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Coffee roasters: drum versus hot air

Roasting is the stage at which the green, tasteless coffee bean turns into an aromatic, brown bean ready to brew. But the way the heat is delivered during roasting has a huge influence on the result - and it depends on the kind of roaster. There are two main types: the drum roaster, the most popular, and the hot air roaster, called fluid bed. They differ in the mechanism of heat transfer, which translates into a different flavour profile of the coffee. The drum gives body and depth, hot air gives cleanliness and brightness. In this post you will get to know both kinds of roaster, learn how they transfer heat, what the difference between conduction and convection is and how this affects flavour. It is a journey into the technology of coffee roasting. Let us start with how heat is transferred during roasting at all.

How heat is transferred

To understand the difference between roasters, one must get to know the three ways of transferring heat in coffee roasting. Conduction is the transfer of heat by direct contact - when the bean touches a hot metal surface, like the wall of the drum. Convection is the transfer of heat by the movement of hot air flowing around the beans. The third way, radiation, usually plays a smaller role. The key difference between roasters lies in what proportions they use conduction and convection. The drum roaster combines both mechanisms: the bean touches the hot wall (conduction) and is flowed around by hot air (convection). The hot air roaster relies almost entirely on convection: the bean is suspended in a stream of hot air, not touching metal. These different proportions of heat transfer give a different result in the cup. Understanding conduction and convection is the key to understanding the difference between roasters. So let us get to know both types, starting with the most popular - the drum roaster.

The drum roaster

The drum roaster is the most popular and most traditional type of coffee roaster. It uses a heated metal cylinder (the drum), which rotates, tumbling the beans during roasting. Heat is transferred by a combination of conduction (the beans touch the hot surface of the drum) and convection (hot air circulates in the drum). Usually convection dominates, but the share of conduction is significant and gives the coffee character. The rotating drum ensures that the beans are constantly in motion, which helps with even roasting. Drum roasting usually lasts longer than hot air - about 15-20 minutes. The drum roaster gives the roaster great control over the roast profile and is valued for its ability to draw out the body, depth and complexity of coffee. It is the workhorse of most roasteries, from small craft to large commercial. The drum roaster is a classic, proven over decades. Its mixed heat transfer is the secret of a full, rich flavour. Let us now get to know the alternative - the hot air roaster.

The hot air roaster

The hot air roaster, called fluid bed, is a more modern alternative to the drum. In this roaster the beans are suspended on a stream of hot air, completely eliminating contact with metal surfaces. The heat transfer here is almost entirely convective - the bean is heated by the hot air flowing around it, like in a hot whirl. This method has several characteristic traits. First, it is much faster: the heat transfer coefficient in air roasting is dramatically higher than in the drum, which lets a batch be roasted in 6-8 minutes instead of 15-20. Second, the bean never touches hot metal, which minimises the risk of scorching and unevenness. Third, the hot air blows the chaff away from the beans during roasting. The fluid bed roaster gives cleaner, more even roasting. It is a modern approach, valuing speed and cleanliness. Let us now get to know how these differences translate into the flavour of coffee.

Differences in heat transfer

The heart of the difference between roasters is the efficiency and character of heat transfer. The heat transfer coefficient in air roasting is dramatically higher than in the drum - studies show values of the order of 14,000-80,000 W/m²K for the fluid bed roaster, compared to just 1,200-3,100 W/m²K for the drum. This huge difference in efficiency means the air roaster roasts a batch much faster. The consequences are significant. The fast, intensely convective roasting in the fluid bed roaster gives even heating without contact with metal, minimising scorching and unevenness. The slower drum roasting, with a share of conduction, gives more time for the development of body and depth, but is also more prone to unevenness if the airflow is limited or the beans pile up. The difference in heat transfer is not a technical detail, but the foundation of distinct flavour profiles. It explains why the same beans can taste different from different roasters. We write more about the roast profile itself in our post on the roast curve and development time.

The influence on coffee flavour

The differences in heat transfer translate directly into the flavour of coffee. Coffees from the fluid bed (hot air) roaster are usually brighter and have less body than coffees from the drum roaster. Air roasting can draw out brighter, cleaner flavours, because it avoids the heavy caramelisation typical of the drum. That is why coffees from the fluid bed roaster are sometimes described as more acidic, clean and expressive in notes. The drum roaster in turn, thanks to the share of conduction and slower roasting, gives coffees of greater body, depth and complexity, with richer caramelisation. Additionally the air roaster blows the chaff away from the beans - and studies show that although chaff contains beneficial compounds, its combustion during roasting can introduce unwanted flavours. The choice of roaster is thus a choice of profile: brightness and cleanliness versus body and depth. These are two different styles of coffee, both with their supporters. The influence of the roaster on flavour is real and noticeable. It is another factor shaping the final cup.

Roasting evenness

An important aspect differentiating the roasters is roasting evenness. The fluid bed roaster, thanks to fast and even heat transfer without touching hot metal, minimises typical roasting defects, like tipping (scorched bean ends) or scorching (surface burning). The beans suspended in the air stream are heated evenly from all sides, which gives a uniform result. The drum roaster in turn is more prone to uneven heat distribution, especially when the airflow is limited or the beans pile up unevenly - this can cause tipping and scorching, leading to bitter or burnt notes. This does not mean the drum roasts worse - well conducted it gives excellent results - but it requires more attention and skill from the roaster. The fluid bed roaster is in this respect more forgiving. Evenness is another axis on which the two types of roaster differ. It is a factor important especially for the repeatability and cleanliness of the profile. Each type has its strengths and weaknesses here.

Coffee roasters in a table

Let us set both types of coffee roaster side by side:

The table shows that the drum and hot air are two different approaches to roasting. The drum combines conduction and convection, roasts more slowly and gives coffees of greater body, depth and caramelisation. The fluid bed relies on convection, roasts fast and gives coffees that are brighter, cleaner and more acidic. It is not a matter of one being better, but of two different styles of coffee. The choice of roaster is a choice of flavour profile. Each type has its advantages and its supporters among roasters.

Why it is worth knowing this

Understanding the difference between roasters enriches the appreciation of coffee. First, it shows that the flavour of coffee depends not only on the bean and the roast level, but also on the technology of the roaster in which it was roasted. Second, it explains why the same beans can taste different from different roasteries - the drum and hot air give different profiles. Third, it helps understand coffee descriptions: information about air or drum roasting is a hint about the profile. Fourth, it lets you appreciate the craft of the roaster, who matches the roaster and profile to the bean. A conscious coffee lover knows that behind the brightness or depth of their favourite coffee may stand the choice of roaster type. Next time, trying a clean, acidic coffee or a full, deep one, it is worth thinking about which roaster might have roasted it. It is knowledge that deepens the understanding of coffee and lets you better match it to your own preferences. The roaster is one of many factors shaping the flavour in the cup.

The key points in a nutshell

Coffee is roasted in two main types of roaster. The drum roaster, the most popular, uses a rotating hot cylinder and combines conduction (the bean touches metal) with convection (hot air), roasts more slowly (15-20 minutes) and gives coffees of greater body, depth and caramelisation. The hot air (fluid bed) roaster suspends the beans in a stream of hot air, relies almost entirely on convection, roasts much faster (6-8 minutes, heat transfer coefficient many times higher) and gives coffees that are brighter, cleaner and more acidic, minimising scorching. It is not a matter of a better roaster, but of two different flavour profiles. Want to compare coffees from different roasters and record your impressions? Keep tasting notes in the GustoNote app. See also our posts on coffee roast levels and on the roast curve.