8 Facts About Coffee Roasting

We’ve all seen different roasts in coffee shop menus and on the packs of coffee we buy. But how many of us really understand what makes a dark roast different to a light one?

Or even why coffee is roasted in the first place? I want to help you understand how coffee roasting works and how it affects the flavor of your favorite drink.

When you’re finished, you’ll be much closer to choosing the right roast to suit your tastes.

So read on, find out more about this fascinating process – and perhaps even give it a try yourself!

1. Roasting brings out the flavor of the coffee bean – but also kills it

Roasting brings out the flavor of the coffee bean – but also kills it
Image: Lucky Belly

Strange though it might seem, the roasting process both unlocks the flavor of the coffee bean and starts chemical reactions that eventually lead to it becoming stale.

So how does it work?

During roasting, coffee beans are heated to an internal temperature of about 400 degrees Fahrenheit.

This intense heat causes sugars and amino acids inside the bean to decrease while gases build up.

The bean hardens and turns from green to brown, and a fragrant oil called caffeol is released.

It’s this oil that replaces the grassy aroma of green beans with the delicious coffee smell we know and love (1).

So far so good.

Over time, however, the roasted beans continue to release more carbon dioxide gas.

It’s this gas that all too soon makes the coffee taste stale.

Unfortunately, grinding the beans speeds up the staling process still further.

That’s because it increases the surface area of the coffee, making it easier for the gases to escape.

If that wasn’t enough, the research reviewed by the Specialty Coffee Association of America found that roasting releases a few substances that are responsible for the majority of coffee aroma loss.

Two of these – methanethiol and 2-methyl propanal – give the most intense aroma but are lost just two hours after roasting.

For this reason, coffee beans are usually exported in their green state and only roasted at their final destination.

2. For the best flavor, freshly roasted coffee beans need time to degas

For the best flavor, freshly roasted coffee beans need time to degas
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With all that talk of staling, you might think you should use coffee beans as soon as they’ve been roasted.

If only life were that simple…

We’ve heard that the roasting process releases gases from the coffee bean.

This is known as degassing. Whilst it continues for many days, it happens fastest soon after roasting.

If you try brewing the beans too soon, you’ll find the gas gives it a metallic flavor.

Unfortunately, while the gases are being released, the substances that give your coffee its wonderful aroma are also being lost.

The key to a perfect cup is getting the right balance between degassing and saving the aromatics in the bean.

So how do you do that?

There are a few things to bear in mind here.

Dark roasts usually degas more quickly than light roasts and longer roasts degas more quickly than faster ones.

The majority of carbon dioxide – about 40 percent – is released in the first 24 hours after roasting.

This means the best time for grinding and brewing your beans can be anywhere from 2 to 12 days after roasting.

The simplest approach is to ask your roaster or barista how long you should wait to get the best results.

Many coffee retailers won’t sell their product until it’s ready to go.

3. Drum and hot-air: the two main types of roasting machine

Drum and hot-air the two main types of roasting machine
Image: Lucky Belly

Roasting plays a hugely important part in creating the flavor of coffee.

Perhaps it’s no surprise, then, that a variety of different machines have been developed to do the job.

Today, most roasters fall into one of two categories:

  • Hot-air roasters (also known as fluid bed roasters)
  • Drum roasters

Drum roasters, as their name suggests, involve a rotating metal drum into which the beans are poured.

The beans are then heated by an external source, usually sitting beneath the drum.

This heat source can vary from electricity to natural or liquid petroleum gas, or even wood.

Drum roasters are favored by small and medium-sized producers.

Depending on the size of the drum, they can roast between 5 and 120kg per batch.

The roasting process is generally slower and gentler than in hot-air roasters.

As a result, it is considered to produce a better flavor (2).

Hot-air roasters, on the other hand, focus on maximizing the throughput of coffee beans and minimizing the human effort required.

You won’t be surprised to hear that they’re the roaster of choice for industrial producers.

Hot-air roasters heat the beans quickly using hot air flowing through perforated grids.

In some machines, the air stream inside the machine moves the beans.

In others, a conveyor belt keeps the beans moving. The process is much faster than in drum roasters.

It’s so fast, in fact, that it’s possible to roast beans in as little as 90 seconds.

4. Roasts are divided into 4 colors

Roasts are divided into 4 colors
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Coffee roasts are categorized according to their visual appearance:

  • light
  • medium
  • medium-dark
  • dark

As a general rule, the darker the bean, the longer it’s been roasted.

That’s because the natural sucrose (sugar) inside the bean goes from sweet to caramel to burnt during the roasting process.

Coffee experts usually recommend light roasts for milder varieties of coffee.

That’s because they retain more of the original flavor of the bean.

As you’d expect, they are light brown in color and will have no oil on their surface.

During roasting, light roasted beans reach an internal temperature of between 356 and 401 degrees Fahrenheit.

It is at 401 degrees Fahrenheit that the beans pop and expand, a process known as the “first crack”.

Medium roasts are a shade darker but still have no oil on the surface of the bean.

They balance the original flavor of the bean with more of the flavors from the roasting process.

For this reason, this type of roast works well for coffees with more body and sweetness.

Medium-dark roasts are darker again, and may have some oil on the surface of the bean.

Here, you’ll be able to taste even more of the roasting flavors.

The result is a coffee with more body and spice than lighter roasts.

Finally, dark roasts are almost chocolatey in appearance, with a sheen of oil on the beans.

With these roasts, the original flavor of the bean is hidden by the flavors from the roasting process.

You’ll get a more full-bodied coffee as a result.

5. Light roast Vs dark roast – Which has more caffeine?

Light roast Vs dark roast – Which has more caffeine
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While most people assume darker roasted beans have more caffeine than their lighter counterparts, others say the opposite is true.

So who’s right?

The surprising answer is no-one – or at least, it depends on how you measure it (3).

The truth is that the level of caffeine stays largely stable throughout the roasting process (4).

That, however, is not the end of the story.

What does change during roasting is the density of the coffee bean?

Broadly speaking, the longer it’s roasted the less dense – and therefore the less heavy – it becomes.

That, in turn, means the same amount of coffee by weight will contain a different number of beans, depending on whether they are light or dark roasted.

One amateur coffee-loving scientist put this to the test by measuring out equal weights of light and dark roasted coffee.

Then he counted the beans. In a ten gram sample, he found he had sixty-five light roast beans and sixty-seven dark roast beans.

In a nutshell, darker roasts weigh less.

If you measure your coffee by weighing it out – as most coffee connoisseurs recommend – you’ll get more dark roasted beans than you will light.

With roughly the same amount of caffeine per bean, that means you’ll get slightly more caffeine with a darker roast.

In truth, the difference is so small you’ll probably struggle to notice it.

6. The post-harvesting process affects how coffee should be roasted

The post-harvesting process affects how coffee should be roasted
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Coffee beans are actually the pips of the coffee fruit, also known as the coffee cherry.

The cherries need to be processed to get to the pips and dry them out ready for roasting.

Producers use a range of different processing methods. Sometimes they leave the cherries whole to dry in the sun.

Others strip away the skin or even the pulp before spreading the beans out to dry.

Research shows the interaction between processing and roasting techniques can have a major impact on coffee aroma (5).

That’s why artisan producers adjust the roast profile to the processing technique to highlight the different flavors of the coffee.

Coffee made from beans dried inside the cherries is said to have a fruitier flavor that suits a shorter roast.

Beans dried after they’ve been stripped from the cherries, on the other hand, have vanilla, chocolate, caramel and fudge flavors.

A longer roast with a steady level of heat intensifies those flavors.

But roasters will take into account more than just the processing method when deciding on the roast.

They also consider the coffee’s country of origin and the altitude at which it was grown.

Coffee variety is important too. Varieties such as Bourbon, and Catuai can take quite a lot of heat.

Others, like Geisha and Mokka, can lose their fruit flavors if exposed to very high temperatures.

7. An important part of roasting is…cooling

An important part of roasting is…cooling
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It might sound strange, but cooling the coffee beans after they’ve reached the right temperature is almost as important as heating them.

After roasting, the machine releases the beans into a cooling tray.

This tray rotates to keep the beans evenly distributed.

At the same time, air is drawn through small holes to cool them down at a steady rate (6).

While the beans are rotating, the roaster will keep a close eye on them to spot defects.

One issue to look out for is the presence of beans known as Quakers.

These are beans that have a low sugar content so don’t change color as much as other beans.

While they may look under-roasted, they will actually taste burnt.

They’re fairly common, so watching out for them is an important part of quality control (7).

8. You can roast your own coffee in a popcorn maker

You can roast your own coffee in a popcorn maker
Image: Lucky Belly

Perhaps all this talk about roasting and flavor has made you want to try it for yourself?

If so, here’s what you need to know.

Start by sourcing some green coffee beans.

You’ll probably have to track down a specialist retailer for this.

Once you’ve found one, don’t be afraid to buy in bulk: properly stored, green beans can produce delicious coffee after as long as a year of storage.

There are coffee roasters on the market that are designed for home use, but these can be expensive.

You can try using a skillet or cookie sheet in the oven, but it will be hard to get an even roast.

The best option if you’re just starting out is to use a popcorn maker.

This will keep the beans moving as they’re heated, in almost the same way as a professional roaster.

The process isn’t as difficult as you might imagine.

Depending on your machine, the whole thing should take about 4 minutes.

When the beans are cool, wait for 12 hours before storing them in an airtight container.


We hope you’ve enjoyed our look at the coffee roasting process.

We’d love to think it’s given you the information you need to find the roast that’s right for you – and perhaps even to try your hand at home roasting!

Please comment and let us know what you think.

And if you’ve liked what you’ve read, share the love and help more people unlock the mystery of coffee roasting!

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