Roasting coffee and how it works

A coffee bean consists of approximately 300,000 to 400,000 cells. During roasting, very complex chemical reactions take place when heat is applied. During this process, sugars and amino acids combine in new ways, so that an estimated 1,000 new aromas or chemical compounds are formed in each individual cell during roasting. Coffee is thus one of the most aromatic foods of all and has significantly more aromatic substances than wine.

We roast our coffees in our coffee roastery in Berlin Wedding in the traditional Probat drum roaster. This allows the coffee aromas to fully develop. Our coffees are roasted in batches of 40 to 60 kg with roasting times that vary between 12 and 19 minutes. The roasting time depends on the desired roasting result. The final temperatures range from 200°C for light filter coffee roasts to 220°C for our Grano Gayo Italian roast.

The energy required for roasting is obtained indirectly through contact with the externally heated roasting drum and the heated air. After completion of the roasting process, the coffee must be cooled down quickly. This is done by introducing cold air on cooling sieves and completely without water. The weight loss of the coffee, which occurs due to the loss of water in the beans and chemical transformation processes (also called Einbrand), can be up to 18% with this traditional method.

The roasting plays a decisive role in the quality of coffee.

We roast our coffees over a longer period of time and at comparatively low heat, so that the flavours can develop clearly. Only the desired, fine fruit acids that contribute to the character of our coffee are retained. We roast our espressos a little longer than our filter coffees. In this way, unwanted acids disappear almost completely and make way for the strong taste typical of espresso. All in all, the drum roasting process is a complex and demanding process that most effectively breaks down the aggressive chlorogenic acids that irritate the stomach wall.

The initially pale green and hay-scented green coffee is placed in the preheated roaster. The heat dries the remaining moisture out of the beans and the colour of the beans gradually turns yellow. After about 7 to 10 minutes, the beans start turning brown and a spicy smell develops, reminiscent of popcorn or freshly baked bread. Due to the new gases (carbon dioxide and monoxide) forming in the beans, the beans increase significantly in volume.

After about 12 to 15 minutes, the pressure in the beans is so great that the cell walls of the coffee bean audibly burst open. This moment is called the “first crack”. From now on, the entire spectrum of taste diversity unfolds. The fruit acids are broken down, sugars are caramelised and the cell structure is weakened. At this point, the Roastmaster has to watch the process very closely, because the beans are only 2 to 4 minutes away from charring. During this period, the roasted coffee’s colour changes from light brown to medium brown, dark brown and black-brown.

When the roasting process is complete, the coffee falls from the roasting drum into the cooling sieve. Here the hot coffee is cooled down by means of cold air and an agitator. The beans then pass through a „de-stoner“, where possible small stones, which are sometimes present in the green coffee, are removed. The roasted coffee is then packed and needs a few more days to de-gas. Only when it has been given this „rest period“ of about five to ten days can it develop its full aroma.

The Roastmaster’s art is to determine how to control the heat input to the roasting machine, thereby ensuring optimum aroma development and getting the most out of a green coffee. Once we have developed an optimal „roasting curve“ for each coffee, the roasting machine technology helps us to reproduce this profile as consistently as possible. During the roasting process, even a few seconds difference in roasting time or minimal temperature deviations during roasting can lead to differences in taste. With increased roasting time, the acids are reduced and the roasting aromas increase. The longer the coffee is roasted, the stronger and more bitter it becomes. The coffee’s individual taste recedes more and more into the background. The Roastmaster determines the degree of roasting and the final temperature depending on the purpose for which the beans are being roasted. When roasting espresso beans, a stronger taste with less acidity is usually the desired outcome, which is why espressos tend to be roasted longer. For filter coffee, on the other hand, a slight acidity contributes to the liveliness of the flavour, which is why it is roasted for a shorter time. With most of our „Limited Edition“ coffees, which are characterised by their light roasting, the Roastmaster stops the roasting process shortly after the First Crack. In Scandinavia, for example, this type of roasting is very widespread. If you drink a filter coffee there, it probably reminds you more of a cup of tea than of traditional coffee. On the other hand, if you visit a café in Italy, you will find strong roasting aromas and a bitter taste in your cup, because the Italians prefer a dark roast, comparable to our Grano Gayo coffee.