Welcome back to our series leading you through the coffee journey, from farm to cup. Last week we looked at what happens at the very beginning, way back at the farm when coffee is harvested and subsequently exported. Now let’s take a closer look at what happens to your beans when they are roasted…

There are three equally important milestones on the coffee journey that will ultimately affect your enjoyment when it comes to drinking coffee - harvest, roasting and brewing. The transformations that your beans go through at each stage are really unlike most natural products. Just think, they start on a beautiful plant, from which cherries are grown, then the seed from that cherry is pulled out and dried to become a raw green bean. During roasting the green beans are transformed into what will eventually resemble what we all know to be a ‘coffee bean’.

Weighing and Blending

There are two schools of thought when it comes to blending coffees, you can do it before or after it has been roasted. Different roasters will be able to give you different reasons why it’s better to do it one way over, the but the principle remains the same for both. We create blends to combine the best characteristics of two or more origins, to create a new unique flavour. For example, you might like the sweet nuttiness from a Brazil but adding a little freshness to the mix could really balance it out, so you might go 70% Brazil to 30% Colombia.

Espresso roasts have traditionally contained some Robusta and some Arabica (especially for Italian style coffee, it's about 30/70), but it's becoming less popular now in the UK as our tastes are evolving. So blending can be an important first step for the roaster and it’s usually weighed out in food safe buckets.

What is a Coffee Roaster?

Traditional coffee roasters are relatively simple machines - imagine if an oven married a tumble dryer and you’re pretty much there. They can come in all sorts of shapes and sizes from a 1Kg sample roaster to huge industrial models designed to roast hundreds of Kg’s at a time.

The key goal of the roaster is to transmit heat to each individual bean evenly and consistently - hence the turning drum. The drum is commonly suspended over a heat source that will either heat the drum directly or heat the air around the drum, which one you use really depends on your preference - it’s a bit like choosing to drive a BMW or an Audi, they both do the same thing, just in their own way.

Roasters are usually powered by gas but you can still find old fashioned wood burning models, although these aren’t very popular as you might expect. Quite imaginatively, the person who operates the coffee roaster is called... the coffee roaster!


Once the roaster is up to temperature (that’s the machine not the person), the beans are dropped in a hopper at the top of the machine - most production facilities will have a mechanical loader that works a bit like a hoover, sucking the beans from the loading tray directly into the roaster.

There is a lot of information out there with regards to temperatures and colour scales that can be a little confusing, as it really depends on the beans and the equipment you're using. But as a rough guide here’s the changes that your beans will go through as each stage of roasting in our own very scientific fashion…

“That smells like hay…”

Yes it does doesn’t it! That’s because at the first stage of roasting, drying as it’s commonly known, moisture is escaping your beans very quickly and releasing steam with a grassy aroma. This will usually start when the bean’s internal temperature hits around 225℉ and the beans will still look quite pale.

“Now it smells like popcorn…”

Mmm, my favourite bit! Coffee roaster’s (the people) will record each roast to plot what they call ‘the curve’, which essentially represents the internal temperature of the beans throughout the roast. This used to be done on graph paper but there’s now software capable of delivering accurate data. If you look at the curve on a coffee roast profile you’ll always see that it looks just like a hill that goes, down, then up, then down again.

Up until around 350℉ the beans take on heat from the roaster, but what happens next is that the beans release their own heat energy. As moisture starts to evaporate very quickly from deep inside the bean it quite literally explodes resulting in a loud ‘CRACK’, just like popcorn - we call this ‘First Crack’. At this stage the beans tend to resemble the colour and shade of roasted peanuts.

Flavour Development

From here on, the roaster needs to be very careful maintain a steady reduction in heat that doesn’t cook the beans too much too soon, but also allows the flavours (really the caramelisation of the sugars) to develop - if you’ve had coffee that tastes a bit flat and grassy it’s probably been underdeveloped.

This stage is very important and as we’ve said before, speciality coffee requires a little love and tenderness when it comes to preserving those delicate refined flavours. By now the beans will be brown, how brown however really depends on the bean and the roast - this is why it's unreliable to measure your roast on colour alone.

Dark Roast

A darker roast is often what people might refer to as ‘strong’ coffee and associate with high caffeine content… that strength of flavour usually comes from the over-development of the beans (like char-grilling food) and caffeine content is not actually affected by how dark coffee is roasted - myth busted!

If you leave the coffee roasting for long enough after first crack, the moisture content in the beans goes so low that you actually start to draw the natural oils out of the beans too, which is why dark roasted beans look all shiny and leave more oily residue floating at the top of your brew - don't worry it’s perfectly safe to drink!

When the internal temperature reaches around 425℉ the bean will crack again, because the actual proteins and carbohydrates (the chemical makeup) of the bean transforms again. Naturally the beans will be very dark at this stage regardless of what beans you have used.

Chill the Beans

Once the roast is finished the coffee is dropped onto an air fan assisted cooling tray to stop the beans from cooking in their own generated heat - it’s tempting to put your hand in and feel them like warm pebbles but trust me, they're like little pellets of fire! This is also a great opportunity to inspect the beans to make sure they’re all about the same colour and to pick out any ones that are burned to a crisp or haven’t cooked enough.


It’s important to let roasted coffee rest and de-gas before brewing it. This can take around 5 days for the beans to release all of the carbon dioxide that they contain after roasting. Carbon Dioxide is gross, it makes coffee taste very stringent and can overpower all of the good flavours in the beans - even with perfect extraction during brewing your coffee can still taste like battery acid if it hasn’t been rested for long enough after roasting.

This is also why coffee bags have valves on them to release carbon dioxide without letting any oxygen in.

So, that’s our brief guide through what happens to your coffee when it’s roasted. There really is a great deal of science and detail that can be applied to the roasting process, all in the name of perfecting the roast profiles to deliver you with a delicious brew. Why not ask your barista or coffee merchant what different roast types they have in stock and taste a few this week?