Stages in Roasting December 27 2013
We though it might be a nice idea to outline the coffee roasting process, giving you an idea of how the bean changes throughout the roast.
We begin by charging the roasting drum – in other words we heat it to our desired starting temperature. We have a digital probe inside our roasting drum and this gives us a constant reading of the temperature in the drum.
During the roast this temperature will be monitored by our software to allow us to replicate successful roasts in the future. In the early Carvetii days this was all done by pen and paper, with a spreadsheet used to give us a visual image.
Once we reach the desired temperature the beans are released into the drum.
The cold mass of the beans reduces the temperature in the drum quite quickly. We are quite gentle with the heat at this stage and eventually, after a minute or two, the temperature stops dropping and begins to climb. The beans are still green in colour at this stage and we are slowly drying out some of the moisture held within them.
The beans begin to change colour, initially to a brighter green, and we ramp up the heat. We want to roast the beans quite quickly from this point. The beans now change in colour to yellow, and then onto tan and light brown. Each of these colour changes represents a specific bean temperature.
Once the beans have reached a brown colour, they are nearing a point in the roast called first crack. There are various gases within the bean, including steam, and during the roast there is a build up of pressure inside the bean. First crack is the point where the gases can no longer be held within the bean, and they literally pop.
By now we’ve been controlling the heat applied to the beans. This is a bit of a balancing act – the beans lose energy during first crack and if we reduce the heat too much the coffee bakes rather than roasts. Similarly if we don’t get the roast under control, it can run away with itself and we end up with over roasted coffee.
Once first crack is over, there is a short window before second crack begins. Second crack is the actual structure of the beans breaking down, and it sounds like twigs breaking. The first few pops of second crack is pretty much as far as we take our roasts. Up to this point the predominant flavours in the coffee are very much linked to the origin characteristics in the bean. At around second crack and beyond, more roasted flavours begin to come through. As the roast continues these roasted flavours begin to eclipse the origin flavours. We’d rather our coffees don’t have roasted tones to them.
Once we’ve reached the appropriate stage in the roast, the beans are dropped into a cooling tray. Cool air is drawn through the tray and the aim is to cool the beans as quickly as possible.