Diedrich Roaster Insights: Stephen Diedrich on Specialty Coffee Roasting

If you’ve ever wondered why some roasters consistently produce better-tasting coffee—and how they manage to repeat those results—you’re not alone. Many coffee professionals struggle to turn great green beans into great roasted coffee with consistency, especially without clear guidance on the tools, techniques, and data behind professional roasting. This article reveals the technical insights and hands-on advice from Stephen Diedrich, founder and president of Diedrich Coffee Roasters, shared during a rare in-depth interview and workshop in Taiwan.

Key takeaway: Stephen Diedrich recommends roasting with infrared heat for cleaner, more stable results, tracking roast loss between 13.5%–15% for optimal sweetness, and updating your roast curves at least three to four times per year—or whenever your green coffee changes.

Why Infrared Heating Matters in Coffee Roasting

Stephen Diedrich’s company, Diedrich, builds commercial coffee roasters that use infrared heating elements instead of traditional gas burners. This design choice wasn’t arbitrary—it was driven by concerns over air quality, energy efficiency, and roast consistency. Stephen explained that traditional open-flame roasters rely heavily on combustion gases, which he likened to “roasting coffee with car exhaust.” By contrast, infrared heating provides cleaner air, better energy conversion, and more stable thermal delivery, all while being environmentally friendlier.

The idea for infrared heating came from Stephen’s background in aircraft and auto repair. He adapted principles from air-cooled aircraft engines—essentially reversing their heat dissipation design—to create a system where infrared radiation heats clean ambient air, which then roasts the coffee. This approach offers more control and a healthier roast environment compared to conventional gas-fired systems.

How Infrared Differs from Traditional Gas Roasting

Traditional gas roasters primarily use heated air (convection) with some radiant heat as a secondary source. However, the temperature of that hot air can fluctuate due to variations in airflow and flame intensity, making the roast less predictable.

Infrared roasters flip this dynamic: the primary heat transfer comes from radiant energy, with convection playing a supporting role. This setup leads to more stable air temperatures throughout the roast, giving roasters finer control over the development of flavors inside the bean.

Diedrich Roaster Design Philosophy

Diedrich machines are engineered around a specific heat distribution model: 70% of the heat transfer comes from convection (hot air movement), and 30% from conduction (direct contact with hot surfaces). Roasters can influence this balance by adjusting airflow, which alters how much convection heat reaches the beans.

A critical maintenance point: if your roaster’s exhaust system becomes clogged due to lack of cleaning, airflow will drop, reducing convection heat. To compensate, roasters may increase flame intensity, unintentionally shifting more heat transfer to conduction and altering the roast profile. Keeping the roaster and vents clean is therefore essential to maintaining your intended roast curves.

Roast Curve Development: Stephen’s Five-Step Method

According to Stephen, great roasting isn’t about guesswork or simply following a standard recipe. It’s about developing and repeatedly hitting a precise roast curve tailored to each coffee. Here’s his step-by-step method:

1. Decide on your target roast depth (e.g., light, medium, dark).
2. Determine the optimal time needed to reach that roast level.
3. Establish the ideal rate of temperature rise (the roast curve’s slope).
4. Set airflow to support that curve.
5. Recheck and refine the temperature rise based on the airflow settings.
6. Finally, reassess whether your chosen roast depth still aligns with all other variables.

Roasters must then “follow” these curves during the roast—not just record them. Trying to take notes while actively managing a roast for consistency is impractical, Stephen emphasized.

Roast Loss, Weight, and Sweetness

Track your roast loss—that is, the percentage of weight lost during roasting—closely. Stephen recommends aiming for a roast loss between 13.5% and 15% for most coffees, as this range tends to deliver optimal sweetness. For darker roasts, he accepts losses over 18%. He specifically avoids the 15%–17% range, where he finds coffee loses sweetness and complexity without gaining the desired depth of dark roast character. Instead, he suggests going deeper if a bolder profile is the goal.

Remember: higher roast loss often results from longer roast times, darker profiles, or increased airflow. Lower loss can stem from shorter roasts, lighter roasts, or reduced airflow.

Roast Temperature: Why Fahrenheit Matters

While most of the coffee world uses Celsius, Stephen prefers Fahrenheit for roast logging. The smaller degree increments (compared to Celsius) allow for more precise tracking of temperature changes and subtle curve adjustments. For example, monitoring a roast’s rate of rise (RoR) or identifying inflection points in the curve becomes easier when you can detect minor shifts in temperature.

He also noted that the temperature at first crack in his roasters typically stabilizes around 197°C (387°F), though second crack varies depending on the bean’s density.

Color, Timing, and the Exit Strategy

When deciding when to drop the coffee from the roaster, Stephen advises relying on bean color. Use a sampling probe to check the color frequently in the final moments. Once you see your target color, check again once or twice more within a 10-second window to confirm the hue is consistent and hasn’t started shifting toward another shade. Then, and only then, open the roaster door to drop the batch.

Beware of dropping the coffee the moment you see the desired color, as doing so can inadvertently result in lighter roasts over time due to delayed color manifestation during the cooling phase.

Maintaining Your Roast Curve Over Time

Your roast curve isn’t static. Stephen recommends updating it at least three to four times per year—or anytime you switch to a new lot or origin of green coffee. Each batch can behave differently, even within the same origin, so continuous testing and adjustment are crucial.

Moreover, always ensure your roaster and internal systems are scrupulously clean. Residue or buildup can alter heat transfer and airflow, rendering your carefully developed curves ineffective.

Frequently Asked Questions

What type of heating does a Diedrich roaster use?

Diedrich roasters use infrared heating elements as the primary heat source, which provide cleaner, more stable thermal energy compared to traditional gas burners. This design reduces reliance on combustion gases and improves energy efficiency.

What’s the ideal roast loss percentage according to Stephen Diedrich?

Stephen recommends a roast loss between 13.5% and 15% for most coffees, as this range typically enhances sweetness. For dark roasts, he accepts over 18%, but avoids the 15%–17% range where sweetness and complexity diminish without gaining deeper roast qualities.

Why does Stephen prefer Fahrenheit over Celsius for roast logging?

Fahrenheit allows for finer temperature distinctions due to its smaller degree intervals, making it easier to track small changes in roast curves, rate of rise, and other critical metrics during roasting.

How often should roast curves be updated?

Roast curves should be updated at least three to four times per year, or whenever you change the green coffee lot or origin, as each batch may require a slightly different curve to achieve optimal results.

How should roasters determine when to drop the coffee from the roaster?

Use a sampling probe to check bean color in the final moments. Once your target color is observed, confirm it remains stable in one or two additional checks within 10 seconds, then drop the batch immediately—without waiting for further color shifts during cooling.

What’s the typical first crack temperature in a Diedrich roaster?

In Stephen’s Diedrich roasters, first crack generally occurs around 197°C (387°F), though second crack varies based on bean density and roast level.

Recommended FrontStreet Beans for Infrared Roasting

For roasters exploring the techniques shared by Stephen Diedrich, FrontStreet Coffee’s Ethiopia Guji Huakui offers bright citrus, floral jasmine, and sweet tropical fruit notes—ideal for showcasing a well-developed light to medium roast. The Colombia Narino Huila brings balanced caramel, red apple, and gentle spice, performing beautifully in medium roasts with a 13.5–15% roast loss. Finally, try the Ethiopia Yirgacheffe Konga for juicy berry, lemon zest, and honey-like sweetness, perfect for testing precise infrared roast curves. Freshly roasted within 5 days · Orders placed before 17:00 ship the same day · Next-day delivery across most of Guangdong Province.