How Are Coffee Beans Roasted?

Welcome, coffee lovers! If you're here, you probably already know how delicious coffee can be, but did you know that the roasting process is essential for giving coffee beans their signature flavour and aroma? In this article, we'll take you through the basics of coffee roasting and explain why each step matters.


Green Coffee Beans

Hands holding green coffee beans

Did you know that before they are roasted, the coffee beans are actually green and have a grassy aroma? In fact, green coffee beans don’t smell like coffee at all. The coffee flavour develops when the beans are roasted, unfolding anywhere from 800 to 1,000 different aroma compounds. Which rivals wine. The green beans are also tough and strong before roasting, which is not ideal for trying to grind them up for a delicious cup of coffee.

Preparing for Roasting

Small-scale coffee roasting is possible at home, but it's a simpler process. We roast our beans in a large drum at commercial scale, which is more exciting and takes longer. First, the drum is preheated to 210-240 degrees Celsius, depending on the roast type.

The Roasting Process

In simple terms, the roasting stage is where the coffee beans change from green to brown. There are different ways and levels to the roasting that have a big impact on the flavour. The steps below are the basic events of the process. We go into more detail with the 3 stages further down.

Step 1: The bean loses moisture and shrinks in weight (the percentage will depend on the degree of roast).

Step 2: The bean physically expands.

Step 3: The bean will then turn from green to brown.

Step 4: It loses caffeine, as well as certain proteins and acids.

Step 5: The sugars are caramelised, which creates an oil on the surface of the bean.


The bean needs to lose moisture and shrink in weight (the percentage will depend on the degree of roast). This process tends to take 4-8 minutes, it's important at this stage that you don't burn the coffee beans. Once the temperature reaches around 160⁰C it then gets to the next stage.


Drying continues throughout this stage but the beans will start to turn brown. This is called the Maillard reaction, sugars and amino react to make different aroma and colour compounds. At the end of this stage the beans start to pop and this is called the first crack. 


Once the beans have gone through the first crack, you get into the development and roasting stage. The aromas and compounds develop the most in this stage and you have to start looking at slowing the roast down. If you don't slow the roast down at this stage you can burn the coffee and start to get a smoky taste. 

Cooling and Resting

Like cooking, cooling and resting coffee beans is essential for developing their flavour. As with all good things, this takes time. Coffee beans are typically rested for 5-14 days to allow their flavours to open up and bloom, resulting in a cup with more clarity and depth.

Roasting Techniques and Profiles

When it comes to coffee roasting, there are two main methods: drum roasting and hot air roasting.

Drum roasting is the traditional method, where coffee beans are roasted in a large rotating drum. The heat from the drum is transferred to the beans through conduction, which can lead to uneven roasting and a smoky flavour. Additionally, drum-roasted beans can vary in quality from batch to batch, as there is no guarantee of how long each bean is in contact with the hot metal.

Hot air roasting is a more modern method that uses a convection current to roast the beans evenly. This results in a more consistent roast with a cleaner flavour. Hot air roasting also allows for more precise temperature control, ensuring that the beans are roasted to perfection.

Drum roasting vs. hot air roasting: Which method is better?

Both drum roasting and hot air roasting have their own advantages and disadvantages. Ultimately, the best method for you will depend on your personal preferences and the type of coffee you are looking for.

If you are looking for a traditional coffee with a smoky flavour, then drum roasting is a good option. However, if you are looking for a more consistent and flavourful cup of coffee, then hot air roasting is the better choice.

Impact on Flavour and Aroma

Roasting is where flavour and aroma come alive. Both temperature and roast time will have an impact on how the coffee will taste. A lower temperature will result in a lighter roast which is typically more acidic and darker (higher temperature roasts) are more on the bitter side. There’s a very scientific reasons for this and it comes down to a very long complicated sounding compound called 5-hydoxymethylfurfural. It sounds confusing but it’s simple; it's a fruity compound and the longer the beans roast for, the more this compound breaks down. The less there is, the less fruity the roast.

The roast temperature is the biggest factor in determining flavour, but the time spent in the roaster is important too. A fast roast will preserve more of the aroma compounds as there is less time given for the compounds to break down. Think of it like flash-frying a steak to seal in the juices.


How long does coffee need to rest after roasting?

It’s longer than you might think, depending on the roast, the beans need anywhere between 5-14 days!

Is it better to roast coffee fast or slow?

Coffee that is roasted quickly, tends to have a stronger, acidic taste. For a rounded, natural, and smooth flavour slow roasting coffee is the way to go.

What temperature is best for roasting coffee?

Depending on the level of roast (light, medium, dark) coffee should be roasted at temperatures between 210-240 degrees Celsius. For example, a medium-dark roast is achieved with a temperature of 225 degrees Celsius.

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