Coffee roasting: From kernels to coffee beans

A fascinating transformation

Coffee roasting is a fascinating process. Roasting coffee can transform a simple green kernel into a complex and aromatic coffee bean. An INCREDIBLE MUCH happens to the coffee during roasting, and it takes an INCREDIBLE LITTLE to thwart the process.

Below we review the most important phases of coffee roasting, and you will learn to distinguish between light, medium and dark roasted coffee. It helps you understand what the different roast profiles do to the taste of the coffee.

General about coffee roasting: A matter of time

Most roasters have their own approach and philosophy when it comes to coffee roasting. Depending on who you are, roasting coffee can have different purposes. Some roasters roast coffee because they want to highlight certain qualities in the coffee bean. Conversely, for others, roasting is about hiding unwanted aftertastes.

When you define the roasting profile of the coffee bean, it is typically based on the colour of the coffee – i.e. whether it is light, medium or dark in colour. But you must not overlook the time, which is at least as important a flavour factor. If we take two identical green coffee beans and roast them both at the same temperature and light-roasted colour, but do so at different tempi, the taste of the two coffee beans will differ significantly.

You can roast coffee in anywhere from 90 seconds to 20 minutes. The big coffee companies use industrial ovens that roast the coffee beans in a few minutes, while others use the oven at home, where the roasting takes longer.

Although it can be difficult to say something general about roasting time, there is a general consensus that coffee beans lose more and more of their acid the longer they are roasted. However, a longer roast will, on the contrary, produce more bitterness in the coffee, especially if it is roasted dark. And unlike acidity and bitterness, the coffee’s sweetness will rise and fall like a bell curve through the roasting process. Therefore, it can be an extremely difficult balancing act and a bit of precision work to hit a balanced flavour profile.

However, the best roasters have become so skilled at manipulating the roasting that they can make a coffee that contains both lots of acid and sweetness at the same time

The key phases of coffee roasting: Know them inside out

There are several key stages during coffee roasting. How quickly the coffee beans go through these phases is decisive for the roasting and flavour profile of the beans. Most roasters have full control over parameters such as time and temperature, which we reviewed earlier, so they can repeat the roasting with the same result time and time again.

We briefly review the phases that coffee beans undergo during roasting and how they affect the final result.

Evaporation: To the naked eye, the most obvious change coffee beans undergo during roasting is the change from green to brown. But the coffee beans will not turn brown until all the water has evaporated. Green coffee beans are already dried, but in raw form contain approximately 10% water. A large part of the initial roasting, therefore, consists of evaporation.

“Yellowing”: As soon as the water is gone, the first browning begins. Professionals call the phase “yellowing”, after the initial yellowish colour of the beans. Soon the beans will begin to expand and thin, protective skin will peel off and be sucked out of the roaster.

If the coffee beans are not dried properly in the initial stages, the beans will not be roasted evenly. A possible mistake cannot be corrected, so it is important to get off to a good start.

“The first crack”: A chemical process has slowly begun inside the coffee bean, where gas now accumulates and causes the bean to expand. When the pressure becomes too great, the beans pop and double in size in an instant. This is called “the first crack”. It is now and then that the coffee flavour begins to develop in the beans.

“Roasting profile”: It is after the first crack that the colour of the coffee beans is determined. Lightly roasted coffee roasted to a temperature of around 200°C, medium roasted coffee to approx. 215°C and dark roasted coffee to approximately 225°C.

From now on, the acid in the coffee bean will decrease in line with the roasting time, and the coffee’s bitterness will conversely become more prominent.

“The second crack”: If the roasting continues, the coffee beans reach a point where they crack for the second time. Here the oils will be extracted and appear on the surface of the beans. From here, the coffee will begin to char and its own unique taste will disappear.

If you have coffee beans of inferior quality, they are typically roasted to a second crack, where off-flavours are hidden and drowned out by a more generic roast taste. Some coffee drinkers like extra dark roasted coffee, but it will typically consist of coffee of inferior quality.

The difference in grating profiles

Light-roasted coffee: At VikingRoast, we roast our light-roasted coffee to a temperature of between 200-210°C. It gives the beans a light, light brown colour. By roasting lightly, we preserve as much as possible of the coffee’s natural aromas and taste. In particular, you will taste a lot of acids and fruity sweetness. At the same time, the coffee gets a lighter body, and light-roasted coffees typically don’t take up as much in the mouth.

Medium roast coffee: Our medium roast coffee reaches a temperature of between 210-220°C, but typically at the low end of that scale. Here, the coffee beans get a really nice brown colour and more body than the lighter roasted beans. Medium roast coffee is a classic in filter coffee because it is typically a very balanced coffee that contains the best of both worlds.

Dark-roasted coffee: We roast our dark-roasted coffee at 220-225°C until the coffee beans take on a deep dark brown colour. The colour goes well with the bittersweet aftertaste that is revealed in the coffee. Dark roasted coffee is typically used to brew espresso, but it is up to the individual what they prefer.

With dark roasted coffee, you typically get a lot of body, and if the coffee is of good quality and roasted correctly, you can also sense a rich sweetness.