What happens during coffee roasting?

When you hold a raw coffee bean in your hand - small, hard, green, and pea-like - you realize how much it needs to change to become one of those beautiful brown beans that we love so much. Here are the most important roasting phases summarized briefly for you to know.

What happens during the roasting of coffee?

More than 800 aroma compounds have been identified by researchers in a roasted coffee bean. This is twice as many aromas as in wine. Coffee is therefore one of the most complex and aromatic natural products. These aromas are released during roasting. Roasting is actually not rocket science - it is simply dry heating without water and fat. Just like when you roast nuts, for example (and if you have never done that - do it!). You can also easily roast coffee beans in a pan or on a baking sheet and observe what happens. However, the beans probably won't taste too good because to fully develop the flavour notes, you need to follow a few steps and apply more or less heat at the right time.

The roasting process consists of the following phases

All phases at a glance

Drying the coffee beans

Green coffee contains up to 11 percent water, which is distributed in the bean and must be evaporated before roasting. Therefore, in the first phase, the roaster is heated and energy is supplied to the beans until the water evaporates (at around 20 - 130°C). This phase is very important and must be done in such a way that the entire bean is free from water; otherwise, it will roast less inside and acquire an unpleasant, grassy taste afterwards.

The Maillard Reaction

From approx. 130°C, the roasting process begins. The beans slowly turn yellow and change colour to a beautiful caramel-like brown. Chemical processes take place in the bean, especially the so-called Maillard reaction. The beans noticeably increase in size, and the fine skins dissolve. At the Gene Roaster, these skins are automatically blown away by the airflow, while with the Hottop Roaster, you should turn on the ventilation at this point to prevent the skins from remaining between the beans and burning.

First Crack / the first crack

The big moment for you as a roaster. In the bean, carbon dioxide and steam build up pressure to the point where the bean bursts. You can recognize this by a cracking sound (first crack) or crackling. From now on, it's your time as a roast master, as the beans are now drinkable, and you determine how dark your roast will be by the duration. Now you need to apply some heat again to allow the beans to develop nicely.

Second Crack / the second crack

If you roast for filter coffee, you have probably completed the roasting before the second crack. For espresso or darker roasts, the second crack is an important moment. It is a second popping of the bean; however, the cracking sounds are slightly finer than the first crack. From now on, oils emerge from the bean, which you can recognize by the slightly shiny surface. The acids and characteristic flavour notes of the coffee are now only very weakly pronounced, but the beans now have more roasting flavour and become noticeably more bitter.

Cooling the roasted beans

This is also a very important step in the roasting process, as the beans need to be cooled quickly to stop reactions and prevent further roasting of the coffee. With the Hottop coffee roaster, this is easily done at the push of a button - the beans are rapidly cooled. The Gene Café coffee roaster lacks such a cooling device, so you can either build your own cooling system following instructions from the internet or cool them, for example, with a sieve over a fan.

Coffee Storage

After roasting, carbon dioxide still escapes from the beans. This directly affects the taste right after roasting. Therefore, you should wait a few days before grinding and drinking the coffee. In our experience, the coffee is at its best between the third and tenth day after roasting. After that, the essential oils that give coffee its flavour have dissipated, resulting in a noticeably duller taste. It is best to store the beans in a bag with a valve (also available in our shop) or in a container with a valve, allowing carbon dioxide to escape while preventing oxygen from entering.

Tip: the Roast Loss Formula

You will be amazed, the coffee has significantly increased in volume due to roasting - simply try putting the coffee back into the same container as before roasting. At the same time, the coffee has lost weight. Write down the percentage of weight loss (the so-called roast loss formula): Weight loss divided by the weight of the beans before roasting (e.g. 40 grams / 250 grams = 16%)

You will notice that beans with a similar weight loss taste similar. Typically,

approx. 12 - 15% for light roast,
15 - 17% for medium roast,
18 - 24% for dark roast.

Tip: the Roast Loss Formula

You will be amazed, the coffee has significantly increased in volume due to roasting - simply try putting the coffee back into the same container as before roasting. At the same time, the coffee has lost weight. Write down the percentage of weight loss (the so-called roast loss formula): weight loss divided by the weight of the beans before roasting (e.g. 40 grams / 250 grams = 16%)

You will notice that beans with a similar weight loss taste similar. Typically