Exploring the different types of heat used in coffee roasting

 

There are many different styles, methods and types of coffee roasting equipment that are used to turn green, unroasted coffee into a product that is able to be ground, brewed and enjoyed. 

Whether by using gas, halogen, air or another method, the essential principle of coffee roasting is using heat to turn green coffee into something that is soluble. Roasters seek not only to create a product that can be brewed, but to highlight specific characteristics of each different coffee that are the result of varietal, processing, terroir and more. 

In the following, we explore the main types of heat we use in our coffee roasting approach, to provide an understanding of how heat is used to create great coffee.

The fundamentals of coffee roasting

Coffee roasting is a craft unlike any other – it involves an acute knowledge of different coffee growing origins, bean density, moisture content and the development of acids and sugars, not to mention an understanding of basic thermodynamics. 

When a roaster receives a new coffee, they may decide to take various approaches in order to achieve a desired result. Some of the variables to consider with each coffee may include:

• Density – How dense is the coffee bean as a result of altitude or varietal? 
• Moisture – Does the green coffee have an ideal moisture content for roasting?
• Size – How small/large are the green coffee beans? 
• Characteristics – What are the main (favourable) characteristics of the coffee, and how can we use roasting to highlight them? 

A coffee roaster is essentially a big oven, and the green coffee beans are introduced into this oven to be cooked, or roasted. However, unlike an oven, most coffee roasters are equipped with the means to adjust variables such as heat, air flow and drum speed (the speed at which the main body of the roaster, the drum, rotates). 

 Adjusting variables

Making adjustments to variables such as heat is the real work of any great roast profile; due to the density of the green coffee and the moisture contained within it, heat needs to be applied in such a manner that the coffee is evenly roasted. 

If you placed a cake into an oven that was 300 degrees Celsius, you’d soon have a cake that was burnt on the outside and uncooked in the middle. However, unlike a cake, each coffee bean contains moisture in its centre that needs to be gently removed as it is roasted evenly, inside and out. 

When room temperature (green) coffee beans enter a hot roaster, they carry with it their temperature. Invariably this will cause the internal temperature of the roaster to initially drop, and over time it will climb back up as we apply more heat. As we do this, we must take into account the coffee’s density, the moisture contained within it and what we want as the desired outcome eg. roasting for espresso/filter, or highlighting certain characteristics of a coffee.

In order to ensure that this is done properly, we use a combination of conduction and convection heating when creating profiles and roasting our coffee. This provides us with the ability to remove all the internal moisture, roast the beans completely evenly, and with a high degree of control. 

Conduction vs convection heating 

Conduction heat is the transfer of energy (heat) by the collision and movement of particles and electrons – in simple terms, this is the transfer of heat from the roaster itself to the green coffee beans. Convection heating, on the other hand, is a form of heat transfer via moving liquids or gases – in the case of our roasting process, this is the hot air within the roaster. 

We roast all of our coffee in a cast iron Probat roaster, which provides us with the ability to control the variables to utilise these different types of heat. As green coffee enters the roaster, it carries its lower (room) temperature and cooler air from outside the roaster.

During the initial stages of roasting, our focus is primarily on utilising the heat of the roaster to heat the green beans as fast as possible, via conduction heating. Once the coffee has soaked up the conductive heat we’ve applied, we then turn our attention to using convective heat.

To do this, we increase both the flow of air (airflow) within the drum and the speed the drum rotates (drumspeed). Unlike conductive heat, which transfers the heat of the coffee roaster directly to the coffee beans’ surface (outside to inside), this convective heating method effectively ‘bakes’ the coffee evenly, both inside and out.  

Why do these different types of heat matter? 

We apply both conductive and convective heat with each coffee we roast. Depending on the density of the green coffee and the characteristics we want to highlight, we will change the ratio/relationship of those two types of heat application. 

These different types of heat matter as they allow us to ensure consistent roasting throughout the entire coffee bean. If we use too much conductive heat, then the coffee will be more roasted on the outside than on the inside; likewise if we focus too much on convective heat, then it will be difficult to bring the coffee up to an ideal temperature in time to both dispel all the moisture it contains and roast it in a way to highlight its characteristics. 

If you’re interested in learning more about Fjord’s roasting style and methods, visit the Roasting Approach page or explore the current range of single origin coffees.

Fjord Coffee Roasters is a Berlin-based coffee roastery dedicated to seeking out rare and exotic coffees from across the world.