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Coffee roasting what happens

What happens during coffee roasting?

If you hold a raw coffee bean in your hand - small, hard, green and pea-hardy - you realize how much it needs to change, that it turns into one of those beautiful brown beans that we love so much. Here you will find a short summary of the most important roasting stages that you need to know.

So far, researchers have identified more than 800 flavourings in a roasted coffee bean. That's twice as many flavours as in wine. Coffee is one of the most complex and aromatic natural products of all. These flavours are released by roasting.

Roasting is not really a witchcraft - it is simply a dry heating without water and fat. Just as when you roast nuts (and if you have never done so - do it!). You can also roast coffee beans in a pan or on a baking tray and watch what happens. The beans will probably not taste too good, because to fully develop the flavours you have to take a few steps and add more or less heat at the right time.

Roughly, there are the following phases during roasting:

Drying the coffee beans

Green coffee contains up to 11 percent water, which must be distributed in the bean and evaporated before roasting. Therefore, in the first phase, the roaster is heated and fed to the beans until the water evaporates (at about 20-130 ° C). This phase is very important and must be done so that the whole bean is clear of the water, otherwise it will roast less inside and get an unpleasant, grassy taste.

The Maillard reaction

From about 130°C it starts with the roasting. Now 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 are significantly larger and the fine cuticles dissolve. With Gene roaster these cuticles are automatically blown away by the air flow, the Hottop roaster you should turn on the ventilation here at the latest so that the pellets do not remain between the beans and burn.

First Crack

A big moment for you as a roaster. In the bean, carbon dioxide and steam develop a pressure that bursts the bean. You can recognize this by cracking (cracking) or crackling. From now on, your time as a roast master or roast master, because the beans are now drinkable and you decide with the duration of how dark your roasting. Now you have to add some heat again, so that the beans develop nicely.

With the degree of roast you now decide on the right balance between acidity (more about the acidity in coffee, by the way, you will find here) and bitterness. There are names for level of roasting that are common and you should know as a home roaster. You can find more about this here.

Second Crack

If you're roasting for filter coffee, you've probably finished the roast before the second crack (second crack). For espresso or dark roasts, the second crack is an important time. It's a second bursting of the bean; the pops are a bit finer than the first crack. From now on, oils emerge from the beans, which you can recognize by the slightly shiny surface. The acids and also the characteristic flavours of the coffee are now only very weak, but the beans now have more roasted taste and are also significantly bitter.

Cooling the roasted beans

Again, this is a very important step in roasting, as the beans need to be cooled down quickly to stop the reactions and stop the coffee from roasting. With the Hottop coffee roaster, this is easy at the touch of a button - the beans are quickly cooled. The Gene Café coffee roaster lacks such a device for cooling, here you can build with instructions from the Internet your own cooling device or you cool them, for example with a sieve over a fan.

The storage of coffee

After roasting, carbon dioxide still exits the beans. This affects the taste immediately after roasting. Therefore, you should wait a few days until you grind the coffee and drink. In our experience, coffee is best between the third and the tenth day after roasting. Thereafter, the essential oils that make up the taste of coffee, have already gone so that the taste is significantly fluffier. The best way to store the beans in a bag with valve (you can find them in our shop) or in a can with valve, so that although carbon dioxide can escape but no oxygen comes in. 

Tip: the burn-in formula

You will be astonished, the coffee has increased in volume due to the roasting - just try to put the coffee back in the same container as before the roasting. At the same time, the coffee has lost weight. Write down the weight loss percentage (the so-called burn-in 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 about:

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


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Ingo Albrecht Kaffee

More questions? We're here for you.

Do you have any further questions about the roaster? I'm a trained roaster and a member of the Specialty Coffee Association and the Roasters Guild of Europe, working with the machine every day. Please contact me at any time.

Telephone +41 76 261 97 17 or Email ingo[at]

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