Coffee roasters are often painted as the rocket scientists of the specialty coffee world, but what do they really do to unlock the world of flavour that lives inside our coffee beans?
Although roasting coffee well requires knowledge, passion, perseverance, and the ability to experiment, the reality can be explained in relatively simple terms.
The variety of your coffee, the growing conditions that nurture it and the processing method the producer applies to it all have an impact on its flavour potential. But it's not until the roast that those flavours are unlocked.
A simple analogy: how is popcorn related to coffee?
I’m pretty sure most of you will have used a microwavable popcorn sachet. Think of the microwave as your roaster and the popcorn as your green coffee.
If you keep them in the microwave longer than needed, the popcorn becomes darker in colour, with smoky flavours developing, and eventually becomes burnt and unpleasant.
On the other hand, if you don’t microwave them for long enough, not all of your kernels will fully develop and pop. If you get your timings just right, you’ll get beautifully sweet and fluffy popcorn.
OK, so coffee can be sweet, but it’s not fluffy. And a roaster wouldn’t get far with just a microwave.
But it’s a useful starting point if you’ve never thought about it before.
Green coffee and flavour
La Bastilla Coffee Estate in Nicaragua
To understand the effect that the roasting process has on coffee beans, first we need to understand a little bit about green coffee, how it’s grown and processed, and how that affects flavour.
First up, there’s species and varieties. There are four main coffee plant species:
But only two of these are used in production across the world. Arabica and Robusta.
Arabica vs. Robusta: How do they differ?
You can find a full post on arabica vs. robusta beans, but the main differences are:
- caffeine content
- how and where they can grow
Robusta has a high caffeine content but less interesting characteristics – lower acidity, high bitterness and a heavier body.
Arabica, the domain of specialty coffee, has half the amount of caffeine as Robusta, requires more attention and specific conditions to grow, but produces a much more complex and varied flavour profile when roasted.
Arabica is generally sweeter, more acidic, and lighter bodied than Robusta, but can produce 800 different aromas (that’s significantly more than wine) that span the categories of fruit, floral, sweet, cocoa and nuts and more.
Within Arabica, there are over 50 known varieties – some that you may have come across include Bourbon, Caturra, Heirloom, Gesha and SL28 – each of which have the potential to produce different levels of quality and types of flavour.
Where it's grown
Around 40% of the world’s coffee is Robusta and most of this is grown in Vietnam and Indonesia, where it’s the primary type of coffee produced. Other places, like India, Brazil and certain African origins, produce both. While elsewhere in South and Central America, Arabica is much more dominant.
Arabica’s ability to create more complex flavour profiles, combined with it’s comparative neediness, means it commands a higher price. It’s why you’ll often find Robusta in cheaper blends and instant coffees.
Growing and processing: how they affect flavour
Alongside species and variety, how and where a coffee is grown and processed really matters. Coffee that’s grown in Brazil will taste completely different to coffee that’s grown in Ethiopia, and within those two countries there’s lots of variation at a regional, local and even farm level.
Why is that?
It’s partially due to what the farmer does to the coffee once it’s been grown and picked (more on that in a second), but the conditions under which it’s grown have a big influence too - some of the biggest factors include growing altitude, temperature, rainfall, amount of shade and soil composition.
Once a coffee has been grown under the influence of these conditions, it’s processed - the step in the chain where the producer takes coffee cherries and turns them into green coffee beans.
There are 3 widely used methods, which produce distinctly different flavour characteristics when turned into roasted coffee.
- Natural process, where cherries are dried with the fruit and mucilage still intact, resulting in coffee that’s generally high in sweetness, lower in acidity and heavier in body.
- Washed process, where cherries are pulped and washed with water to remove the fruit and mucilage before drying, producing coffee that’s generally higher in acidity, lower in sweetness, and lighter in body.
- Honey process, where coffees are pulped, but not washed, leaving just the sticky mucilage in place for drying, producing coffee that retains the sweet, full bodied characteristics of a natural, but with a brighter acidity.
Why and how we roast coffee?
Given all the factors that affect flavour at origin, then why do we need to roast?
Green coffee is actually pretty flavourless, so when we talk about varietal, origin, growing conditions and processing all affecting flavour, we’re talking about the flavour of the coffee once roasted. It’s only through the roast that these flavours are unlocked and revealed.
In the roast, coffee has to experience an incredible amount of change through a range of chemical reactions, but for this to happen, the only thing that needs to be applied is heat. Though this heat needs to be applied under controlled conditions, which is the essence of the art and science of roasting.
Our Giesen W6A drum roaster
Roasting coffee requires a machine, and although there are a few different types, the principles remain similar across them all.
The key component is heat, which is transferred from a heat source to the beans through one or more of these methods, depending on the roaster.
- Conduction (contact) – heat transfers from a hotter to a colder body (such as from a hot drum to cold coffee beans)
- Radiation – energy, in the form of electromagnetic radiation, is emitted by a heated surface in all directions and travels directly to its point of absorption at the speed of light (this happens with all surfaces within a roaster)
- Convection – heat is transferred from one place to another by the movement of liquids and gases (in certain roasting machines, hot air is blown onto the beans)
The most common machine type is a Drum Roaster, which combines convection and conduction through a solid, rotating drum with an open flame located below.
As the drum rotates evenly, the beans absorb heat from its surface as well as the hot air inside, and eventually, each other.
We use a Giesen W6A drum roaster as we find it’s consistent, has loads of variables to play with and provides a really clean roasting environment.
With a Fluid Bed Roaster, on the other hand, hot air is blown into a chamber, suspending, rotating and roasting the beans through convection heat.
They’re more eco-friendly than drum roasters due to their lower CO2 emissions, but because their primary vehicle for heat transfer is hot air, and that aiflow also has to float and rotate the beans, it’s less easy to control the all important variables required to produce lightly roasted specialty coffee.
Finally, Recirculation Roasters can have a drum, or a chamber. They recirculate roasting exhaust air within the system - meaning that the hot air from the roasting chamber goes back into the burner to be used again for heating the next batch of coffee.
These roasters are the most energy efficient, but the main disadvantage is the risk of smoky flavours developing due to the reused air.
Stages and characteristics of the roast
So how do we roast coffee?
Before we begin, we’ll create a roast profile, which is sort of like developing a recipe for coffee roasting. In fact, we’ll usually create and try out a number of different roast profiles on a coffee, tasting the results of each, before deciding on THE ONE.
To develop the profile, we’ll consider:
- the green bean’s key characteristics – like its origin, process, bean size and density – and
- the flavour profile we’d like to achieve with the roasted coffee.
Then we’ll decide when to apply heat, how powerful that heat needs to be, the colour we’re aiming for and the degree of roast development – the key factors that take us from a to b.
Although there are lots of variables to play with during the process, coffee beans will always go through a number of key stages:
During this first stage, the moisture content of green coffee is around 8 – 12 per cent, so it needs to be dried before roasting begins. This takes around 2-4 minutes, up to a temperature of about 120 degrees.
As the temperature rises above 120 degrees, then the coffee beans experience the Maillard reaction, where sugars and amino acids react to form compounds called melanoids, which cause changes in colour, flavour and aroma. Here we see the colour of the beans evolve from green to yellow to brown.
Checking the beans mid-roast
Towards the end of the browning stage, moisture trapped within the beans changes from liquid to gas, pressure builds up which is then released with a popcorn like sound. This is called the first crack, and can result in the beans doubling in size as their inner structure expands.
During drying and browning, the beans have been collecting energy, but the development stage is where aroma compounds develop and the key characteristics of the coffee are defined, such as acidity, sweetness and body.
There are lots of variables to play with here – long, short, dark, light and various increments in-between – and this stage requires careful monitoring as a matter of 30 seconds can result in a completely different tasting coffee.
Once the roast is complete, then it’s important to cool the coffee to room temperature as quickly as possible to stop the process and avoid unwanted characteristics developing.
Light vs dark roasts
The evolution of coffee characteristics as roast temperature increases
When we use the terms light or dark roast, we’re referring to the colour of the coffee bean after roasting, but that colour has a significance beyond the visual.
Amongst specialty coffee roasters, it’s commonly agreed that roasting light or medium-light is the best way of drawing out the natural flavour characteristics of good quality coffee, but here we’ll cover what to expect with light, medium and dark roasts.
In your hand, light roasted beans will feel dense, dry and textured. When ground, they will be aromatic on the nose. When brewed, a good quality coffee will have a high acidity and sweetness, a low bitterness, and pronounced flavour notes that you could describe as sweet, fruity, or floral. The body will be light and delicate.
For coffees that are naturally sweeter and heavier bodied, then a medium or light-medium roast might be the best way to dial up those attributes. In your hand, the beans will look a little darker, a little smoother but still solid and dense. You’ll still get the hit of aromas when the beans are ground. But when brewed, the result will be slightly different – with a heavier body, less of a punchy acidity, and a richer sweetness more characteristic of chocolate, caramels or dried fruits.
The higher level of body and more muted acidity means that medium roasted coffees often work well as single origin espressos, or as a component for espresso blends, as they tend to also work well mixed with milk.
Although we’d never recommend roasting dark, it does have a place in today’s market, as millions of high street coffee chain customers will attest to. It’s typically used to mask the characteristics of poor quality or defective coffee, as the coffee is roasted to the point that bitter, roasty notes take the place of any of the coffee’s natural characteristics.
In the hand, dark roasted beans will brittle with a dark brown, smooth and oily appearance. When brewed, the coffee will be low acidity and low sweetness, with a heavy body – and few detectable flavour characteristics other than the bitterness imprinted by over-roasting.
Roasting for filter vs espresso
A subject that causes some debate is whether coffee should be roasted differently for filter and espresso brewing.
Traditionally, espresso has aimed for a lower acidity with a bigger body, so it can work as a black shot, or blended with milk in an array of drinks.
And as espresso is very much like coffee under a magnifying glass, with small differences in flavour being amplified, there’s an argument that a darker roast will be more forgiving with a wider margin for error under pressurised extraction.
Times and tastes have changed though, and some people (including us) love a juicy acidic espresso as much as a balanced and full bodied one.
Certainly with single origin coffees, we believe there’s good reason to aim for bringing out the coffee’s best characteristics regardless of brewing method.
More often than not, that will result in a coffee that we can recommend across filter and espresso, but if we think it’s best for one or the other, we’ll tell you so.