Updated: Nov 15
Last time round we looked at how we plan our roasts and how we make the necessary preparations prior to the actual roasting of the coffee. This time we will take a closer look at the changes that occur within the coffee bean during the roasting process.
This article is aimed to the consumer who wants to know a little bit more. Fellow coffee roasters will be only to aware that this article barely scrapes the surface of the science of coffee roasting.
When the green coffee is released into the roasting chamber the air temperature inside the roaster falls rapidly as it comes into contact with the much cooler beans. The flame reignites.
At this stage the flame will typically be on a high setting and the beans begin to lose moisture and mass. As they do so, water vapour can be seen escaping from the chimney stack.
The person in charge of the roasting process is called the Roast Master. He (or she) will take air and bean temperature readings at 1 minute intervals throughout the roast and record this information on a chart. This record of the roast is known as the roast profile. It may be used to repeat the same roast at a later date or as a basis for planning a variation of the same roast.
Roastmaster @goldenhbrowne checks the readouts
After around 2 minutes the air and bean temperatures bottom out then begin to rise. This is known as the turning point. Complex chemical changes are taking place inside the bean. Physical changes in the beans can be observed through a spyglass in the front of the roaster or through a small sample which can be removed using a scoop.
The first noticeable change is that beans take on a yellow colour. About a minute and a half later, the yellow becomes a cinnamon brown colour. The roast rapidly gains momentum and from here on in things start to happen quite quickly.
A certain amount of moisture is retained in the bean and as this heats up it expands. This combined with carbon dioxide, generated during the roasting process causes the beans swell and almost double in size. As the beans expand, the chaff (papery parchment) around the outside of the beans flakes off.
After a short while the heat build up within the beans is such that it causes the beans to pop or crack. This is a key roasting stage known as 1st crack. There is an audible cracking sound (similar to pop corn 'popping') and a sudden increase in smoke and roasting aroma from the chimney stack.
In theory, the roast can be stopped at any time now. It is quite common for the flame height to be reduced at this stage, to slow the roast down and extend development time (the point from first crack until the roast is ended). Increasing development time may contribute to body and complexity in flavour in the cup.
Even if the flame is turned off, the momentum within the beans can still increase the temperature of the bean mass by around another 10 degrees Celsius and the beans will continue to become darker in colour.
When the Roaster Master determines that the time is right he opens a gate on the front of the roaster and releases the beans into a cooling tray. The cooling tray draws air through the beans whilst stirring them. This cools them down rapidly which is important so as to stop the beans will continue to roasting under their own momentum.
Light-medium roasts often have more acidity, they can be delicate and have complex flavour profiles. Medium roasts often have chocolate and caramel notes. If the Roast Master continues to apply the flame, after first crack, he is probably looking for a darker style roast. As the roast becomes darker and the heat levels increase, more caramelisation of the sugars within the bean will occur. Too dark and the coffee may take on pungent and smokey characteristics and the end result is likely to include some bitterness.
Roasting darker reduces the differences in appearance, taste and aroma between types of bean. Many speciality coffee roasters will not roast beyond the beginning of what is called 'second crack' (when the bean pops for a second time).
At Bean Smitten we run several test roasts for each new bean or blend. The idea is to discover the exact roast profile that we think brings out the best from the bean. Our measure of what is 'best' will be different for a single origin coffee intended for cafetiere or filter use than it will be for and espresso blend. We cup (taste) samples from each test roast to help us decide.
We hope you have enjoyed this overview of the roasting process at Bean Smitten. It is likely that we shall return to this subject area at a future date. In the meantime we recommend trying as many different coffees as you can. Discover your preferences and remember that taste is a personal thing.