Roasting Coffee: How Does it Work?


Before working in a roastery, I had no idea what the job entailed. It was only towards the end of my stint as a barista that I really grasped the importance of roasting and how it affects the coffee. I feel there is a lack of a somewhat simple explanation of what we do, so in this post I will give an overview of what our jobs as roasters are. I won’t get into the science part, as then it would be a book rather than a blog, and I’m not even nearly knowledgeable enough yet to try and share the chemistry that takes place during a roast. If that is something that interests you, I would recommend reading Scott Rao’s book “The Coffee Roaster’s Companion”.

I started in a roastery over two years ago now, and one thing has become clear: it’s a complicated process. In terms of time, speciality coffee roasting is only a blip on the scale. We are still in the very early stages. We are where wine was centuries ago. This is why the job is so exciting; there are so many secrets left to figure out. So many things have no definitive answer, and it’s quite amusing, albeit rather very interesting, listening to a lot of industry leaders in coffee making completely contradicting statements. Both statements may be true, but in my opinion, the common theme is: do what works for you. Every roastery is different and will have different ways of achieving the best results.

Roasting Machine


By Visitor7 (Own work) [CC BY-SA 3.0 (, via Wikimedia Commons

A roasting machine is actually quite a simple one. How it works is, you have the hopper on the top to load the coffee into. To empty the beans into the drum, where they will then be roasted, you usually push a handle or press a button attached to the hopper (making a guess on that), but this depends on the model being used. The drum is like the inside of a washing machine; it constantly turns and inside there are paddles to help rotate the beans to help ensure an even roast. Under the drum is the gas burner, which heats the air in and around the drum. You can change the power of the burner during the roast to help control the temperature of the air and drum. This air is constantly being pulled through by a fan. The air goes through a pipe and out the back of the roaster. You can control how quick the air is being pulled through; this is called air flow. When the roast is finished the coffee is emptied into the cooling tray, which has cold air being dragged through with the use of another fan.

Roast Profile

So when people come into the roastery for barista training with us at Cloud Picker, we give them a brief overview of what’s happening during a roast. The way I explain it to people who are completely unfamiliar is: essentially, our job, as roasters is to create cooking instructions for the coffee. Then the most important part is tasting the coffee and editing the instructions to try to get it perfect. Below, we have what is called a roast profile. This, ladies and gents, is our cooking instructions.

roast profile.png

This graph shows us our roast profile. The graph has two u shaped curves, one red, one blue. The blue u shaped curve is the temperature in the drum which we call our bean temperature. The red u shaped curve is the temperature of the air leaving in the pipe just after exiting the drum. This is called the air temperature.  The temperature of the two lines are shown by the numbers on the left, which are displayed in degrees celsius. The blue n shaped curve is the rate of rise of the bean temperature. This monitors the rate at which the temperature of the drum is changing. The reading shows us how many degrees the temperature is going up or down every 30s. So a reading of +3.9 means the temperature is rising by 3.9 degrees every 30 seconds. The last line, which is the curvy red one is the Air temp rate of rise. Which is monitored and displayed in the same way. You can ignore the shaded lines in the back.

Creating a Roast Profile

When we articulate a roast profile we have to take different things into consideration. The main two are: density of the bean and water moisture in the bean. When making a roast profile, the first thing you have to decide on is the temperature that you need the drum at; this is called our charge temperature. How do we decide on charge temp? Well, we find this by taking what we know about the bean, which is the density and the moisture content. The beans we work with are from around the world, in many different climates. So beans that are grown at higher altitudes take longer to grow, as temperatures are colder. Because they take longer to grow, they have a higher density. So the higher density in the bean the harder it is for the heat to get to the middle. As a result, we need to charge at a higher temperature. The optimum moisture content of a green bean is 10-11%. However it can vary from 8-11%. The moisture in the bean acts as a good heat conductor, so the higher the moisture the more heat you can apply.

Turning Point

After dropping the beans into the drum at the desired (charge) temperature, the first thing that will happen is the temperature will drop. This is because the beans you have just dropped into the drum are room temperature. The first main part of the roast is called the Turning point. This is when the heat in the drum stops decreasing and starts to increase again. This is a good indicator of how well you did with your charge temperature. The turning point should be around the 60-90 seconds after dropping the beans in. The rate of temperature change in the drum is monitored and is displayed by a curve which you can see in the graph above, which we call the rate of rise; this is the blue dotted line on the graph. The rate of rise is very important as this is what we look at while we try to anticipate what is going to happen before it happens.

Yellowing Point

               Next we have the Yellowing point. The yellowing point is when the beans go yellow…. this happens at around 150°C and indicates the start of the Maillard reaction. This is the chemical reaction that is so important in coffee, as it develops most of the flavour in the final cup.

First Crack

The First crack is when the beans physically crack within the drum. The pressure that has been building is violently released, releasing CO2 and water vapour. The beans expand in size and lose weight. The reaction is like when you have popcorn in a microwave and it starts to pop. Any time after first crack is what we call the development time. The development time is important to make sure the core of the bean has received sufficient energy to roast it. If not, you will get unpleasant, grassy/vegetal notes. After all, a coffee bean is a fruit! The ideal development time should be roughly 20% of the overall roast time.

Finally, we are ready to dump the coffee out of the roaster. We empty the beans into the cooling tray, where a fan cools them immediately. This is important as coffee is like an egg; if you don’t cool it the inside will keep cooking and the coffee will become baked and taste flat.

This is the end of the roasting process. To sum it up: we are trying to reach a desired predetermined end temperature, within a set amount of time, without applying too much energy to burn the coffee, but not so little that it doesn’t bake the coffee, which would result in a bland taste.

We then cup and cup and cup, as many times as we need to, to see how we can improve our cooking instructions to get the best possible outcome with the beans that we have. IF you are unfamiliar with cupping, please have a look at my previous post here


Cover photo By Dan Bollinger (Own work) [CC BY-SA 3.0 ( or GFDL (, via Wikimedia Commons

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