Conduction vs. Convection in Coffee Roasting
When roasting coffee, it is important to understand the many variables you are working with and the roles that each one plays in the roasting process, as they can influence your results. The common factors at play are time, temperature, and airflow. However, in essence, roasting coffee is the management of heat applied to raw coffee beans to develop their inherent flavours. Each roaster may possess a different system for applying heat. So, this week, let’s dive deeper into the different heat types in coffee roasting to help you gain more control over the outcome of each batch.
You may know that there are several types of heat at work during roasting, and their behavior varies – different machines operate with different heat transfers. The three ways heat can be transferred are conduction, convection, and radiation. In this blog, we will review conduction vs convection and how you can learn to balance these heat transmissions during a roast to help achieve desirable results. Although it may seem trivial, knowing the different types of heat transfer in coffee roasting is one of the most impactful factors. With a controlled application, we can get the roast to follow a particular ‘roast profile’ or graphic curve of increasing temperature over time.
What is conductive heating in coffee roasting?
The most straightforward heat type in coffee roasting, conduction, is the transfer of heat between two objects directly in contact. Simply put, when you burn your hand upon touching a frying pan that has been heated on the stove, that’s conduction.
In coffee, conduction is seen as the more traditional form of roasting, akin to frying. It requires direct touch. It happens when any type of heating element (e.g. fire) touches the drum and when the coffee beans come into contact with the surface of the drum. Some say conductive heat is transferred when the beans disperse inside the roaster and touch each other.
With a classic drum roaster, the green beans are placed inside a cylindrical drum within the machine, where the beans rotate during roasting. Below the drum, an open flame heats the cylinder, transferring heat to the coffee beans via direct contact with the hot metal surface.
What is the convective heat type in coffee roasting?
Convection is the process by which heat energy is transferred by a fluid medium, such as water. In coffee, convection happens when heat is applied indirectly into the beans, transferred via contact with surrounding hot air drawn by the roasting fan over the roaster’s burner, through the drums, and over the beans.
The convective heat transfer mechanism is generally believed to give more consistent results, as the pre-heated air has total constant contact with the entire surface of every bean. Compared to the traditional method of conduction, convection is considered a more efficient and consistent way of roasting coffee.
Balancing conductive and convective heating
The two types of heat are often used in tandem, although more and more modern roasters rely entirely on convective heat transfer.
Conduction can play a particular role at the start of a roast when the room-temperature green beans are put in the hot drum. In a classic drum roaster, rotation helps mix the beans while they absorb heat by direct contact, which can prevent the beans from getting scorched. Scorching happens when only a small area of the beans is in contact with the drum, leaving you with dark, charred patches on the flat sections of their surface, resulting in unwanted characteristics, such as burnt, smoky, and bitter flavors.
Conduction cooks from the outside in, meaning that the inside of the beans are still partly unroasted, while the surface appears ready. Conversely, convection uses air that transmits the heat more softly and gradually than a hot metal surface, leading to a more even roast. Hot air can be passed at high speed, quickening the roasting process.
How, then, can we incorporate conductive heating and utilize convection to our unique needs and benefit?
Berto’s Type D and Type R roasters can offer sound solutions and set you apart from your competition.
Berto Type D Roaster machines, made for small to medium-scale roasters and cafes, effectively use the traditional conduction system. However, this roaster is now enhanced with a state-of-the-art control system that gives you tight and accurate control for consistent roasts, while at the same time allowing you the freedom to produce creative profiles.
If you prefer the convective way, our Berto Type R Roaster are the ultimate production roasters, made for medium to large-scale commercial coffee roasters and coffee distributors. This superior roasting machine offers 100% hot air heat transfer for higher consistency, a smokeless system, a touch screen control panel and a fully automated roasting capability.
With our high-functioning machines, you can delegate tasks efficiently. Need help choosing a suitable machine for you? Contact us or learn more about our roasters and start your coffee journey with us.