The ultimate guide to coffee roasting

When cupping is over and the coffee roaster has decided to actually purchase the beans, it is time to roast the batch. Dive deep into the world of coffee roasting with our comprehensive guide. Uncover the intricacies of bean selection, roasting techniques, and flavor profiling. 

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1. Coffee roasting
2. Tools
   - batch roaster
   - Computer controlled tools
   - Colour meter
3. Coffee roasting and it's influence on flavour
4. The roasting process
5. Roasting phases
   - Green phase
   - Yellow phase
   - Cinnamon phase
   - The first crack
   - End phase
   - Cooling the bean
6. Roasting supermarket coffee vs. specialty coffee
7. Roasting for espresso vs. filter coffee
8. Blends & roasting
9. Packaging coffee

The characteristics of the coffee bean only develop truly during roasting. And of course we don‘t need to tell you that this process is pure craftsmanship. The roaster can raise the quality of the bean to a higher level or completely mess it up. Naturally, there is a big difference between roasting samples and a full production roast. The result will never be the same as the sample because, firstly and foremost, two completely different roasters are involved, and secondly because the amount of beans used for a sample roast – only 100 grams – can’t be compared with the amount in actual production roasts. So, the first roast of a new batch of beans can sometimes give the roaster a surprise.

Finding the right roasting process is not a simple task and there is often a lot of experience and skill involved. You don‘t become a Michelin-starred chef overnight, and the same goes for a good coffee roaster too. And every roast is different. The ambient temperature differs from summer to winter, the humidity varies, etc. A non-industrial roast takes, on average, between nine and fourteen minutes. Depending on the type of batch roaster and its size, the roaster has to perform several roasts, one after the other. Obviously, the batch roaster becomes much hotter over time than it was at the start of the day. That also influences the roasting process.

The coffee roaster must always take careful account of the actual temperature of the batch roaster, to achieve the desired result every time. That is why roasters start by roasting the most fragile coffees at the start of the day, because he or she has the temperature of the batch roaster more under control at the beginning of the ‘roasting day’. So, the coffee roaster must get to know and master the batch roaster installation from top to bottom. After the first roasting of a new batch of coffee, the coffee is cupped again, and there is possibly some fine tuning for the next roasting. Passion and subjectivity must be kept to a minimum during this phase. The roaster must, as it were, suppress all subjective feelings he or she has about a particular bean, plantation or grower. Objectivity must always prevail during roasting. Coffee is a science. To be able to practice the art of roasting to its full extent and to get the best out of the bean, you need the right tools – and a skilled roaster of course. 

To be able to practice the art of roasting to its full extent and to get the best out of the bean, you need the right tools – and a skilled roaster of course.

We work with two batch roasters, a 22 kg Probat and a 60 kg Probat, both made from cast iron and operating on natural gas. There are of course many other different types and sizes. The coffee beans are roasted in a rotating drum – to spread the heat well – and are then moved on to a special type of cooling grid. The temperature of the batch roaster must be controlled manually and this requires the necessary know-how. On average it takes about nine to fourteen minutes to roast 15 kg of coffee beans, which are then cooled on a cooling rack. By contrast, industrial machines only take about three minutes to roast 500 kg of beans, which are subsequently cooled with water.

In addition to the batch roaster itself, the coffee roaster also has computer-controlled tools that support every stage in the process, both to adjust and to record progress. We use these tools to maintain as much control as possible over every step of the process and to help us in our quest for the most consistent results possible when roasting.

The colour of the bean is a parameter we use to measure roasting objectively. Coffee roasters use a device known as a colour meter to distinguish small nuances in the shade of the coffee beans and to determine their precise colour. In addition, the coffee roaster can double-check his or her roasting results. At the end of the roasting process, a sample is always immediately taken and ground. Every sample is placed under the colour meter, which detects the colour and translates it into a graduated scale. This instrument helps us ensure consistent bean quality. But colour does not tell us everything. You can of course achieve a certain colour in various ways, and every adjustment to the roasting curve will have a direct influence on the flavour.

In the traditional method, green coffee beans are roasted in an enclosed rotating drum, where the heat is provided evenly and precisely controlled externally. This method does not permit roasting big batches of beans together, but it does properly roast each single bean. The drum rotating on its axis keeps the coffee beans constantly moving and the heat can work its way into them evenly. Roasting coffee beans is primarily a matter of finding the right balance between the flavour components, particularly acidity, sweetness and bitterness.

Before starting, the roaster must decide which way he or she is going to roast the beans. This is determined based on the results of the first cupping and the way the coffee will be brewed afterwards (e.g. filter or espresso). Espresso actually needs a different roasting from coffee for use with a filter. Some roasters swear by separate roastings for filter and espresso coffees. Others opt for an ‘omni-roast’, which means that they apply one type of roasting to each batch of beans, whether or not they will be used for filter or espresso.

Green coffee beans actually have no flavour of their own. Coffee beans only get their unique aroma, flavour and colour once they are roasted. The coffee beans are poured into the batch roaster when it is at an average temperature of 200 °C. The cold beans bring the temperature down to around 100 to 130 °C. The coffee roaster must then stoke up or ‘turn up the gas’ until the beans begin to evaporate moisture. Next all the excess moisture pops out of the beans. We call that the ‘first crack’. Then the beans start to turn brown, which is also called the Maillard reaction, and begin caramelizing.

Acidity is one of the first criteria we use to assess coffees. This term is often misinterpreted. To a layman, acidity is synonymous with ‘sour’. But nothing could be less true. The acidity in coffee is related to the freshness and the liveliness that one finds in it and it is closely connected to the coffee variety and its origin. African coffees usually have very high scores on acidity: in fact, acidity provides the ‘sparkle’ in the coffee and is really not negative at all. You can read more on acidity in the following blog:

As we have mentioned, coffee roasting is done in a large, slowly rotating drum, which is heated. The green coffee beans lose moisture through heating, which means a reduction of 15 to 20% of their original weight. So the roaster loses 15 to 20% of the original weight of the batch of beans he or she purchased – yet the beans get bigger as the temperature rises.

At a certain moment, the beans ‘pop’, just like popcorn. We call this the ‘first crack’. Then the bean starts to turn nicely brown (the Maillard reaction). The coffee also becomes drinkable after the first crack, and therefore soluble in water. However, it is too early to remove the bean from the batch roaster because the coffee still has a sour flavour at this stage. But don’t confuse this sourness with ‘acidity’, because the sourness (which you could call acidity) at this stage of the roasting process actually does have a negative connotation. Just after the first crack, it is the acidity in the bean that is emphasized.

As the roaster continues the roasting process, the coffee bean caramelizes and its sweetness becomes emphasized. At that moment, the acidity is somewhat tempered. If the roaster roasts the beans even longer, the bitterness primarily gains the upper hand and you only have a burnt taste left. All the other aromas and flavours are lost if this happens. You can’t put a fixed time on this, since every bean is different. It is mainly the roaster’s instinct or intuition that ensures that the beans are in the batch roaster for exactly the right time. So we highlight once again that the lighter brown the roasted coffee is, the fresher and fruitier its aroma will be. The darker it is, the harder and bitterer its flavour will be.

To achieve a good result, the roaster looks at, smells and listens to the coffee beans constantly. After a certain time the coffee will pop again, and that‘s the ‘second crack’. At this temperature the coffee bean is almost black; the outside of it looks like it has been oiled. and it now has only a burnt flavour. It is a great shame if a speciality coffee undergoes this dark roasting. It causes the bean to lose almost all its sparkle and sweetness, and the bitter and, later, burnt flavours dominate. The downside of lighter bean roasting is that other negative traits may also come to the fore. This means that the roaster must be absolutely certain that the beans are of high quality. It is in fact easier to cover up bad qualities when a bean is excessively dark-roasted. That is why we always roast samples of coffee beans lightly, so that we can detect any less-than-good qualities and therefore get a very true and clear view of the bean’s quality.

This is how the beans look at the start of the roasting process. In this phase, the temperature of the batch roaster is stoked up high immediately. After a couple of minutes, you can smell the aroma of freshly mown grass.

Once the beans reach this colour, the drying phase begins. You can smell the aroma of hay or dried grass. In the next stage, we bring the temperature down.

This is the phase in which you smell the aroma of freshly baked bread. If you are in the neighbourhood of our roastery, this is the aroma you will notice. This is the phase in which the Maillard reaction occurs.

In this phase, the bean can’t absorb the heat any more, and it starts to crack or pop. The remaining moisture in the bean comes out. The bean exudes heat instead of absorbing it. The roaster needs to reduce the temperature further at this stage.

We use the colour to decide when to stop roasting. This is the moment when the coffee roaster demonstrates his or her craft. Roasting slightly longer after the first crack ensures that acids are converted into sugars. The exact duration of roasting depends on the roasting profile, the type of bean, and the intended brewing method. A darker roast eventually breaks down all the flavours and results in a high level of bitterness.

The way the bean is cooled after roasting is also very important. After roasting, the bean goes onto a grid with cold air blowing through. At the moment when the coffee roaster decides that the roasting is perfect, the process must be finished off as quickly as possible. The bean must now cool immediately, because the oils in the coffee would make the bean smoulder, which could lead to a bitter flavour.

Unfortunately, most coffees are roasted excessively to eliminate the beans’ negative properties, and to create a flatter and therefore commercially more interesting coffee. The supermarket consumer expects the package from his or her usual brand to always taste the same, year after year.

With speciality coffee we work the other way around. We are starting with a natural product and, by its very nature, a coffee bean from the same plantation can’t always have the same flavour year after year. We are actually trying to bring all the exceptional aromas and flavours to the fore. Coffee is also a seasonal product. The picking season varies per country. ‘When it’s run out, there is no more’ is our logic. So coffee from a speciality roastery is actually the latest harvest. Once it is used up, the next region has its turn.

Speciality coffee is never roasted too far because the roaster is confident that he or she is working with top quality, and because over-roasting will completely eliminate all the good characteristics such as freshness and sweetness. Speciality coffee can’t guarantee a constant flavour, but that is precisely what makes it so fascinating. A light roast requires finesse. It is a balancing act to ensure that the coffee bean is not too lightly roasted but certainly not too much.

Espresso is like a small taste bomb, in which, under the influence of pressure and intensity, the dominant flavours are emphasized. If a coffee that already has a lot of acidity by its character is removed from the batch roaster right after the first crack, when the acidity is highest, and is then brewed with an espresso machine, the dominant factor in that coffee, in this case the acidity, will be excessively dominant, and the coffee will lack balance. That is why we have to roast beans for espresso differently from those for filter coffee. So the roaster needs to know beforehand the target brewing method for the coffee he or she is roasting.

Let‘s talk about blends. And no, blends are not just made by mixing anything and everything together. You need to invest a lot of time to find a beautifully balanced blend. Mind you, some blends are indeed a mixed bag of goodand poorer-quality beans. It goes without saying that we will not be talking about those here. A good blend requires time, effort and a lot love. Roasting a single-origin coffee is easier than composing a good blend. The roaster first needs a clear picture of the final product he or she wants to create. For example, different sizes of coffee beans demand different roasting methods. When the roaster is composing a blend with beans that differ in density and size, he or she will opt to roast them separately and combine them afterwards. But similar beans can possibly be roasted together. The advantage of a well-composed blend is that it produces a more complex coffee with a broader flavour profile. To achieve this, the coffee roaster will look for beans that complement and strengthen each other’s qualities. If he or she succeeds, the reward is a distinctive coffee with great character.

It goes without saying that the further the coffee is away from the roasting process, the less fresh it is. It is important that coffee is not too old when consumed. We recommend consuming it within a month for optimal enjoyment of all the bean’s finer characteristics. The coffee will not actually be bad after a month, of course, but the strength of its aromas depletes as time goes by.

Naturally, the method of packaging also plays a role in keeping the coffee fresh. Bags with a one-way valve offer a good way to pack coffee. The valve ensures that the coffee can de-gas without air entering the bag. Coffee packed in this way is always super fresh. So you can’t drink coffee immediately after the roasting process, because it needs time to de-gas sufficiently first. In the best-case scenario, the beans rest for four to five days in the packaging – in which they can de-gas – before they are consumed. Only then do they show their true nature. The darker the bean, the more gases it contains. That is why beans for espresso should rest for a week, while those for filter coffee can be brewed and drunk earlier.

Here are some tips & tricks to store your coffee beans, even if the best way to store them is not at all!