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Heat transfer methods in coffee roasting

Jason Jin
February 16, 2024
min read
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Conductive Heat

A photo of baking a pizza in a brick oven using conductive heat
A photo of baking a pizza in a brick oven using conductive heat

Conductive heat is extensively used in our daily lives. It is present in cooking pans we use for frying, heating pads for steaming, and electric blankets for warm sleeping during cold nights, among various other applications. All these involve the movement of heat, something we often take for granted. But how does this phenomenon of heat conduction occur in coffee roasting?

Concepts of Conductive Heat

Though the terms conduction, conductive heat, and thermal conduction may seem similar, they are distinct yet related concepts. It's crucial to understand their precise meanings.

  • Conduction: The movement of energy within an object.
  • Conductive Heat: The heat (energy) transferred through conduction. While conduction refers to the phenomenon itself, conductive heat is about the heat (energy) that has been moved.
  • Thermal Conduction: The phenomenon of heat (energy) being conducted. It might seem like conduction and thermal conduction are the same, but different types of energy can undergo conduction, such as electrical conduction for electricity.

Conduction in Roasting Machines

Conduction occurs not just within an object but also between different objects. When two objects with different amounts of thermal energy come into contact, heat moves from the warmer object to the cooler one. This principle allows coffee to be roasted in roasting machines. To maximize conductive heat during roasting, the charge temperature may be increased, and the drum speed may be slowed down to extend the contact time with the green beans.

You might wonder, "How much conductive heat should be used during roasting?" However, this question cannot be answered with an absolute value. The flavor and texture of the coffee are affected by the ratio of conductive heat, convective heat, and radiant heat used during roasting. After tasting the outcome, adjustments can be made to either increase or decrease the use of these heat sources to achieve the desired taste.

Furthermore, each roasting machine has its preferred methods of heat transfer, so it's important to understand the characteristics and strengths of your roasting machine model and incorporate this knowledge into your roasting practices.

Convective Heat

Convective heat is extensively utilized in our daily lives, evident in almost every space through air conditioners, radiators in old buildings, and air fryers used for cooking, among various others. However, convective heat can be generated through natural convection or forced convection depending on the situation. One might wonder what type of convection is employed in coffee roasting.

Concepts of Convective Heat

  • Convection: The phenomenon where molecules within a fluid move due to diffusion or differences in physical properties such as temperature or concentration.
  • Convective Heat: The heat (energy) that is transferred through convection.
  • Types of Convection
  • Natural Convection: The flow of fluid caused by density differences occurring naturally.
  • Forced Convection: The flow of fluid that is forcibly moved using mechanical power.

Convection in Roasting Machines

In roasting machines, forced convection is typically generated by a blower located above the cyclone. The blower creates a flow of hot air, heated from the heat source, that passes through the drum, transferring heat to the green beans.

The degree of convection can be adjusted in roasting machines capable of controlling the blower's output (RPM) using a control valve, or in other machines, by adjusting a damper located between the drum body and the cyclone.

To secure more convective heat during roasting, the output of the blower may be increased or the damper opened wider. However, hastily increasing convection without adequate firepower can lead to excessive air contact with the beans without sufficiently transferring heat internally, resulting in undercooked beans with rapidly evaporated moisture. Therefore, finding an appropriate exhaust value that is neither too fast nor too slow, matching the characteristics of the green beans, is crucial for roasters.

While increasing the drum speed of the roasting machine is sometimes believed to secure more convective heat, the fundamental factor affecting the roast is the proper setting of exhaust values according to the characteristics of the green beans, rather than significantly influencing through drum speed alone.

Radiant Heat

The reason you feel the wood fire as warm is due to radiant heat
The reason you feel the wood fire as warm is due to radiant heat

How is radiant heat utilized in everyday life? As it involves heat, it's commonly used for warming things up or in heating devices. For example, there are electric heaters that resemble fans.

Having conducted coffee training for over five years, I've found that many people struggle to understand radiant heat. To help with this, I often use the analogy of a candle. If you place your hand slightly above a candle, it feels warm due to convective heat. However, if you move your hand to the side, it still warms up, even though it's not directly touching anything, indicating that it's not conductive heat. The heat being transferred in this situation is radiant heat. Similarly, when we sit around a campfire outdoors, we feel warm due to radiant heat. So, how is radiant heat applied in coffee roasting?

Concepts of Radiant Heat

  • Radiation: Inside atoms, electrons shift from their original energy levels to different ones when they gain or lose heat, emitting or absorbing electromagnetic waves in the process. This allows heat to be transferred directly from a hotter object to a cooler one without needing a medium. (Source: Doosan Encyclopedia)
  • Radiant Heat: The heat (energy) generated (transferred) through radiation, with higher temperatures resulting in more significant radiant heat production.

Radiant Heat in Roasting Machines

During roasting, radiant heat occurs everywhere in the roasting machine. Since all objects above absolute zero (-273°C) emit radiant heat, and roasting machines are preheated before roasting coffee, it can be said that all parts supply radiant heat. However, the following three sources have the most direct impact on the coffee:

  • Drum: Positioned closest to the green beans and being the hottest part, it directly transfers the most radiant heat to the beans.
  • Beans Themselves: Radiant heat moves from hotter to cooler objects. Although it might seem like all beans within the drum heat up uniformly, variations in type, density, and size mean some beans heat up faster and, in turn, transfer radiant heat to cooler beans.
  • Heat Source: Direct-fire roasting machines, where the drum is perforated, allow coffee to be directly exposed to the heat source, receiving radiant heat from it directly. For example, Fuji Royal roasters.
  • Machines with an additional radiant heat source (like halogen) installed inside the drum, such as Stronghold roasters, directly transfer radiant heat from that additional source.

Understanding and utilizing radiant heat in coffee roasting is vital for achieving desired roasting profiles and enhancing the coffee's flavor characteristics.

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Jason Jin
Firescope Co-founder

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