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All the basics to know about coffee roasting

June 30, 2021

All the basics to know about coffee roasting - Impact Roasters


Impact Roasters was created after our founder’s passion for coffee and roasting, and we would like to share it with you. Here is our easy, straightforward guide to coffee roasting, a no-fuss, no confusing technical stuff. Only the basics and what we deem important to know about roasting, so you can get a good understanding of what we do! So let’s get into this! 


As you probably know, coffee beans are the seeds inside the coffee cherry. Once they are processed, meaning that the fruit has been removed from the seeds, the beans are green. They are sold and stored that way to preserve them and their flavors.

To put it simply, roasting coffee is cooking the raw beans so they can be brewed. The roasting process will bring out all the aroma compounds inside the beans and give your coffee its flavor – and its characteristic brown color. Basically, the roasting process is the necessary final step to get your wonderful coffee.

During the roasting process, the beans undergo a complete transformation. First, there is a chemical transformation during which the sugars and acids in the fruit are changed and brought out to balance each other. In a way, this process replicates the ripening of the coffee fruit but for the beans.

Secondly, there is a physical transformation, as the beans change colors and go from green to brown. Basically, the beans become charcoal as they burn under the heat and water evaporates. This is also the process that leads the bean to explode and crack during the roasting: the heat causes pressure to accumulate inside the beans, and when they can’t take more, well, they explode - literally. Their walls crack to let off some steam, and that’s what causes the crack you see on your beans.


Roasting is done in a special machine called a roaster that will heat the coffee between 180° and 230°C inside a furnace for about 15 minutes. The beans will be stirred continuously during the process so they can all roast evenly. We can break down roasting into four main stages: drying, browning, development, and cooling. The timing and settings described below are based on the machine we use, the Giesen W60, so it may vary from one roaster to another, the beans you roast, or from one roasting style to another.

First is the drying stage. As you may have guessed, it implies drying out the beans to draw out the moisture inside the green beans. The cold beans are dropped into the hot drum that’s been preheated for 20-30 minutes. The beans will turn into a yellow color as the moisture goes down. This stage will usually last for about 6 minutes.

Then comes the browning stage, or Maillard phase, which lasts 5 minutes. The temperature is going up, and the heat starts to develop the aroma compounds locked inside the beans. The Maillard reaction takes place and causes the brown color, hence the name of this phase. The beans keep on getting darker and darker until the next stage. Flavors start to develop, and caramelization begins as the sugars inside the beans cook.

The development stage starts at the 11th minute and lasts two minutes (minutes might slightly differ depending on the bean profile). The beans pop in the roaster because the carbon dioxide inside the beans is released, so they explode, giving them their characteristic crack in the middle. This first crack is crucial in roasting: it indicates the passage from the first phase to the second. All the flavors developed before are locked and will be developed from this point on.

The first crack is also the starting point of the second phase: roasters can fine-tune the desired taste for the coffee. All the acidity, aromas, and bitterness develop in waves. Master roasters decide when to stop the cooking of the beans when they reach their desired roast degree.

Finally, at the end of the roast, the beans are taken out of the drum and dumped into a cooling tray. On our Giesen W60, the beans are released from the drum into the cooling fan, which has a stirrer. This may seem simple, but the ending point is actually super important as it determines the final depth of all the flavors developed during the process. It also determines the roast level of the coffee. Basically, missing the ending point can make all the precious efforts go to waste, so it mustn’t be looked down on.

The beans are left to cool for 3 to 5 minutes. It is important that they cool quickly to prevent the beans from baking. Baking will develop some off-flavors in the beans and take away some clarity of taste. Put simply, the flavors carefully brought out during the roasting will be muddied up, so again, it’s essential to pay attention to these last steps.


Length of the process and roast degree

Now, let’s talk about some things we haven’t covered up but are part of the basics in coffee roasting: the length and the roast degree.

The time the roasting process takes will depend on the wanted flavor and roast degree. Like everything else, both these elements will impact the flavor of the final cup, so roasters usually choose the flavor profile they aim for and adjust roasting time and degrees accordingly.

A faster roast will preserve and develop most of the beans’ aromas and flavors, but there is a higher risk of burning the beans. A slower roast will suppress some unwanted aroma in the coffee, for instance, the acidity of the beans brewed in espresso. Because the heat reduces the acidity, slower roasts tend to be gentler in taste.

The roast degree is also important to keep track of the flavors developed in the coffee. Basically, there are three roast profiles: light, medium, and dark, going from a light brown color to more dark brown. A light roast is more acidic, with fruity flavors. A darker roast tends more towards a bitterness.

Put simply, a darker roast draws out the acidity in the beans to develop a bitterness, while a light roast preserves the aromas of the raw coffee. Be careful not to roast too dark, though: usually, you can see commercial coffee being roasted very dark, to almost a black color, with a shiny and oily appearance. That is no good, as the coffee has basically lost all its quality.

At Impact Roasters, we choose lighter roasts to preserve all the flavors’ characteristics of our Ethiopian coffees, like most other specialty coffee roasters.

Flavor development in the first and second stages

To get the flavor of the roasted coffee spot on, it is crucial to understand the role of the first and second phases. Though they are linked, they each develop different flavors in the coffee. Each phase must be carefully mastered, as they can’t be fixed or can’t hide the other’s flaws.

The first phase develops the sugars and acids in the coffee. The final cup will be too sweet if it's too long, without much acidity as it will go down, with a bitter end. However, if it is too short, there will be an overpowering acidic taste, as the sugars in the coffee didn’t have the time to caramelize and develop. This leads to an overall granny taste, with a short aftertaste.

The second phase develops the final overall taste, the burnt flavors, and influences how long the taste will last in your mouth. It uses the flavors developed in the first phase and builds upon them. If the second phase is too long, there will be too many baked and burnt notes with an empty taste: the beans have been roasted too dark, and all the sugars and acid turned bitter. If it’s too short, the coffee will lack sweetness, and the aftertaste will be short, resulting in a too-acidic coffee.

Roasting coffee is an art that needs time and practice to be mastered. Though it doesn’t have many steps and seems simple overall, it is the little details that separate a good roast from a great roast. Roasting coffee has many more secrets to uncover, and this is only the beginning of your learning journey.

We hope this article satisfied your curiosity and gave you a good understanding of what we do here at Impact Roasters. We love to show and share our passion with our community, so feel free to stop by and watch us roasting our beautiful coffee in Valby, Langgade, and Peter Bangs Vej roasteries!

If you can’t make it in person to one of our shops, you can find all our roasted beans on our website.


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