Beyond the Profile: Mastering Key Coffee Roasting Variables

While many coffee roasters focus intently on time and temperature parameters—the infamous "roast profile"—there is a complex world of physics happening inside the drum that dictates the final quality of the cup.

Achieving consistency and unlocking potential flavor requires understanding the dynamics that drive thermal transfer. Let’s look beyond the basic curve at the variables that truly shape a roast: drum speed, heat sources, transfer methods, and the critical importance of airflow.

The Debate Over Variable Drum Speed (Agitation)

Years ago, a major "fad" in specialty coffee was the customized shop roaster featuring a variable speed drum controller. The ability to change the RPM of the drum on the fly was seen as revolutionary. Many roasters raved about unbelievable cup quality improvements gained by decreasing or increasing RPM at specific stages.

Of course, industrial-sized roasters have utilized this feature for a long time. It is quite common for agitation speed to change throughout the cycle on large machines. The point of agitation is clear: moving the bean around to prevent excessive conduction or concentrated thermal transfer (scorching).

If a roaster is designed to utilize low speed in the early stages and faster speeds towards the end to manage changes in thermal transfer, then there is a valid argument for agitation speed control. However, for many standard shop roasters, the exaggerated improvements claimed by simply changing agitation speed were likely overstated—perhaps a placebo effect at best.

Heat Source Consistency and Installation

The heat source is another area where many roasters obsess, often misdiagnosing problems.

Consistency is key in roasting. Unfortunately, some shop roasters are designed with barely enough thermal energy to run consistent full batches back-to-back.

Even if the roaster was designed properly, poor installation can ruin its performance. Substandard fuel calibration or inadequate venting can cripple a machine's ability to transfer heat effectively, leading to chasing profiles that are impossible to replicate.

Understanding Heat Transfer: Conduction vs. Convection

To master your roasting device, you must understand thermal dynamics. There are two primary types of heat transfer during roasting:

• Conduction: Heat transferred via direct contact (e.g., beans touching the hot drum surface).

• Convection: Heat transferred via a surrounding medium (e.g., hot air in a fluidized bed roaster).

Unfortunately, there is sometimes a divide between "fluid-bed zealots" who espouse the superiority of convection roasters over old-school drum types. They often stretch the truth, claiming air roasters are "cleaner."

In reality, many modern drum roasters employ both conduction and convection effectively. Most air-roasting pundits would be surprised to discover that some advanced drum roasters can employ very high convection rates that match their so-called fluidized superiority.

Airflow: The Highly Undervalued Variable

Finally, we arrive at airflow. This topic rarely gets enough "air-time," as most people generally refer to airflow merely as the critical function of exhausting fumes, smoke, and chaff.

While exhausting byproducts is necessary, airflow is a vitally important and highly undervalued variable that requires careful management during the roasting process.

Skilled roasters use airflow in tandem with the heat source to provide dramatic changes—either advancing or retarding roasting progress. Airflow offers finer control throughout the roast, especially during critical stages where beans transition from absorbing heat (endothermic) to emitting heat (exothermic).

The Speed of Airflow vs. Heat

The primary advantage of airflow management is speed. Heat sources often have "lag" or latency—it takes time for a burner to heat up or cool down the thermal mass of a roaster. Airflow changes, by contrast, can be almost instantaneous.

If your roasts are tasting flat, dull, baked, or dry, look beyond just your time/temp profile. A very interesting dynamic relating to improper airflow management could be the root cause of your flavor issues.