Roasting 101 | 6. Heat Transfer

In the coffee roasting process, heat transfer plays a crucial role in developing the unique flavor and color of coffee beans. There are three main forms of heat transfer: conduction, convection, and radiation. Each form of heat transfer affects the coffee roasting process differently and will be explained in detail below.

1. Conduction

Conduction occurs when heat is transferred from a region of higher temperature to a region of lower temperature within a material through the movement of atoms and molecules. In coffee roasting, conduction happens when coffee beans are in direct contact with the hot surface of the roasting machine. The heat from the hot surface is directly transferred to the beans, causing them to heat up and begin the roasting process.

For example, when coffee beans are on the hot surface of the roasting drum, heat is transferred from the drum to the beans through direct contact. The effectiveness of this heat transfer depends on the thermal conductivity of the drum material and the coffee beans. Materials with high thermal conductivity will transfer heat more quickly and evenly, ensuring the coffee beans roast uniformly and develop a better flavor. When you hold a can of hot tea, you feel the heat through the can because heat is conducted through the can due to the temperature difference between the hot tea and your cooler hand.

2. Convection

Convection occurs when heat is transferred through the movement of a fluid or gas. In coffee roasting, convection happens when hot air within the roaster moves around the coffee beans, carrying heat and transferring it to the beans. Convection can transfer larger amounts of heat than conduction and is commonly used in roasters that utilize hot air to roast coffee beans.

For example, in a hot air coffee roaster, hot air is blown through the coffee beans, carrying heat and heating the beans. This convection process helps distribute heat evenly across all the coffee beans, ensuring they roast evenly and preventing scorching. When a container of water is heated, conduction heat transfer occurs in the water at the bottom of the container. Additionally, the heated water rises due to buoyancy, and convection occurs along the sides of the container, leading to additional heat transfer.

3. Radiation

Radiation is the process of heat transfer through electromagnetic waves without the need for direct contact between objects. In coffee roasting, radiation occurs when heat from a heat source (such as an oven or infrared lamp) is directly transferred to the coffee beans through electromagnetic waves. Radiation is often used in specialized roasters where the heat source does not directly touch the coffee beans.

For example, when using an infrared oven to roast coffee, the heat from the infrared lamps is directly transferred to the coffee beans through electromagnetic waves. This process helps the coffee beans receive heat evenly and quickly, creating a unique flavor and attractive color.

Practical Examples of Heat Transfer

Sauna

For instance, you can enjoy a sauna with an air temperature of 100°C, but you cannot bathe in water at 100°C. This is because the heat transfer coefficient of water is higher than that of air. Liquid water transfers more heat to your skin compared to the air in the sauna. As a result, you feel much hotter in water than in the air.

Fluid Flow Velocity

Another factor that significantly influences the magnitude of the heat transfer coefficient is the fluid flow velocity over the object's surface. The higher the velocity, the larger the heat transfer coefficient. Therefore, heat transfer due to forced convection is greater than that of natural convection. A strong breeze from an electric fan will cool you faster than when no fan is present.

In coffee roasting, the three forms of heat transfer - conduction, convection, and radiation - all play important roles and affect the quality of roasted coffee beans.

Multiple questions:

1. What are the three main forms of heat transfer in coffee roasting?

   a) Conduction, Evaporation, and Radiation

   b) Conduction, Convection, and Diffusion

   c) Conduction, Convection, and Radiation

   d) Conduction, Evaporation, and Diffusion

2. When does conduction occur in coffee roasting?

   a) When hot air moves around the coffee beans

   b) When heat is transferred through electromagnetic waves

   c) When coffee beans are in direct contact with the hot surface of the roaster

   d) When coffee beans are exposed to cold air

3. Why is a material with high thermal conductivity important in coffee roasting?

   a) It helps the coffee beans to have a better color

   b) It helps reduce the temperature of the coffee beans

   c) It transfers heat faster and more evenly

   d) It helps increase the size of the coffee beans

4. When does convection occur in coffee roasting?

   a) When heat is transferred through electromagnetic waves

   b) When coffee beans are in direct contact with a hot surface

   c) When hot air moves around the coffee beans

   d) When cold air moves around the coffee beans

5. Why can convection transfer more heat than conduction in coffee roasting?

   a) Because heat is transferred through electromagnetic waves

   b) Because the coffee beans generate their own heat

   c) Because hot air moves around the coffee beans, carrying heat and transferring it to the beans

   d) Because water in the coffee beans evaporates faster