How Do They Do It: Coffee Roasting

April 23, 2026

Anyone can roast coffee, but there's a lot more to it than meets the eye. We'd love to delve deeper into this with our essay "How Do They Do It: Coffee Roasting".

It often seems inaccessible, and questions like: How do you roast coffee, how exactly does it work? Can you learn it? And how on earth do you start? are frequently asked. Yet, it turns out to be simpler than you might think, with some expert knowledge.

At Drip Coffee, we have a conscious and distinct vision for how we roast our coffees: to prioritize our quality, we deliberately use shorter roasts (light roast), distinguishing between a roast for espresso and one for filter. We also strongly believe that through these two roasting methods, the aromas are best expressed in their respective brewing methods – for us, there is no one roast to rule them all; however, we fully support the idea that a certain roast can be used for any brewing method and can also be delicious.

Green coffee

Everything starts, of course, with the selection of green coffee: do you opt for a single origin or a blend¹ of different origins? In addition, the process – how the green coffee is processed at origin – will also have a significant influence.
An additional parameter for each green coffee is the moisture percentage, the residual water value (maximum 12%) which is necessary to allow aroma development during roasting: this is measured in advance with a special device.

Depending on the choice of green coffee, you therefore have a different starting situation. In many cases, this selection is also linked to the eventual brewing method of the final product.

The chosen green coffee is weighed in the final phase before roasting to the optimal weight for a specific batch, which depends on drum size, and prepared in the hopper² of the roaster.

We'd love to tell you more about green coffee in a new blog post.
¹ in case of pre-blend
² the funnel on top of the machine

The roasting process

The core business of a coffee roastery therefore takes place on the machine where we, as coffee roasters, still have a great deal of influence on how the coffee will ultimately taste. First and foremost, the roaster must be warmed up before you can let the green coffee into the drum – ideally, the machine should not warm up too quickly; we allocate at least 30 minutes for this at the start of the roasting day.

“𝘦𝘯𝘥𝘰𝘵𝘩𝘦𝘳𝘮𝘪𝘤 𝘳𝘦𝘢𝘤𝘵𝘪𝘰𝘯: 𝘸𝘩𝘦𝘳𝘦 𝘵𝘩𝘦 𝘣𝘦𝘢𝘯 𝘢𝘣𝘴𝘰𝘳𝘣𝘴 𝘵𝘩𝘦 𝘢𝘥𝘥𝘦𝘥 𝘦𝘯𝘦𝘳𝘨𝘺”

Once warmed up, the starting temperature of the roast depends on the type of beans you want to roast: a blend needs a different charge temperature than a single origin, among which there is also a lot of difference due to various processings. On our Probat P12, we often started our roasts at a drum temperature between 180ºC and 185ºC for batches of a maximum of 10kg. The higher starting temperature is necessary because the green coffee is poured 'cold' (read: at room temperature) into the drum.

Due to the high density of unroasted coffee, it is essential to put a lot of energy into the beans in the initial stages of roasting. This energy input is needed to heat the bean to its core, but also to vaporize the remaining moisture within it. We call this the endothermic reaction: where the bean absorbs the added energy.

When all the moisture has evaporated, the water vapor creates enormous pressure within the coffee bean – which loses density due to heating – and this built-up steam literally makes the bean crack. This is the first crack and a very important event within the roasting process, namely the moment when aroma development takes place and the point from which the coffee is consumable.
The nuance requires us to say that in this "development phase" we break down aroma components into the pleasant flavors we can recognize, because at the very beginning of this new stage, not enough breakdown has yet occurred, and you notice this in the high acidity. The more aroma components you break down, the more acidity you remove as a roaster, but also the more bitter notes you develop. The fact that first crack has a big impact is also physically noticeable, as the beans have approximately doubled in size.

“𝘧𝘰𝘳 𝘶𝘴 𝘵𝘩𝘦𝘳𝘦 𝘪𝘴 𝘯𝘰 𝙤𝙣𝙚 𝙧𝙤𝙖𝙨𝙩 𝙩𝙤 𝙧𝙪𝙡𝙚 𝙩𝙝𝙚𝙢 𝙖𝙡𝙡”

The right moment to stop the roast depends on the intended use for that coffee. In our vision, we want to highlight the prominent aromas, possibly with higher acidity, and for this, we use the light roast – here, we deliberately choose a short development time. Our espresso coffees require a "softer" approach, and therefore we allow the aromas to develop longer, to round off the higher acidity – something that the espresso machine would otherwise only emphasize negatively. From first crack onwards, we speak of the exothermic reaction: where the accumulated energy is released from the coffee beans.

After you, as a coffee roaster, have decided that you have achieved the most optimal aroma development, you remove the coffee from the drum by letting it fall into the cooling tray. There, the coffee is cooled by ambient air so that it no longer develops further. Only after this is the entire roasting process complete.

The roasting process itself can be divided into different phases:

Dry phase: the initial phase where the coffee is poured into the drum and energy build-up has yet to occur – endothermic reaction.
Browning stage: due to the heating of the beans, various stages of color change are observable; besides green, also yellow and light brown, where you can recognize the smell of dried grass and toast, respectively.

During the above phases, primarily the endothermic reaction takes place, and the color change is accompanied by the caramelization of sugars and the Maillard reaction (chemical reaction between amino acids and sugars).
1st crack: the point where built-up steam literally makes the bean crack and double in size.
Development phase: depending on the development time, the roaster develops the desired aromas here.

From 1st crack, you get an exothermic reaction where the accumulated energy is released from the coffee beans.
Cooling stage: the phase just before the end of roasting where the coffee is removed from the drum and cooled to prevent further aroma development. After approximately four minutes, the coffee is sufficiently cooled, and the end is reached.

The machine

Besides the parameters you control yourself, a decent machine is also indispensable. And they come with different possibilities, in different designs or for different markets.

The standard machines – primarily historically evolved this way – are drum roasters where a drum is inside the machine and, through slow rotation, "shakes up" the coffee and regulates the heat distribution (gas bed) so that the beans warm up through convection (hot air) and conduction (direct contact with the drum wall and other beans). Our Probat P12 is such a type of coffee roaster. In the same vein as Probat, we also have Giesen and Diedrich Roasters.
A closed circuit machine is a type of machine that reuses the hot air it generates to re-heat the drum. The most well-known brand here is Loring.

The machines discussed above are all drum roasters. There are also fluid bed roasters which have been on the market since the 1970s. Such machines pre-heat the air and primarily transfer energy through convection, and because the beans are constantly propelled into the air, they appear to float. Most of these roasters also operate entirely electrically.

An emerging player in electric roasters is Stronghold, whose machine has a vertical drum and an additional energy regulator, namely halogen lamps. This feature allows radiation to heat the bean deep into its core – which also carries the risk that too much radiation can over-roast the inside of the bean compared to the outside.

For all machines, it is also important to understand the thermodynamics of the machine, as this determines how each roasting profile or roasting plan should be created.

Thanks to years of experience with the Probat, but certainly also due to its reliability and consistency, Drip has chosen this brand.