What happens to the grain during roasting


During the roasting process, the coffee bean changes its chemical structure. As a green coffee, the beans are small, high in density and smell delicately sweet and rather grassy. After smelling them, you wouldn't expect them to smell like the coffee you know and love. All the flavours we love in coffee are encoded in the green beans, but they need to be unlocked. This happens during the roasting process, during which (among other things) 800 to 1,000 aromatic compounds develop in the coffee.


Mastering this transformation requires great experience and knowledge. Just as a barista goes through various types of training and practice, learning how to set up the grinder and operate the machine to create the perfect cup of espresso, so too does a roaster need long-term training and practice.

Roasting coffee is not easy. After all, each coffee needs a different temperature and a different roasting time. Therefore, the roaster normally first roasts a small amount of coffee on a small roaster. He usually roasts at least three different samples. In a small roaster, this process takes about 8 to 12 minutes.

These samples are then tastedtogether . During the tasting, the individual flavours of all the samples are analysed and the best one is selected. This is then matched with other specifications that can be set on the large roaster. The roaster is tasked with finding the best curve, or profile, for the coffee . This means finding the ideal temperature and time sequence. You have to have years of experience profiling coffees, otherwise a lot of coffee goes to waste.


Roasting is another coffee roasting profession that requires you to master the technology and terminology in the first place. In order to roast at all, you need the main thing, and that is the roaster. A big machine with a lot of functions that a professional roaster must of course know how to operate.

There are two types of roasters in use today, the classic - you could almost say retro - drum roaster and the fluid bed roaster. In our roaster we use the Dutch drum roaster Giesen W15A. It has a maximum capacity of 15 kg of coffee which is sufficient for medium sized roasters.

In modern times the roasting process is controlled by computers. Nevertheless, the human element is still indispensable in this process. Throughout the roasting process, the roaster monitors the coffee, takes out samples and eventually adjusts the temperature, air quantity, drum speed and roasting time as needed.

Large roasters, which produce huge quantities of coffee and roast in large roasters, leave this process entirely to computer technology. It's like a barista working with a fully automatic coffee machine. The machine does all the work for him, but the end product is not as good. For this reason, you should alwaysreach for aselection of coffee that you can be sure is fresh and of good quality.

A classic drum coffee roaster from the traditional manufacturer Giesen together with modern technology and an experienced roaster Zdenek is the way to create the perfectly roasted coffee we all love. Image source: Spa Coffee


Different roasting profiles have an impact on the resulting taste and aroma of the coffee. In total, there are three main roasting stages:

  • Drying,
  • the Maillard reaction,
  • development.

These terms describe different stages of chemical and physical changes.


Did you know that green beans contain 10 - 12% moisture? Without a reduction in this moisture, browning of the bean cannot occur. Once the coffee reaches the roaster drum, water starts toevaporate and pressure starts to build up inside the beans. The drying phase ends at 150°C. After 150°C, the second phase begins.

Green coffee beans still contain some moisture. This evaporates in the first few moments of roasting. Image source: Spa coffee


The Maillard reaction is behind the important chemical reactions that take place in the beans at the beginning of roasting. It is the most widely used reaction in the food industry and causes the non-enzymatic browning and the accentuation of typical coffee flavours and aromas. It is a complex series of reactions between amino acids and sugars. Simply put, at different temperatures, the proteins together with the sugars begin to caramelise. In coffee beans specifically, the start of the Maillard reaction can be traced to the beginning of the first crack.

Small changes in temperature and the length of time spent in the Maillard reaction can have a big impact on the final coffee profile. For example, coffee that spends more time in the Maillard reaction will increase its viscosity. A shorter time, on the other hand, can enhance the perception of sweetness and acidity. In fact, if coffee is left in the Maillard reaction for too long, the acids that produce fruity and sweet notes are destroyed.


Strecker degradation is a process that depends on the Maillard reaction. Amino acids react with carbonyl groups to form compounds such as aldehydes and ketones. This reaction is necessary for the formation of aromatics.


After first crack, roasting changes from an endothermic reaction (the grains absorb heat from the drum) to an exothermic reaction (the grains release heat). During this phase the physical transformations continue - the grains increase their porosity, the oils migrate to the cell walls and the colour darkens. Many other chemical reactions occur.


  • airflow
  • temperature and time
  • batch weight


Airflow during roasting plays an indispensable role in the even transfer of heat inside the drum and carries away unwanted smoke and impurities produced during the roasting process. The airflow in the roasting drum must be controlled very carefully as it affects the moisture content of the roasting beans and thus their final sweetness. It alsohas an impact on the overall flavour profile of your roasted coffee ( coffee's cup profile). Preferable roasters are those where the roaster can control this variable, its intensity and the time it takes to affect the beans. Our Giesen W15A roaster can do this.


It is a good idea to monitor the temperature in the drum before the beans are thrown in and roasting begins. According to the experience of the roaster and the weight of the batch of beans to be roasted, adjust/adjust it accordingly. This temperature can significantly affect the heating rate and thus the roasting speed. For example, fresh beans need a lower initial temperature, while beans that have been roasted or are more dry need a slightly higher temperature.

The temperature inside the roasting drum is also an important variable, which suggests different roasting stages and chemical reactions in the beans. The temperature can also be adjusted during the process. If the temperature is too high, the heating process naturally proceeds faster than desired, and it is easy for the beans to spend insufficient time in the Maillard reaction and for the roasted coffee to be bland and tasteless. Likewise coffee that has been roasted at a lower temperature and has barely reached the Maillard reaction.


In order to keep a firm grip on the roasting process, it is essential to have a good estimate of the quantity of the batch to be roasted. Many roasters recommend not filling the roaster to capacity, but working with a batch up to about 80% of the roaster's capacity. Each roaster behaves a little differently, heating up differently, also due to the material it is made of. Also each type of coffee is a little different, differently moist, the beans may be differently sized, rinsed or not. A good roaster must take all this into account when determining the batch.


The moment the coffee beans begin to brown, the Maillard reaction begins. This process starts at a temperature of around 150 °C. The beans are still absorbing heat and continue the exothermic part of the roasting process. The heat causes a reaction between the carbohydrates and the amino acids in the beans. This causes changes in colour and taste. The colour change is caused by the production of melanoidins.

Gases including carbon dioxide, water vapour and some volatile compounds are produced during the Maillard reaction. Theinternal pressure increases enough to break the cell walls of the grain and thefirst crackoccurs - the so-called first crack.

There are high temperatures in the roaster at all times, which change during the roasting process. Thestarches are converted into sugars which gradually caramelise. At a temperature of around 170 °C, the heat causes the large complex carbohydrates to break down into smaller sugar molecules that can dissolve in water. This reaction continues until the end of roasting and contributes to the sweet notes in the coffee. As a result, you can taste notes of caramel or almonds, for example, in the coffee.

Acids are also broken down and new ones are formed. Thecellular structure cracks at the first crack due to the high temperatures and all the moisture and carbon dioxide escapes from the bean. The grain changes size. Gradually, the proteins are broken downinto peptides, resulting in the oils that can be seen on the surface of the grain with prolonged roasting. The burnt grains contain carbon in them. Almost all of the substances contained in the roasted beans are water soluble, allowing us to infuse the coffee in water.


Volatile compounds are generally responsible for aroma, while non-volatile compounds contribute to flavour. But what are these substances and what exactly do they give us?


Volatile compounds are organic compounds that evaporate at room temperature. Many of them are formed in the Strecker degradation process we wrote about above. When the volatile compounds that produce the aroma dissipate, we smell the aroma of coffee. Volatile compounds include:

  • Aldehydes, which give a fruity, green aroma.
  • Furans, which contribute their typical caramel aroma.
  • Pyrazines, which contribute anearthy aroma.
  • Sulphur-containing compounds, including 2-furfurylthiol. Some of these have an aroma that is commonly described as 'roasted coffee', but there are others that do not smell very appealing. (For example, methanethiol smells like rotten cabbage).
  • Guaiacol, which has smoky and spicy notes.


Non-volatile compounds are simply substances that are stable at room temperature. This means that they do not evaporate. Some of these compounds change during roasting, while others remain stable during the process. Many non-volatile compounds contribute to the final flavour profile of the coffee.

Examples include caffeine, which is responsible for some bitter flavours. Caffeine occurs naturally in coffee and remains unchanged by the roasting process. Other non-volatile compounds include sucrose, which provides sweetness, and lipids, which make up the body and resulting mouthfeel. Melanoidins, which we wrote about above, create colour and body and are also non-volatile compounds.


Acids play an important role in creating the final taste. They are very sensitive to heat, and roasting can degrade some acids and create others.

Citric and tartaric acids, for example , produce fruity and sweet notes that degrade during roasting. If you roast coffee beans for too long and at high temperatures, you will reduce the overall sweetness of the resulting profile.

In addition, coffee contains high levels of chlorogenic acid, which breaks down into caffeic and quinic acid during roasting . Chlorogenic acid and quinic acid are thought to give coffee its bitterness and astringency.


The roaster consists of several important parts. It has a funnel at the top into which the roaster pours a precise dose of green beans. At this point, the roaster must be heated to a high temperature.

The roaster's job is all about experience and skill in roasting coffee. It also requires a certain physical fitness to carry the bags of coffee and pour the green coffee up into the hopper of the roaster. Image source: Spa coffee

The green beans go into the roaster drum, which rotates throughout the roasting process. This allows all beans to be roasted equally and evenly. The moment the green beans fall into the drum, the temperature inside the roaster drops rapidly. The beans gain temperature by contact with the heated drum. Roasting takes about 8 to 15 minutes. The beans reduce in weight during roasting as the water evaporates from them.

The coffee roasting process must be controlled for a flawless result. In our company, the roasting process is supervised by Zdeněk, who checks the beans in the roaster throughout the roasting process. Image source: Spa Coffee

Thebeans gradually darken. Around 190 - 200°C you will hear the first crack, the so-called first crack.

The fresher the coffee is, the more cracking you will hear. During this cracking ,the bean noticeably increases in size. You can think of this process as the popping of popcorn .

It isonly now that important chemical changes begin to take place in the kernel. Sugars appear and gradually caramelize, and theacidity gradually subsides. When roasted ideally, you can taste the true flavours of the variety itself in the resulting coffee.

At an even higher temperature it is possible to achieve asecond crack. However, we do not roast for second crack in our roastery. Because if you roast the beans longer, they are overheated. Over-roasted coffee tastes bitter and you only feel the strength of the roast, not the original and original flavours of the coffee variety.


After roasting, the roaster empties the beans from the drum into a special circular tank where the coffee is cooled. It is a wonderful moment that is always eagerly awaited. The hot and brown beans finally fall out of the roaster. The beans go into a cooler where they must cool rapidly so that no further chemical processes take place.

Special rakes circulate in the cooler to stir the roasted grains, allowing them to cool well and quickly. This cooling process is wonderful. Thousands of beans swirl around and the air is filled with the perfect aroma.


After cooling, the beans go into special containers where the coffee is left to sit for a few days, usually around two days. In smaller roasters, for example, the roasted coffee is stored in barrels. Various gases escape from the coffee and the beans await packaging.


Coffee beans from roasters are most often packed in special leak-proof bags that have a special one-way valve on them. This allows excess gases to escape. The valve only works towards the outside of the pack to prevent air from getting in.


  1. Light Cinnamon - A very light brown bean with a taste like the bean has been toasted, distinct sour notes.
  2. Cinnamon - A light brown bean, you can still taste the toasted flavour and strong sour notes.
  3. New England - Light brown grain color with still a sour taste, but not as much of a bready taste. At this point the first crack begins.
  4. American, Light Gity - Slightly darker light color, sweeter taste. First crack ends at this point.
  5. City Plus, Medium - Medium brown grain color, while feeling all the important flavors at once.
  6. Full City - Medium dark brown color, oils start to appear on the surface, sweet bitter notes.
  7. Light French, Viennese - Dark brown colour with more oil flecks, more bittersweet flavours, caramel notes. The grains gradually lose acidity.
  8. French - Dark brown grains, very pronounced oils on the surface, slightly burnt notes, acidity fading.
  9. Halian - Very dark coffee beans, stronger burnt notes, no acidity.
  10. Spanish - Very dark beans, almost black. Oily and you can almost taste the charcoal. Empty, bitter, burnt taste.