6: Heat Transfer in Coffee Roasting

Coffee roasting machines transfer heat to beans by convection, conduction, and radiation. Each roasting machine transfers heat by a different mix of these mechanisms. The following is an overview of how machine design affects heat transfer. I discuss roasting machine designs extensively in Chapter 7.

Convection, Conduction, and Radiation

“Classic” (my term) drum roasters, which apply heat directly to the drum, cook beans primarily by convection and secondarily by conduction. Radiant heating from hot roasting-machine surfaces and between neighboring beans makes a small contribution to heat transfer as well. In a personal communication with me, a representative of a well-known German manufacturer estimated heat transfer in his company’s drum roasters to be 70% by convection and 30% by conduction.

Indirectly heated drum roasters segregate the drum from the heat source to maintain a cooler drum during roasting. Convection contributes a higher proportion of the heat transfer in these machines.

Fluid-bed roasters have no drum, and they roast by keeping the beans aloft in a high-velocity stream of hot gases. Recirculation roasters, such as the Loring Smart Roaster™, capture and reuse a proportion of the exhaust air from the roasting process. Both of these roasting machine designs transfer heat almost exclusively by convection.

At the beginning of a roast batch, charging the beans introduces a large volume of room-temperature beans and air into the hot roaster, sending the environmental temperature in the roaster plummeting. During the first few minutes of a batch in a classic drum roaster, conduction from the hot drum plays a significant role in transferring heat to the beans. As the air temperature in the roaster rebounds after its initial plunge, convection comes to dominate heat transfer. In such a machine the drum acts as a “heat-storage” device that jump-starts development early in a batch. Convection-oriented machines call for the use of hotter charge temperatures to provide adequate heat transfer early in a roast and compensate for lack of a heat-storing drum.

Establishing a high ∆T early in a roast and minimizing it by the end of a roast is essential to creating good bean inner-development and a uniform roast.

Heat Transfer and Temperature Gradient

The first two-thirds or so of roasting is an endothermic process, meaning the beans absorb energy, and heat is conducted from the outer bean to the inner bean. The temperature gradient, or “∆T,” within the beans largely determines the rate of heat transfer. Simply put, a greater ∆T causes more rapid heating of the inner bean. The ∆T early in a roast reaches an estimated 90°F (50°C),¹⁰ peaks there or

slightly higher, and decreases as roasting continues.* In other words, after the first few minutes of a roast, the bean-core temperature should slowly merge with the surface temperature as they both get hotter. In general, ∆T should peak higher in faster roasts and lower in longer roasts.

Note the large ∆T at 2:00.

Heat and Mass Transfer Within Coffee Beans

Beginning at the outermost layer of a coffee bean, moisture evaporates during roasting and forms a “front of evaporation” that moves toward the bean’s center.⁵ The cellulose structure of the inner bean, being relatively cool, remains intact and traps moisture at the bean’s core. The heating of this trapped water produces water vapor, increasing pressure within the bean and forcing its structure to expand. This pressure, estimated by various researchers to peak as low as 5.4 atmospheres (550 kPa)⁸ to as high as 25 atmospheres (2533 kPa),¹⁸ builds until the stresses are great enough to disrupt the cellulose structure, at which point first crack occurs. Once the pressure, steam, and CO₂ escape during first crack, the bean’s core temperature jumps.

Cross section of a green coffee been with mucilage layer.

Heat Transfer and Moisture

Both humidity in the roasting environment and moisture within beans influence heat transfer during roasting. After an initial lag, humidity in the roasting air increases the efficiency of heat transfer and causes faster moisture loss from the beans.⁸ Moisture content within beans has a more complex influence on roasting. Greater moisture content has three major effects on heat transfer within a bean:

It increases heat transfer because moisture increases a bean’s thermal conductivity.

It increases a bean’s specific heat capacity, meaning that the bean requires more heat energy to raise its temperature by a given amount.

I t leads to greater transfer of evaporated moisture out of the bean, inhibiting heat transfer to the inner bean.

The net effect is that temperature rises more slowly in moister beans than in drier beans.⁸ Therefore, machine operators should apply heat more aggressively when roasting moister beans and more judiciously when roasting drier beans.**

* Very fast (2-3 minutes) roasts, such as those often used in laboratory experiments, may show significantly higher temperature gradients. Roast time and peak ∆T are negatively correlated; as the value of one increases, the value of the other decreases.

** I learned this lesson the hard way during my first winter as a roaster. My green coffee had lost much moisture during storage in that season’s cold, dry air, and I found my coffees roasting too quickly. At first I didn’t know why the beans were roasting so fast, but I learned to use less heat in my roasts that winter. The next autumn I installed a humidifier in the roastery and maintained constant

temperature and humidity levels all year to stabilize the green coffee’s moisture content.