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If you ever get the chance to open a sack of green coffee, the first surprise is the smell. Raw coffee smells nothing like coffee. It smells of cut grass and dried peas, it is hard enough to chip a tooth, and if you managed to brew it you would get something thin, sour and vegetal.
Everything you love about your morning cup – the chocolate, the caramel, the fruit, that rush of aroma when you open the bag – is created in a matter of minutes inside our roaster. It is the part of coffee's long journey that we know most intimately, so we thought we would take you inside the drum. No chemistry degree required, but by the end you will know why your beans look, smell and taste the way they do.
Stage One: Drying Out
Green coffee arrives with roughly 10–12% of its weight still made up of water. Once the beans are tipped into the hot, rotating drum, the first job is to drive that moisture out, gently and evenly.
For the first few minutes not much seems to happen. Then the beans turn from green to a pale straw yellow, at around 150°C, and the smell in the roastery shifts from grassy to something closer to hay, and then to baking bread.
It is the least glamorous stage, but it sets up everything that follows. Rush it and the outside of the bean scorches before the centre has caught up. Let it drag and the finished coffee can taste flat and papery, with the sweetness quietly cooked out of it.
Stage Two: Browning, Where Flavour Is Born
Once the beans are dry enough, the real chemistry begins. The headline act is the Maillard reaction – the same family of reactions that browns your toast, gives bread its crust and puts the savoury edge on a seared steak. Sugars and amino acids in the bean react with one another and produce hundreds of new compounds. Researchers have identified more than 800 aroma compounds in roasted coffee, and the vast majority of them do not exist in the green bean at all.
Running alongside it is caramelisation, as the bean's natural sugars begin to break down. Between the two, the beans move from yellow to cinnamon to light brown, and much of the sweetness you taste in your cup is built here.
This is why two coffees from the same farm can taste noticeably different in the hands of two roasters. The green bean holds the potential; how it is guided through these minutes decides how much of that potential ends up in your cup.
Stage Three: Listen for First Crack
At around 200°C, something audible happens. The steam and gases trapped inside each bean build up enough pressure to fracture its structure, and the drum fills with a series of distinct pops. Roasters call it first crack, and it is the single most important landmark of any roast.
The beans swell as it happens, growing by half or more of their original size, and the surface smooths out. From this point on, every second counts. The time between first crack and the end of the roast is known as development, and it is where we make most of our decisions about how a coffee will taste.
Keep development short and you hold on to more of the bean's natural acidity and fruit. Let it run a little longer and the body fills out, the edges round off and the sweetness deepens towards caramel and chocolate. Go too far and you start trading sweetness for roastiness.
Most of our coffees, including our medium roast blends and single origins, are taken out of the drum during this window, at the point where we think each one tastes most like itself.
Second Crack and the Darker Side
Carry on roasting and a second round of cracking arrives: quieter, faster and higher-pitched than the first, as the bean's structure starts to break down further. Oils migrate to the surface, which is why dark roast beans have that glossy sheen, and the smoke coming off the drum turns darker and more pungent.
The further a coffee travels in this direction, the more the flavours of the roast itself take over from the flavours of its origin – smoky, bittersweet, deep dark chocolate. That is a choice rather than a flaw. Plenty of people love a darker cup, which is exactly why Usiku exists as our dark roast blend.
One thing roast level does not change much is caffeine. Darker roasts taste bolder, but they are not meaningfully stronger, a myth we tackled in our post on three specialty coffee myths.
Drop, Cool, Rest
When a batch reaches the point we are aiming for, we drop it out of the drum into a cooling tray, where it is stirred and air is pulled through it. Speed matters here: a bean straight from the roaster is hot enough to keep cooking itself, so cooling quickly locks in the roast we chose rather than a slightly darker one.
Then the beans rest. Roasting leaves them packed with carbon dioxide, which needs time to escape before the coffee is at its best, and it is the same gas that makes your grounds bubble and bloom when the hot water first hits them. We explain why we never skip this step in our post on resting coffee before packing.
What This Means for Your Cup
A little roasting knowledge goes a surprisingly long way at home.
Your bag weighs less than the coffee we started with. Beans lose somewhere between 12% and 20% of their weight in the roaster, mostly as water. To fill a 1kg bag of medium roast, we need to roast roughly 1.2kg of green coffee – one of the reasons roasted coffee costs more per kilo than the green coffee prices you see in the news.
Darker beans grind and brew differently. The longer a bean roasts, the lighter and more porous it becomes, and the more readily it gives up its flavour to water. If you switch from a medium roast to a dark one, you may find a slightly coarser grind or a touch cooler water tastes better. Going the other way, a medium roast can take a little more time or a finer grind.
Oily beans need a little more care. The oils on a dark roast are exposed to the air, so those beans lose their freshness faster. Keep them sealed, cool and away from light, and use them up while they are at their best.
You can taste the roast for yourself. The easiest way to understand everything above is to try two coffees side by side. Brew our medium roast Nyumba next to our dark roast Usiku, both in our specialty coffee blends collection, and notice where the brightness and sweetness sit in one, and where the roast takes the lead in the other.
Roasting is only half of the story, of course. The other half happens at origin, long before the beans reach us – and if you are curious about that side, our guide to washed, natural and anaerobic processing picks up the thread.
Every batch we roast is a short run of careful decisions: how quickly to dry, how long to brown, when to listen for the crack and when to let go. The farmer grows the potential and we try to do it justice in the minutes it spends with us. Next time you open a fresh bag and that first wave of aroma reaches you, you will know exactly where it came from.
