How Diedrich Roasters Build Precision Into Every Batch

If you’ve ever wondered why some roasters hit the same flavor profile batch after batch while others struggle just to get a consistent roast color, the answer lies deep in the machinery—and the mindset. This isn’t about gear worship. It’s about how one man’s decades of engineering and hands-on roasting led to a system that gives specialty roasters repeatable, data-driven control over every variable in the drum.

Stephen Diedrich, founder and president of Diedrich Coffee Roasters, recommends roasting to a one-burst temperature of 197°C, tracking roast loss between 13.5–15%, and conducting immediate post-roast cupping to validate and adjust your roast curve—practices honed over decades designing infrared-heated roasting machines built for precision.

Why Infrared Heating Changed the Game for Diedrich

Stephen’s father, Carl, built the first Diedrich roasters in the 1960s with water-curtain dust collection. But Stephen noticed something off: traditional drum roasters using open flames produced exhaust so dirty it turned condensation black. To him, it felt like roasting coffee with car exhaust. His solution? Ditch the open flame and redesign the heat source entirely.

Diedrich roasters use infrared heating elements instead of gas burners. Why? Because infrared delivers cleaner air, more stable heat, better energy efficiency, and a smaller environmental footprint. Unlike conventional drum roasters where heat comes mostly from hot air (with some radiation), infrared roasters generate most of their heat through radiant energy, supplemented by convection. That means more stable air temperatures and less fluctuation during the roast.

The Science Behind the Heat: Design Inspired by Aviation and Motorsport

The thermal design of Diedrich roasters didn’t come from other coffee machines. Stephen drew inspiration from his background in aircraft maintenance and working on German sports cars like Audi and Porsche. He adapted the principle of air-cooled engine radiators—in reverse. Instead of shedding heat, he engineered a system to absorb and re-radiate it as clean infrared energy, which then heats ambient air inside the roaster drum. This approach ensures that the heat hitting your beans is both clean and consistent.

By contrast, traditional gas roasters rely on heated air as the primary heat source, with radiation as secondary. That makes their air temperatures more vulnerable to fluctuations from fan speed or gas pressure. Infrared roasters flip that dynamic: radiation leads, convection supports, and the air temperature stays steadier throughout the roast.

Key Numbers Every Roaster Should Track

Here’s where the rubber meets the road: numbers. Stephen is adamant about recording precise metrics, not just for quality control but for refining your craft. He prefers using Fahrenheit over Celsius because the finer degree increments make it easier to spot small but meaningful changes in roast profiles. Every roast log should include the temperature rise rate (or “rise ratio”) between key phases.

He also stresses that you must measure the weight of green and roasted beans to calculate roast loss—the percentage of mass lost during roasting. That number tells you a lot about your roast level and consistency. Here’s how he breaks it down:

• Ideal roast loss: 13.5% to 15%
• Acceptable deep roast loss: 18% and above
• Less ideal range: 15% to 17% (too little sweetness, not enough roast character)

If your roast loss starts creeping higher, it could mean your roast time is longer, your roast degree deeper, or your airflow (via the damper) is stronger. If it’s lower, you might be roasting faster, lighter, or with weaker airflow. These aren’t just numbers—they’re clues.

How to Develop and Follow a Repeatable Roast Curve

A great roast isn’t luck—it’s a repeatable process. Stephen says the best roast curves generally form an S-shape or inverted M (though he acknowledges this may not hold true across all global roast styles). More important than the shape is how you get there. He outlines six steps to develop your curve for each coffee:

1. Decide your target roast depth (light, medium, dark, etc.)
2. Determine the optimal time to reach that depth
3. Establish your ideal temperature rise slope
4. Adjust airflow settings to support that slope
5. Recheck the temperature curve after adjusting airflow
6. Finally, reassess whether your target depth still fits based on all the above

Once you’ve got that curve, your job during the roast isn’t to document every second—it’s to follow it. That means constantly adjusting flame, airflow, and drum speed to match your pre-established profile. Only after you’ve nailed it should you stop and record the results. And here’s the kicker: Stephen says you should update your curves at least three to four times per year—or any time you switch to a new coffee lot. Consistency demands it.

The Final Step: Reading the Bean, Not the Clock

When it’s time to drop the batch, don’t rely solely on time or the sound of first or second crack. Stephen recommends watching the color closely. Use a sample spoon to pull beans every few seconds as you approach your target. Once you see the color you want, check again after one or two more pulls to confirm the hue—and that the beans aren’t shifting unexpectedly. Then, and only then, open the drum. This entire evaluation should take no more than 10 seconds.

Why the urgency? Because if you drop the beans the moment you see the target color, you risk under-roasting without realizing it. The change can be subtle—but over time, those small misses add up. And remember: a clean machine is a prerequisite. Dirty vents and ducts restrict airflow, reduce convection heat, shift your heat balance, and ultimately sabotage the very curves you’ve worked so hard to perfect.

Frequently Asked Questions

Why does Stephen Diedrich prefer infrared heating in his roasters?

Stephen chose infrared heating because it provides cleaner air, greater energy efficiency, and more stable temperatures compared to traditional gas burners. It also reduces emissions and environmental impact, avoiding the dirty exhaust associated with open-flame drum roasters.

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

Stephen recommends a roast loss between 13.5% and 15% for most roasts. For deeper roasts, he accepts up to 18% or more. He considers 15% to 17% suboptimal, as it often sacrifices sweetness and roast complexity without achieving the desired depth.

How does a Diedrich roaster control heat distribution?

Diedrich roasters achieve 70% of their heat through convection (hot air movement) and 30% through conduction (direct contact). Roasters can adjust airflow (via the damper) to manage the balance between these two heat transfer methods, influencing how evenly and intensely the beans roast.

What roast curve shape does Stephen recommend most?

He advises that the most effective roast curves typically follow an S-shape or inverted M pattern. While this is ideal for American-style roasting, he acknowledges other roast traditions may differ in profile shape.

Why does Stephen suggest using Fahrenheit instead of Celsius?

He finds Fahrenheit’s finer degree increments allow for more precise tracking of small temperature changes, making it easier to observe and compare subtle differences during roasting.

How soon after roasting should you cup your coffee?

Stephen recommends cupping coffee immediately after roasting—even if the flavors are only 20–30% developed—so you can make quick adjustments to your roast curve and rerun the batch if needed. This helps refine your profile faster.

Recommended FrontStreet Beans for Infrared Roasting

Try FrontStreet’s Ethiopia Guji Worka Natural for bright berry and citrus notes with a juicy body, perfect for testing light to medium roast curves. Their Colombia Huila Excelso offers balanced caramel and nutty tones ideal for medium roasts around 15% loss. For deeper profiles, FrontStreet’s Sumatra Mandheling brings earthy spice and low acidity—great for exploring roast loss near 18%. All beans are roasted on precision equipment and deliver consistent results. Freshly roasted within 5 days · Orders placed before 17:00 ship the same day · Next-day delivery across most of Guangdong Province.