most major coffee shop chains roast their beans to beyond second crack because it is also easier to be "consistent". Let's face it burnt coffee tastes like everyone else's burnt coffee …
A typical roasting profile
So, let's take a look at the graphic above. This graphic is a screenshot from the roasting software that we use to manage and monitor each and every roast. Lewis, our roastery manager, uses this system to ensure that each roast is consistent across our coffees and blends and thus Gunpowder blend today is the same as Gunpowder blend next week.
This particular graphic was actually a recent roasting of our new Rwandan beans. Rwandan beans are at their best with a light roast - that is to say that we don't roast them for too long.
To understand this graphic you need to know a few things
It's a graph and as with each graph there is a X-axis - along the bottom - which is time in minutes.
And there is therefore a Y-axis - the vertical - which is temperature.
You'll see 2 lines coming from the left - a red line and a blue line
The red line is the temperature of the drum, or the environmental temperature
The blue line is the temperature of the beans
As we move along the red line (the drum temperature) drops to around 150 degrees. Remember that it has just come off a previous roast so it is already hot as we have "dropped" the roasted beans at 0 minutes and introduce the new beans. The temperature of the drum quickly recovers at around minute 1 and then climbs steadily to around 180 degrees at 6 minutes and then remains constant at 180 degrees for the remainder of the roast.
The bean temperature (blue line) also has a sharp decline from the previous roast - but then at around 2 minutes it reaches a turning point. We have indicated this as Point 1 in the graph and this is at around 75 degrees. Once the beans have reached their turning point, then they too begin to heat steadily although not as quickly as the drum temperature and eventually reached the 180 degree point at 10 minutes.
Now, there is a 3rd line - so if you stay with us for just a little longer - this line which is also blue confusingly, rises from the horizontal X-axis at around 1.45 minutes. So in other words at the turning point temperature. This line rises very quickly and then declines slowly in almost a even line. This line is the rate of change of temperature of the beans. This is VERY IMPORTANT. As you can imagine we want the bean temperature to rise steadily. We don't want to heat them too quickly. Too quick means some beans get burned whilst others are still raw. A steady rise in temperature is what we want. This 3rd blue line shows that the change of temperature is very quick to start with and thus it rises from 1.45 minutes to around 2.15 minutes .. but then it declines slowly. This is not the temperature remember - the temperature is still rising but the rate of change of that temperature is slowing and so we get an even, steady roast across all the beans. We did warn you that it gets technical ...
At the point that we detect the "first crack" (point 2 in the graph) that is the point that the beans are roasting. As we said above, the cracking point is important.
Lewis, our head roaster, has decided that these beans need a light roast and so after only around 45 seconds after the "first crack" he "drops" the beans (point 3 in the graph). The drop is when the beans are let out of the drum and into the cooling tray.
You will see the yellow shading just after the first crack. The yellow shading indicates the length of the roast and stops once the beans are dropped.
Once the beans have "dropped" then they are cooled and bagged!
The skill of the roaster is to manage all the variables - the temperature of the drum, how fast that temperature increases, the temperature of the first crack, the length of roast, and the rate of rise in temperature. That rate of rise in our graph gives us a smooth almost straight line in rate of change. This is the sign of a good roast.
One more thing - the roasting profiles allow us to experiment and see what happens if we change the variables. If we drop the beans later, or heat the drum higher for a shorter period of time, or conversely heat the drum to a lower temperature for a longer period of time. We can compare roasting profiles against each other and identify improvements and roasts that perhaps did not go to plan.
So there you have it. How to hand roast coffee beans. Any questions, or requests, please send Lewis a message. If you ask really nicely he might include your roasting profile with your order…