Defect Identification

TL;DR: Scorched beans are caused by excessive heat transfer at the point of contact with the drum, creating dark, bitter spots; you can prevent this by managing your drum speed and heat application more precisely.

The Anatomy of a Roast Gone Wrong

You have mastered the Maillard reaction and navigated the First crack. Yet, even the most seasoned roaster occasionally pulls a batch that looks like a Dalmatian—spotted with dark, charred patches against a lighter background. These aren't just aesthetic blemishes; they are flavor saboteurs. When we talk about defects, we aren't judging your character; we are diagnosing the physical interaction between your heat source and the bean.

Most defects occur because the bean spent too much time in direct contact with the hot metal of the roaster drum, or because the airflow was insufficient to carry away the heat. Think of it like cooking a pancake: if your pan is too hot and you don't flip it, the bottom burns while the top stays raw. In coffee, we call these localized burns "tipping" or "scorching."

Identifying the Usual Suspects

To troubleshoot, you must first learn the language of the defect. Each mark tells a story of a specific thermal failure. By observing the bean surface, you can backtrack to the exact moment in your roast profile where the mechanics failed.

Scorching vs. Tipping

• Scorching: These appear as dark, irregular patches on the flat sides or edges of the bean. This is usually caused by excessive conductive heat from the drum wall before the bean has enough moisture to act as a thermal buffer. If your drum speed is too low, the beans sit against the metal for too long, essentially frying in place.
• Tipping: This is more specific. You will see the tips of the bean—the pointed ends—turn dark black while the center remains underdeveloped. This happens when the bean is subjected to high heat too quickly during the early stages, causing the edges to char before the internal structure has properly expanded.
• Uneven Development: This is a broader issue where some beans look light and others look dark. This often points to a lack of agitation or inconsistent airflow, meaning the beans aren't "dancing" enough to distribute heat evenly.

The Diagnostic Toolkit

When you pull a test batch and see these defects, don't just toss it. Use it as a roadmap. If you see scorching, your first instinct might be to lower the heat, but consider your drum speed first. Increasing the drum RPM (rotations per minute) keeps the beans moving, preventing them from lingering on the hot metal surface. If you see tipping, you likely need to "stretch" your roast, applying heat more gently in the initial stages to allow the bean's cellular structure to expand without burning the delicate tips.

By treating the roaster as a dynamic system of airflow and conductive surfaces, you move from being a passive observer to an active conductor of thermal energy. You are no longer guessing; you are engineering the environment. The bean is simply reflecting the reality of the environment you built for it.

Defect identification is the art of reading the bean's surface to reveal exactly where your heat application or agitation speed failed the roast.

Now that you can read the story written on the surface of a bean, we are ready to move beyond simple troubleshooting and into the complex world of flavor profiling.